Seric

The Offer

• Check your Wells Fargo Go Far Rewards login for a cashback offer at select merchants.Readers are reporting seeing deals at the following merchants for 1-5% back
  • Office Depot – 2%
  • CVS
  • Sam’s Club – 5% (up to 10,000 points)

The Fine Print

• Valid until January 15, 2019

Our Verdict

R.I.P those sweet months they offered 40-50% at some merchants. I guess the Sam’s Club deal might still be useful for some readers, just not as useful as the previous deals.

The post Wells Fargo Go Far Rewards: 1-5% At Select Merchants (Office Depot, CVS & Sam’s Club) appeared first on Doctor Of Credit.

There are a lot of right ways to do optical testing. The gold standard is to put the lens on a $150,000 optical bench or run it through a well-equipped Imatest lab. But unless you have an optical bench or Imatest lab handy, that’s not practical.

We have to optically test around 400 lenses a day, which is more than our Imatest lab can possibly handle. So over the years we’ve learned a lot about practical ways to test lenses. We’ve constantly double-checked our methods using Imatest and an optical bench, refining our optical testing.

We’ve developed a simple set up that is about 98% accurate in identifying lenses that are decentered or optically misaligned. A lot of people could do this at home themselves. Certainly any camera club could make an identical setup. 

Judging from the emails I get, a lot of people want to be able to test their lenses optically and few know how to do it, so this should be useful for them. 

But let me be clear about this is not, so I don’t waste your time if you wanted something else. This is not going to let you test 6 copies of a lens and determine which is the absolute sharpest copy on your camera. It’s doesn’t let you compare two different lens models and decide which is sharper. For that you do need Imatest, a bench, or just a lot of photographs.

But if you have a lens that isn’t performing like you think it should this will let you determine if the lens is optically aligned and centered properly. It can be helpful to know if the problem is with the optics of that copy, rather than the focus system, technique, or simply that the lens isn’t designed to perform any better than that. 

Nobody needs to do optical testing, of course. You can just take pictures and if you don’t like the lens send it back. But that’s not always convenient or possible. Testing is a lot faster than trying out 4 copies before you decide a lens just isn’t for you.

A Few Things Before We Start

You will need a tripod.

Some of my most experienced techs can test hand held. It usually takes about 6 months of testing 75 lenses a day before they can, though. So I recommend testing your first 9,000 lenses on a tripod. After that you can try it hand-held if you like, although you will lose accuracy.

Being Perfectly Square to the Test Chart is Required

This is the main reason a tripod is needed, although not the only reason. If you are tilted at a slight angle to the chart, you’ll get a lot of the same results you would get if the lens were decentered.

Center Sharpness is the Least Sensitive Test of a Lens

Most lenses can be pretty badly decentered before center sharpness is affected very much. Eight out of every 10 lenses we fail as ‘optically decentered’ would pass fine if center resolution was the only thing we checked. Those that do fail in the center always also fail off center.

You Can Learn a Lot About a Lens by Observing it Slightly Out of Focus

Of course, they’re blurry when slightly out of focus, but the way that they blur can be quite revealing.

Equipment You’ll Need

Here’s all you have to have:

• Tripod
• Test chart
• Camera output to a computer monitor 

I don’t think I need to say anything about tripods, but I’ll go into a bit more detail on the other two requirements.

Test Chart

We’ve tried basically every type of optical testing chart that exists and by far the most useful is the ISO 12233 chart. A lot of people use AF1951 charts stuck in multiple places for testing. These aren’t completely useless, but they are very insensitive. Some fairly bad lenses look fine when you just use a bunch of AF 1951 charts to test them. Remember, they were designed to test things back in 1951.  Hopefully our standards are a bit higher today.

(As an aside, I know of several service centers that use only a modified version of AF 1951 bars to test lenses optically. Which explains why they send lenses like this back, saying they are optically fine.)

The variable targets in each corner provide great corner-to-corner comparison, which is the most sensitive indicator of an optically decentered lens. The long gradual change in line spacing shows a lot more than the short bars of an AF 1951 chart. It also provides identical horizontal and vertical test areas in each corner. While not a true test of astigmatism, differences in the horizontal and vertical targets strongly correlate with astigmatism and other aberrations. The thick, slant-edge boxes and bars show chromatic aberration and blurring in a way an AF1951 chart doesn’t do.

High-quality ISO 12233 charts can be pricey. A good quality, 40″ wide ISO 12233 chart costs about $250. A top quality (photo paper) 60″ chart like we use in our test lab is about $700. But you can download a free version of the chart courtesy of Stephen Westin, enlarge it, clean it up in Photoshop, and print it yourself. It isn’t quite as sensitive as a professional quality chart, but it is good enough.

One thing you will want, though, is a thick black border, perfectly square, around the edge of the test chart. I’ll show you why later. If you’re printing your own, just add a rectangular border in Photoshop. If you’re buying one, it will almost certainly come with a border.

You’ll also want the biggest chart you can get. You need the chart to fill the photo frame completely when testing, so using a small chart to test a wide angle lens gets too close to minimum focusing distance for accuracy. I recommend testing at least twice the minimum focusing distance, and further away is even better.

There is one other test chart we find exceedingly useful, the Zeiss version of Siemens Star Chart, which you can buy for under $30. (Or download and print courtesy of John Williams, who has posted a pdf on Wikipedia Commons.) We only use the very center part: the black dot, surrounding white circle, and a bit of the surrounding star.

In order to keep things simple, we cut the center 2″ out of the Star chart and glue it over the existing center of an ISO 12233 chart. Sometimes we’ll add stars in the corner, too, just outside the vertical/horizontal line cross.

Lighting

Lighting isn’t critical for this kind of testing; you simply want to light the chart with reasonably bright continuous light. It doesn’t have to be extremely bright. Too much light can actually be a problem. We do a lot of this testing in live view mode and too much light will cause the camera to close down the aperture. Try your ambient room lighting before you set anything else up; it will probably be fine.

Industrial quality fluorescent fixtures will cause the usual banding problems and should be avoided if possible. If your testing area has standard fluorescent fixtures (like ours does) then you’ll need some additional lights to overwhelm the room lighting. In this case a couple of incandescent or photo-quality fluorescent softboxes are inexpensive and perfectly adequate.

A Monitor

The goal here is simply to be able to see the camera’s live view output on a decent sized, good quality monitor. For most cameras we simply run an HDMI cable from the camera’s HDMI out port to a freestanding monitor. No computer necessary. We prefer a large monitor for the comfortable working distance, but that’s just a preference, not a necessity. You could use a laptop or tablet with a high-resolution monitor, or a video monitor if you have those handy.

The HDMI output isn’t going to have resolution as high as an actual photographic image, but it’s good enough for this testing. We always confirm with an actual photographic image, of course, just to be certain. But it’s really rare that the photo shows something we missed in live view.

If your camera doesn’t have video output, or you don’t have a monitor around, there are lots of workarounds. Any tethered shooting setup will give you the same capabilities and can be run to a laptop or tablet. Built-in camera Wi-Fi or a Wi-Fi card may do the same thing, as long as it gives you live-view focusing capabilities. If your camera just isn’t capable of putting out live view to a monitor, you can try doing this using a series of photographs, but you lose a lot of information and it’s much, much slower.

Here’s a picture of one of our ‘portable’ setups, like the one I’ll bring to WPPI. (The chart is mounted in a non-portable light housing that we use for multiple purposes. We need the lighting to wash out the industrial fluorescent lighting in our building, but you probably won’t). Chart, camera on tripod, and HDMI monitor are all that is necessary.

Someone will ask if they can do this simply using the camera’s LCD. Yes, you can. But it will take a lot more time, be more prone to error, and you’ll go blind after a while.

Setting Up

Setting up square to the monitor is the final step, and it’s actually pretty quick to do. The first step is centering the camera.

1. Place the tripod and camera close to the test target, raise the camera so it’s the exact height as the center of the target and lock the column (or legs) in place.
2. Gaffer tape or draw a line on the floor at right angles from the center of the test target. Now as long as your tripod is centered over your line, your lens will be centered on the test target.

The second step is eliminating any tilt and rotation of the camera, which is also simple.

1. Center the tripod over the line you made at right angles to the test chart, at the spot where the test target fills the image on the monitor. (You should just see the black edges around the chart.)
2. Tilt the camera right or left, up or down so that the rectangle is squared. If the camera is tilted the rectangle will look like a trapezoid. (Obviously if the lens has pincushion or barrel distortion it won’t quite be a rectangle, but you get the idea).
3. Rotate the camera clockwise or counterclockwise so the top and bottom lines are perfectly horizontal

If you want to double check your final setup take a picture, download it in Photoshop and measure the length of opposite lines (they’ll be equal if the image is square) and their angle. A degree or two of tilt and rotation is fine for this work (unlike Imatest, where 0.5 degrees is critical).

If you look back at the image above you can clearly see from the picture on the monitor that we haven’t squared things yet (look at the white band outside the top and bottom of the test chart’s black box). The camera will need a couple of degrees of clockwise rotation and a bit of turning. We’re using a geared head here, but a ballhead or pan-tilt head works just fine.

Basic Testing

I’ll give a lot more examples of testing in the third part of this series, but the basics are quite simple.  The first point is that we test with the aperture wide open. Even a badly decentered lens may look reasonably good when stopped down.

For this reason you don’t want too much light on the target when testing in live-view mode: the camera will stop the lens down and override your aperture setting, making the lens look better than it really is when looking at it live. Then when you take a photograph of your test image, you’ll wonder why it looks so much worse. (If you have a question if this is happening, just walk around to the front of your setup and look into the lens barrel. You’ll be able to see if the aperture is stopping down.)

Focusing and the Center Star Chart

The star chart in the center of the test target makes it very easy to focus in live view, but first examine the pattern of the chart just out of focus at maximum magnification. On a well aligned lens the star should blur into a fuzzy oval like three of the images below do. A badly decentered lens will blur with a flare going in one direction or another, like the image on the lower left below.

If you look carefully, you’ll notice the image on the upper left has just a bit of flare going upward, toward 12 o’clock. This amount of flare can occur on some normal lenses, but the amount on the lower left is never normal.

When you get the star sharply focused, the pattern should be circular, like the image below. In some decentered lenses the gray circle, where the individual star lines blur together will be oval.  In other types of decentering, you may notice the more vertical stars are much sharper than the horizontal stars, or vice-versa.

I suggest taking a photograph with the center both in and slightly out of focus. Once in a while, examining the photograph will show an  abnormality of the star pattern or a flare that wasn’t obvious in live view.

Checking the Corners

Once the lens is perfectly focused in the center, it’s time to examine the corners. If you have a really good, large monitor and a camera that outputs good live view images you can  get a pretty good idea about the corners looking at the entire image in unmagnified view. Most of the time, though, you’ll be much happier doing this at 5X magnification and moving the cursor around the screen to look at the corners individually.

The idea is NOT trying to measure how well the lines resolve, especially in live view. It’s whether the lines all appear equally sharp in each corner. If all four corners look like the close-up of the ISO chart I posted above, you’ve got a great lens. No further testing is needed, go take pictures. It probably won’t, though. So let’s talk a bit about what you’re more likely to see.

Normal Field Curvature

Most lenses have some field curvature, so the corners aren’t at their sharpest when the center is in best focus. When you examine them, the corners all look a bit soft, but equally so. The second step is to zoom in on one corner and manually focus the lens to bring the corner into best focus. If you look at how far you have to turn the focus ring to get the corner in best sharpness you’ve learned something about the lenses field curvature: was the corner in best focus closer than the center, or further away? And by how much? That can be useful to know when you’re out taking pictures later.

Once any corner is in sharpest focus, move the cursor around to check the other three corners and see if they are all equally sharp again. Field curvature, which is a normal part of lens design, means the corners will have a different focus point than the center. But since our alignment is square and centered to the chart, the 4 corners should all focus at the same point. As long as they do, this part of the test is normal.

Abnormal Field Curvature (Tilt)

If some corners have sharpest focus at a different place than others, that’s not field curvature, it’s tilt. Until now I’ve been using the generic term ‘decentered’ for any lens that is optically out of sorts. In reality, there are three types of misalignment: decentering, tilt, and spacing errors. If you’re interested in the difference, I describe them here. A badly aligned lens often has more than one problem and the symptoms overlap somewhat, but we can make some generalizations.

A mildly tilted element often causes two corners to be different than the other two corners. (It can be side-to-side, top-to-bottom, or opposite corners.)

How do we tell this is simply tilt, rather than another form of decentering? With tilt, changing focus often makes the top two corners sharp, but the bottom corners will get fuzzy (or left and right, etc). With other problems, the top two corners usually never get sharp, they always remain softer than the bottom corners. But that’s a generalization that isn’t always true and really doesn’t matter unless you’re optically adjusting lenses.

More Fun with Corners

A mildly decentered lens may only have one bad corner rather than two like the example above. In that case center resolution is always still excellent, although on an optical bench or in an Imatest lab you might notice the sharpest point of the lens has moved slightly off center, away from the bad corner.

In other cases the problem affects only the horizontal or vertical line pairs, like the image below. It is subtler than the example above, but it’s still significant enough to cause a loss of sharpness on that side of a photograph.

One other interesting finding that we sometimes see in this situation: by slightly changing focus you may be able to make either the horizontal or vertical lines sharp, but not both at the same time in the affected area. There are some lenses that will show this astigmatism-like behavior in all 4 corners. That’s just the way that particular lens is designed. But if only 1 or 2 corners show it, the lens has issues.

When All 4 Corners Look Bad

A badly tilted element, especially if near the rear of the lens, can make the entire image soft, even in the center. Spacing errors, and significant decentering of an element can have the same effect. Sometimes a decentered element leaves the center of the lens quite sharp, but all 4 corners very soft.

The question becomes how do you tell if a lens has a significant optical abnormality, or it’s just a bad design that is soft no matter what copy you have? Usually when this happens, the lens is so bad you don’t need any kind of testing to see the problem. There are halos, you can’t identify anything in the image, the camera won’t even autofocus with the lens, etc.

It’s rare, but there are cases where all 4 corners are bad, or the entire lens is somewhat softer than it should be, but not so bad as to be immediately obvious. A little common sense helps in this situation. You should expect 10X consumer grade zoom to be rather soft at the long end. An f/1.2 lens is probably going to have slightly soft corners at widest aperture. Even good telephoto zooms often have soft corners at the extreme telephoto end.

There are several things that help identify the ‘optically bad copy’ from ‘badly designed lens’ in this situation. First and foremost is the center star target. A lens decentered this badly will almost always have a significant flare, like we demonstrated above, when just slightly off focus. Some will even show flare at their best focus. If you examine the corners carefully they may all be soft, but one is usually quite a bit better than the others, and one quite a bit worse. Examining the thick black boxes will often show chromatic aberration at the edges and it will be of odd pattern: It might be away from center on one side and toward center at the other, etc.

If you’re still not certain, retest the lens with the aperture closed down 1 stop. Obviously all lenses will be a bit sharper stopped down. But with an optically decentered lens you’ll usually see some corners get much sharper stopped down than others, and the pattern will look like the ones we’ve described above.

I want to emphasize that this ‘all over’ or ‘all corners’ softer decentering is really rare. Out of every 100 decentered lenses,  we find 2 or 3 of them have this kind of pattern.

Limitations

Zoom lenses have to be checked at 2 or 3 spots. Most optically challenged zooms are bad throughout the range, but there are definitely some that only have a problem at the long or short end. Obviously you have to move the tripod to reframe the image at a different zoom range and realign the camera, but that shouldn’t take more than a few minutes.

Because of the limitations of chart size, testing at a given focal length is only done at one focusing distance. There are lenses, although they are rare, that have problems at certain focusing distances but not others and this test might miss that problem. But optical benches and Imatest have the same limitation; they only test at certain distances. You can overcome that to some degree by making charts of different sizes, but that’s a lot of trouble to go to for something you’ll probably never see.

One thing I should note: this test may over read a bit, particularly on wide-angle zoom lenses and large aperture, wide-angle prime lenses. Most of these have a little bit of flare in the center even when perfectly adjusted and all 4 corners will rarely be completely identical, although they should be close. If you look carefully enough, for example, you’ll see slightly softer horizontal or vertical lines in almost every copy of a 16-35 f/2.8 zoom or a 35mm f/1.4 prime shot wide open.

That’s one reason I suggest this testing setup for a camera club. If a friend brings another copy of the lens, you’ll feel a lot better comparing yours to his when you have a question, and you should be able to do that in 10 minutes.

Summary

A lens that’s optically awful, bad enough to affect sharpness everywhere, will generally show a lot of flare of the Siemens Star in the center when slightly defocused. All four corners may be blurry, but some corners will be worse than others, or the horizontal and vertical resolution will be different.

Less severely decentered lenses may look fine in the center and may, or may not, exhibit center flare. They will show differences in the corners that are readily apparent, however. If a lens has no flare in the center and the corners of the ISO 12233 chart all look identical, it’s optically fine.

We’ve been testing exactly this way for several years, identifying 60 to 70 lenses a month as decentered. We retest all of those on an optical bench and rarely find one that was actually OK. In other words, the test is nearly 100% specific. If it says a lens is decentered, it’s almost certainly decentered.

We also run QA checks using Imatest or the optical bench on lenses that have passed this optical testing. When we recheck these lenses, we do find that about 0.5% of  are actually decentered. However, if we repeat this optical chart test on those same lenses, they almost always fail the repeat test. In other words human error, not the test itself, let the bad lenses pass.

There are lots of other ways to test lenses, of course. But we’ve tried most of them and this is by far the most accurate in our experience, short of setting up an Imatest lab. And testing 8,000 to 9,000 lenses a month, experience is one thing we have a lot of.

The next article will be a few days in the making, but we’ll have a lot more examples of problem lenses, and will probably do a simple optical adjustment using nothing but this test system.

Roger Cicala

Lensrentals.com

February, 2012

Addendum:

I linked to the seven-hour video file from Trustycon, the convention held as an alternative to RSA's annual security event, inspired by the revelation that RSA took money from the NSA to sabotage its own products.

Now Al has broken down the video into the individual talks, uploading them to Youtube. This is very handy -- thanks, Al!

TrustyCon Videos Available (Thanks, Al!)

Finally, a sport that tests your biology, design and photography skills, along with your patience. Aquascaping—competitive aquarium design—is a completely real thing and the finished products are amazing. Hundreds of competitors flock to The International Aquatic Plants Layout Contest year after year to show off their water gardening skills.

The landscapes come off more dream-like than anything else—only when you notice tiny fish and other aquarium dwellers in the nooks and crannies of the photos that you're convinced it's real. I bet many of you, like me, shudder at the thought of how long it takes to clean the tank; an award-winning aquascape can take months to years to complete.

A few days ago I learned about the Metabones Speed Booster. For both of you who haven’t heard yet, this is an adapter containing optical elements and electronic controls that allows you to mount Canon EF lenses to Sony NEX cameras (other versions are planned for other lenses and cameras). The quick summary is the adapter is the opposite of a teleconverter.

A teleconverter spreads out the light leaving the lens so that only the center portion reaches the sensor. The result is the focal length of the lens seems longer (the image is magnified), but at the cost of reducing the amount of light (effective aperture) of the lens. The Speed Booster compresses the light leaving the lens onto a smaller image circle. This makes the focal length seem shorter and actually increases the amount of light reaching the sensor.

The Metabones’ Speed Booster compresses the light leaving the lens into a smaller image circle. Image credit Metabones http://www.metabones.com/images/metabones/Speed%20Booster%20White%20Paper.pdf

The EF to NEX Speed Booster, for example, changes the effective focal length x 0.71, and increases the maximum aperture by 1 stop. A Canon 50mm f/1.2 lens effectively becomes a 35.5mm f/0.9 NEX lens, for example. Videographers all over the internet were singing Hosannah and laying palm leaves along the path of it’s introduction.

I went into my office, shut the door, and sobbed quietly for a while. Why, you ask? I’ll tell you why. About once a day, we get an email saying somthing like, “I just rented a Canon 5D Mk III and shot video of my daughter’s high-school graduation. My footage doesn’t look anything like Vincent LaForet’s. Obviously the camera was defective and I want my money back.”

This adapter was, I thought, going to result in another 50 emails saying, “I just shot video using a generic 50mm f/2.0 lens I bought on eBay with the Speed Booster adapter and Sony NEX VG20 I rented from you. The footage doesn’t look anything like the footage shot with a Zeiss 35mm T1.5 Super Speed shot on a RED Epic. Obviously the equipment is defective and I want my money back.” I know, like I know the sun is going to rise in the east tomorrow, that you don’t put some more glass between a camera and a lens and get a better image.

But marketing hype will be marketing hype and people who want to believe in magic will believe it – and be disappointed when the magic doesn’t happen.

But Then, There Came a Ray of Hope

Then I flipped over to Metabones’ white paper on the Speed Booster and spit coffee. The primary designer of the adapter is Brian Caldwell. If anyone could make optics do magic, he could. He designed, for example, the Coastal Optics UV-VIS-IR Macro lens, an amazing thing that is the gold standard for forensic macro photography. So I read the white paper carefully and it made perfect sense. The White Paper explained how:

1. The Speed Booster introduces zero (none, nada) spherical aberration, even with an f/0.9 output. That’s amazing. The very complete graphics in the White Paper do show it adds a bit of astigmatism and distortion, though.
2. Where teleconverters magnify lens aberration, a focal reducer would reduce aberations basically because it would shrink them.
3. The adapter is physically smaller than a standard, non-optical EF to NEX adapter.
4. Corner illumination is improved.

After reading the white paper, I became convinced that these things were true. And these are all good things.

There was an additional claim made in the white paper, that MTF (modulation transfer function -  acutance and resolution) was also improved. This one I struggled with. To be blunt, I found this section to be, shall we say, selective, in the comparisons made. I was left with the feeling that it might be using some very specific examples to suggest a general conclusion. The section was a bit more carefully worded than other parts of the white paper, and some information in the graphs, that didn’t quite agree with the claim, was downplayed in the text of the section.

I was willing, however, to be convinced that an FX lens mounted to the Speed Booster would have better corner resolution mounted to a Micro 4/3 than the same lens mounted to a full-frame camera – after all, those Micro 4/3 corners are a lot closer to the center of the image. And that it may, or may not, have better corner resolution mounted to an NEX camera compared to a full-frame camera.

So after my research, my impression was this will be, at least a very useful tool. It may be nearly as good as people hope it will be. In other words, it will, like all other imaging gear ever made, follow Roger’s Law of New Product Introduction (pathway A).

Let’s Do Some Testing Boys and Girls!

OK, first and foremost, this is not going to be a bunch of video samples. That’s not what I do. I’m a testing geek that writes words and makes graphs. But for those who have bravely come this far without a picture to ease the heavy burden of reading, let’s have a look at the Metabones Speed Booster.

Mounted to an NEX-7 (don’t mind the dust, it’s a testing camera)

Note the front glass nicely reflecting the right hand of your humble author.

It’s a nice looking bit of kit – solidly made and well put together. There’s a solid mount for tripods or shoulder mounts underneath. It mounts with a most satisfying thunk and clamps tightly to camera and lens. The optics are close to the surface, though, and some care in handling will be necessary to make sure they don’t scratch.

Let’s Shoot a Few Test Charts

There are a lot of ways to compare lenses with the Speed Booster and not a lot of time, so I tried to choose things that seemed practical. Or cool. Or both.

Starting with cool, I mounted a Canon 14mm f/2.8 II lens, which with the adapter should give us the equivalent of a 10mm f/2.0 NEX lens. That sounds cool to me. Plus I thought we should look at things as extreme as people are likely to get.

We’ll start by comparing simple shots of an ISO1223 chart shot with the lens on a plain adapter and on the Speed Booster. To even things up I moved my position so that the chart filled the image each time, so that we could compare resolution directly. First an overall picture of the chart, followed by center and near corner crops with each adapter.

Canon 14mm f/2.8

ISO12233chart

Center and near-corner crops from 14mm f/2.8 image

Center and near-corner crops from 9.8mm f/2.0 image

Again, the images were from different distances so that the chart filled the image with both shots. It’s not an optically critical test, but I’d call it a complete success for the Speed Booster. Even spotting the original image 1 stop of light, there’s no significant difference in resolution to my eye.

The change in perspective is impressive. These are shot from the same location with the Canon 14mm f/2.8 mounted to a straightforward adapter first and the Speed Booster second.

14mm f/2.8 on NEX-7

14mm f/2.8 on NEX-7 via Speed Booster

Canon 50mm f/1.2

That was pretty impressive, now lets stress things a little bit more. The Canon 50mm f/1.2 lens brings a few aberrations to the table and with the adapter will be an f/0.9 equivalent. I can’t think of anything that would stress an adapter more than f/0.9. Again, I’ll reposition myself so both shots fill the frame with the chart. We’ll compare the 50mm f/1.2 on top to the Speedboosted 35.5mm f/0.9 below.

Canon 50mm f/1.2 on NEX-7

Canon 50mm f/1.2 on Speed Booster and NEX-7 (35.5mm f/0.9 equivalent)

Again, these are not critical tests, but are carefully lined up, best focus of several shots. And again, the Speed Booster comes out very well.  It may be the illumination boost but the acutance in the center, at least seems a little better with the Speed Booster. I would draw your attention, though, to the difference between vertical and horizontal lines in the corner crop of the Speed Booster image. That’s not an artifact of the shot or alignment. With this lens and camera, at least, the astigmatism seems to be showing up a bit. I also note that the image looks oversharpened, but it’s an unsharpened JPEG and this appearance was consistent on multiple shots.

 Canon 135 f/2.0

I wanted to try the other extreme, and made the assumption that this would be about the longest focal length people would want to use the Speed Booster with. I may be wrong about that, but was running out of time today.

Canon 135 f2 on NEX 7

with Speed Booster (94.5 f/1.4 equivalent) on NEX-7

Again, if there’s any deterioration in image quality with the Speed Booster, even though it’s a stop of aperture wider, I’m having difficulty seeing it. I also don’t notice the astigmatism with this combination.

But let’s test the resolution a bit more critically.

Imatest results

We don’t have multiple copies of the Speed Booster yet, so this is what we did. We took a Canon 50mm f/1.2 lens and tested it on a Canon 5D Mk II camera. Then we tested that same copy on an NEX-7 using a standard adapter. Finally, we tested those same copies (camera and lens) with our Speed Booster.

Usually when talking about Imatest results I’m sampling dozens of copies and give you the average (mean) resolution in the center and a weighted average of all the test points on the lens. But usually we NEVER test lenses on adapters if we can avoid it. Why? Because even the very best adapter still introduces and extra variation in tilt and centering between the lens and the camera.

Let me word this more carefully because it’s important. When the imaging sensor is placed in the camera, it is carefully lined up to be completely parallel to, and centered with, the lens mount of the front of the camera. A tilt of 20 microns may be visible on a very wide angle lens. A tilt of 40 microns almost certainly will be visible. From repair manuals we know that the sensor can be made parallel  to the lens mount within a few microns so that’s taken care of.

But when the big, heavy lens mount rotates into the big, heavy camera mount, chances are it’s not accurate within a few microns. Let’s assume it’s getting close to the 20 micron limit, because we know with high-quality, wide angle lenses we can often see some side-to-side variation. Sometimes obvious with some pixel peeping, sometimes not at all, but frequently enough that I’ve assumed we’re getting close to tolerance.

This is one of those sources of lens-camera variation I talk about so often. Lens 12345 looks great on camera 54321 but not so great on camera 112233. It may tilt a bit more on that second camera mount.

When we add an adapter we’re adding another heavy duty mount and making it more likely there’s a bit of tilt. It’s rarely apparent (with a high quality adapter) at standard or telephoto ranges, but often can be detected with high resolution wide-angle lenses. It may cause no harm at all. It may create too much tilt. I mention all of this because it’s going to explain some of our test results.

Canon 50mm f/1.2 on a Canon 5D Mk II

On a Canon 5D Mk II and our lens shot at f/1.4, the MTF50 was 590 line pairs / image height in the center; 460 lp / ih averaged over the entire lens, and 265 lp / ih in its worst corner with a barrel distortion of 1.2%.

Canon 50mm f/1.2 on a Sony NEX-7

On the NEX-7 with a standard adapter shot at f/1.4, it resolved 625 lp / ih in the center, 485 averaged over the entire lens, and 210 in its worst corner with a barrel distortion of 0.825% (smaller sensor). The difference in the center and overall isn’t surprising – the NEX has a higher pixel density and is using the ‘sweet spot’ from the center of the lens. The fact that the lowest corner is worse is a bit surprising until you see the overall graph of results:

Graph of MTF50 results on NEX camera with standard adapter

Notice the upper left corner is worse and the upper right better, and that the right side of the image resolves better than the left. The lens, which behaved very nicely on a Canon camera, is tilted when shot on this particular NEX-7 with this particular adapter. So, of course, we went and got another adapter. It tilted the other way. And we were out of time. From experience I can say the center resolution number is going to be accurate, the actual weighted average should be a few points higher and the worst corner about 275 or so rather than 210 (275 is the resolution on the less affected corner).

So it is with adapters. And before you scream that the adapter was bad, it wasn’t. The guys put those adapters on other cameras and lenses later and they were either dead even or tilted another way. It is what it is. Also remember we’re churning MTF 50 numbers. You need a big difference to be able to actually see the difference in a photograph, and an even bigger difference for it to affect video.

Canon 50mm f/1.2 on NEX-7 with Speed Booster

On the NEX-7 with Speed Booster adapter and shot at f/0.9, it resolved 720 lp / ih in the center, 410 averaged over the entire lens and 230 in the worst corner. Stopped down to f/1.3 the MTF 50 increased to 800 center, 510 weighted average, and 300 for the worst corner. As advertised, the MTF 50 increased compared to the same lens on no adapter.

However, barrel distortion increased to 1.9%. Remember, however, this is in effect now a 35mm lens, so that number isn’t as big a jump as you would think at first glance. Just to note, there was no sign of tilt with this adapter on this camera.

One thing to note – there was definitely a bit of astigmatism, with horizontal and vertical resolution quite different along the edges of the lens. One other interesting note – we measured primarily MTF 50 as this is the number we work with most frequently. We also checked the MTF 20 numbers and for these combinations the Speed Booster had a similar effect: slightly improved but with greater astigmatism.

A reminder for everyone again – we had one copy of the adapter to play with and limited time. But I’ll have to admit that it seems the folks at Metabones pulled off what they claimed: with the adapter a lens is wider, faster, and even a bit sharper.

A Few Images

There’s not a lot of photogenic material around the Lensrentals Lab, but how often do I get to shoot with a 35mm f/0.9 lens? I’ve included some 100% crop areas in the corners of the scaled down shots.

Now, to get completely subjective, there’s something about the images as far as photography that I don’t like, but it’s hard for me to put a finger on it. But if you look at the crops there’s a bit of a glow around highlight areas, both in-focus (in Sarah’s shirt) and out of focus (the rubber bands).

My first thought was perhaps shooting on an NEX -7 could be the issue, since we know that camera has had some problems with adapted retrofocus lenses. Shooting with a Canon 50mm f/1.2 may also be the culprit – that lens is, well, different. But we repeated the rubber band shot with an NEX-6 and then with both cameras and a Canon 85 f/1.2 and the Speed Booster. To my eye (and remember I’m a techie, so I don’t do subjective all that well), there’s a real tendency for highlights, in and out of focus, to bloom a bit at the widest apertures. It seems to go away by f/2.0 affective aperture.

I’d also add, for those who plan on using it, that the autofocus worked well as far as accuracy. Eventually. You won’t be catching any moving targets unless they are turtles, though. I doubt this is of great import to many people, though.

Conclusions

I think it was pretty obvious that I came armed for battle, ready to slam this product as some marketing overhype. I was wrong less correct than I might have been. The Speed Booster does what they claimed it would do, much to my shock and surprise. It creates a wider-angle, greater aperture lens while retaining resolution and acutance.

It does increase astigmatism a bit, although I doubt this will cause anyone problems unless someone is trying to shoot landscape photography with it. It also seems to create some highlight blooming at very wide apertures. Again, nothing that can’t be worked around and probably not something that will be noticeable with anything but the widest aperture lenses.

It is going to take a while and a lot of people experimenting before we find out what combinations of lenses and cameras are awesome with it, which are fairly good, and which fairly bad. They won’t all be the same. But I suspect most of them are going to be pretty good. And this is going to be a very useful tool. 

Most of the little foibles I’ve seen (including the part about adapter tilt) really only apply to photographers trying to tweek every drop of resolution out of their high-resolution sensor. Video, even 5k video, is more forgiving of a slightly weak corner or a bit of astigmatism.

Roger Cicala

Lensrentals.com

January, 2013

This is episode 7 of Boing Boing's, Tell Me Something I Don't Know. It's an interview podcast featuring artists, writers, filmmakers, and other creative people discussing their work, ideas, and the reality/business side of how they do what they do.

Jeff Smith began writing, drawing, and publishing Bone in 1991, through his company, Cartoon Books. He championed self-publishing in the 1990s with other independent cartoonists known as the Spirits of Independents and continues to self-publish through Cartoon Book. Since 1991, Bone has become a world-wide phenomenon, published in nearly 30 languages. In 2005, Scholastic reissued Bone in color through their Graphix imprint, inspiring an entire generation of young cartoonists who found his work through traditional book stores, comic book shops, schools, and libraries. He followed the Tolkien-esque, Bone, with Shazam! The Monster Society of Evil (DC Comics) and RASL (Cartoon Books) - a sci-fi noir about a dimension-hopping art thief. Smith recently announced his next project, Tüki Save the Humans, about the first human to leave Africa during the Ice Age.

Tell Me Something I Don't Know is produced and hosted by three talented cartoonists and illustrators:

Jim Rugg, a Pittsburgh-based comic book artist, graphic designer, zinemaker, and writer best known for Afrodisiac, The Plain Janes, and Street Angel. His latest project is SUPERMAG.

Jasen Lex is a designer and illustrator from Pittsburgh. He is currently working on a graphic novel called Washington Unbound. All of his art and comics can be found at jasenlex.com.

Ed Piskor is the cartoonist who drew the comic, Wizzywig, and draws the Brain Rot/ Hip Hop Family Tree comic strip at this very site, soon to be collected by Fantagraphics Books and available for pre-order now.

Follow TMSIDK on Twitter

Subscribe to the Tell Me Something I Don't Know podcast | iTunes

The KUKA YouBot Omni-Directional Mobile Platform with Arm is a small, arm-on-a-skateboard type of robot that can perform simple tasks. Recently Ross Knepper, a robotics reseacher at MIT, and his team hacked up a couple of them to assemble store-bought IKEA furniture. While it was primarily an exercise, versus designing a commercially-viable product, we were pretty impressed by his solution for screwing the legs into the Lack sidetable that you'll see here:

The YouBot doesn't come with an "end effector" that can perform the rotating motion you and I would do with two hands to get that leg into the table. Knepper's team devised an elegant workaround, using rubber bands attached to two different rings:

Entekaphobia – fear of the number 11

Or. . . How I Learned to Appreciate Small Aperture Photography

If you read my blog much, you know I’m a resolution fanatic. I test every new lens for resolution. For personal use, I’ll choose the lens with higher resolution over the one with creamy bokeh every time. When choosing a camera, I have a (yes, I’m ashamed to admit it, but it’s true) strong tendency to want the most megapixels. I’m a resoholic.

Being a resoholic, I’ve always been somewhat fanatical about apertures. Whenever possible I shoot with the lens stopped down at least one stop to wring the maximum sharpness out of my lens. But I’m always careful not to stop down too far because I was taught, soon after I picked up a camera, that if you stopped down too far the dreaded diffraction softening would kick in.

With today’s high-pixel density cameras, that meant f/8 was as far as I would ever stop down. My mental map of aperture sharpness was like the ancient maps of the world – past f/8 there was nothing but the notation Here Thar Be Monsters. Or the equivalent label in Latin or Olde English, just because that makes it seem much cooler.

Detail from The Carta Marina by Olaus Magnus (1490-1557).

Go to f/11 and the diffraction monster would come and eat the resolution right out of your photographs. The diffraction monster loves to snack on some tasty resolution. When testing I really never checked past f/5.6 or f/8. That’s where the maximum resolution would be. Any further, and, well, you get it by now.

But I knew there were excellent photographers who shot their landscapes and macros at f/11 or even f/16 because they needed the depth of field. I heard rumors of photographers in far off lands who even actually took photographs at f/22. I considered them sort of like those guys who jump off cliffs in batsuits and fly around for a while before pulling their parachute rip cords. It was fascinating to know people did that, but made me a bit queasy. I was certain the survivors would eventually learn the error of their ways.

But lately, some people like Tim Parkin at Onlandscape.com started opening my eyes (by repeatedly beating on my head). They claimed to be shooting at f/16 and even f/22 with high-pixel-density SLRs, carefully postprocessing their images, and getting very nice detailed results. I shook my head sadly at first, hoping they would come to see the light (pun intended). But then I looked at Tim’s recent article The Diffraction Limit and had to admit, their f/22 images didn’t look bad at all.

So I decided it was time to open the closet door and see just how bad the diffraction monster really was.

Preliminaries

Before we get into all of this, let’s remember we’re looking at two simultaneous events when we stop a lens down. I am not going to get into lengthy discussions of Airy Discs, Raleigh Criteria, and other arguments here. You can read about them elsewhere. This is the simple overview of what’s going on.

• 1) As we narrow the aperture (higher f/number) diffraction occurs which causes some loss of resolution.
• 1a) Smaller pixels are affected more than larger pixels since diffraction causes a spread of the point into a disc. A disc of small size might still fit nicely on a large pixel, but might cover two small pixels. The math is mildly complex, it’s not linear, and I’m not going into it more than that.
• 2) As we narrow the aperture, the lens resolution increases.
• 2a) The increase is different for different lenses.
• 2b) The increase may occur at different rates for the center, middle, and corners of a lens.
• 3) Decreasing aperture is sort of a race between these two effects. When we first stop down, the lens sharpening is greater than the diffraction softening. As we stop down further, lens sharpening slows or stops, but diffraction softening continues.

Some Resolution Testing

I’m not one to really believe what I see in online exampless; given enough postprocessing an online jpg can look pretty sharp if the lens was the bottom of a beer bottle. I want at least a side dish of numbers or some comparative crops with my reviews, thank you.

I decided our current Nikon lineup gave me a great opportunity to look at diffraction effects. By shooting the same lenses on a D700, D3x, and D800 I can look at full-frame sensors with 12, 24.5, and 36 megapixel sensors. That gives linear pixel densities of 118, 168, and 204 pixels per mm, respectively.

I decided to use 50mm lenses because we have 3 choices that are quite different in how they behave at various apertures.  The Zeiss 50mm f/1.4 (the schizoid fiftoid) is very soft and dreamy looking wide open, but becomes razor sharp once stopped down to f/5.6 – it’s like two lenses in one. The Zeiss 50mm f/2 Makro planar is quite sharp even wide open, but seems to maximize it’s center resolution by f/4. The Nikon 50mm f/1.4 G is reasonably sharp wide open, but seems to keep getting sharper the more you stop it down.

So I tested all 3 lenses on all 3 bodies at apertures from wide open to completely stopped down in our Imatest lab.

The Effect of Stopping Down on MTF 50

I started right in the middle of my selections: the Nikon D3x with the very predictable Nikon 50mm f/1.4 G lens. Here are the MTF 50 values in line pairs / image height for the center point, weighted average of 13 points, and average of the 4 corner points. Please note that the plotted average is NOT just the average of center and corners, so if the ‘average’ value is near the center, you know the lens stays fairly sharp in the middle regions, while if it’s nearly as low as the corners the lens falls off rapidly away from the center point.

Nikon 50mm f/1.4 on D3x at various apertures

I have to admit I was a bit shocked. Just as expected, the resolution starts to decrease after f/8, but it doesn’t decrease all that much. Even at f/16 the resolution is still quite a bit higher than it was at f/1.4.

The next step was to see how things look with lower and higher pixel density cameras. So I shot the same lens on a D700 and D800.

Nikon 50mm f/1.4 on D700 at various apertures

I was a bit surprised here, too. I had expected the lower pixel density of the D700 would shift the peak resolution a bit, perhaps to f/11, but that wasn’t the case, although the drop after f/8 did seem less severe.

Nikon 50mm f/1.4 on D800 at various apertures

Things weren’t as different as I expected on the D800, either. The center does seem to peak around f/5.6 with the corners peaking at about f/8, which isn’t surprising. The other cameras show only a slight increase in resolution at the center between f/5.6 and f/8 so it makes sense there would be a bit stronger diffraction effect on the D800. I had really expected more than this. The lens still improves in the corners strongly between f/5.6 and f/8 and the improvement is greater than the diffraction softening.

The message I took away, though, is that diffraction softening is real, it occurs where it is supposed to, but it’s really not as severe as I had thought. Even on the D800 resolution is as high, or higher, at f/16 than it was at f/2.8. At f/11 the resolution is as good, or better, than at f/4. And at both f/11 and f/16 resolution is clearly higher than it was wide open. Perhaps the diffraction monster’s teeth aren’t as long and wicked as I thought.

Some Different Lenses

Diffraction softening is fairly constant, but lens sharpening as the aperture decreases is not. Different lenses behave differently. I compared the Zeiss 50mm f/1.4 and 50mm f/2.0 Makro Planar lenses to the Nikon 50mm f/1.4 G we tested above on all three cameras. In the interest of brevity we’ll just show the graphs for the D3x. The variations for the D800 and D700 were similar to these.

ZF 50mm f/1.4 on D3x

ZF 50mm f/2 Makro on D3x

Let’s start with wide open performance. At f/1.4 the ZF f/1.4 lens isn’t as sharp as the Nikon 50mm was, while the ZF 50mm f/2 Makro is sharper at f/2 than either of the other lenses at that aperture. The f/2 Makro has reached maximum center sharpness by f/4 and then slowly loses resolution. The lens doesn’t reach maximum corner sharpness until f/8. The ZF 50mm f/1.4 gets maximum center sharpness at f/5.6 and corners again at f/8 on the D3x. (In the graph above, you can see the Nikon reached maximal sharpness at f/8 for both centers and corners.

The pattern was unchanged on the D800, but on the D700 the two Zeiss lenses center sharpness shifted just a bit to the right – moving to f/5.6 for the 50mm f/2 Makro, with corner sharpness remaining peaked at f/8.

So there is some difference in stopped down behavior with different lenses. Before you ask me to go test this or that, the work has largely been done already at sites like SLRgear.com and Photozone – they show center, corner and edge sharpness at various apertures in their lens reviews.

Yes, I Took Pictures

OK, the numbers surprised me a lot, so I went and did what had to be done. I actually took photographs stopped down to f/16 and even f/22.

Here’s one picture I shot at various apertures (this was on a Canon 6D).

What I saw mirrored what the numbers said I would see. Below are some 100% crops of the white gazebo just off center and some tree trunks near the left edge. I also tried something I was told was possible, but hadn’t really believed. I took the obviously diffraction softened f/22 image and did my best to sharpen it in Photoshop. (By best, I mean about 45 minutes testing different combinations of sharpness and contrast enhancement in 3 layers before getting the results shown below as ‘f/22 sharpened’.)

I haven’t tried this kind of sharpening before and was feeling my way along. I’m sure it would be better with practice (I blacked out the large tree-trunk for example and the image is about 1/3 stop darker than when I started) but still I found the results surprisingly acceptable.

One thing I found very interesting is that I could perform what was basically postprocess abuse on the f/22 image to a degree that would have been impossible with one of the other shots. Below, for example, is the f/2.8 image above processed with exactly the same settings I used on the f/22 image. The center crop, particularly, looks like a ‘find edges’ special effect filter.

I don’t mean to suggest that the postprocessed f/22 image is going to be as good as a nice f/5.6 or f/8 image at all. Rather I’m suggesting it can be improved to a larger degree than they can, making up some of the out-of-camera difference between them.

Does this mean f/16 is my new f/5.6? No, not at all. But I think I may become a lot more aggressive about using f/8 and f/11 when I’m trying for a larger depth of field. I even might use f/16 if absolutely needed. I don’t think I’ll be shooting f/22, though. That’s just a step too far for me.

Roger Cicala

Lensrentals.com

March, 2013

An aside: I’ll be going on vacation for 10 days at the end of this week, shooting with my new camera at my new-found apertures. I don’t expect there will be any more blog posts until late March.

Obviously I’m a gearhead, so I like to know the traits of the lenses I shoot with. I want to know what aperture gives maximal corner sharpness, for example, whether the plane of focus is curved or flat, where the distortion changes in a zoom, which end of the zoom range or focusing distance is the lens sharper at, and a number of other things you may not care a bit about.

Does it improve my composition and technique? No. But knowing this stuff can be helpful. For example, when I want to shoot a landscape at 70mm and f/5.6 will my corners be sharper with my 24-70 f/2.8 or a 70-200 f/2.8? Or which will have less distortion for an architectural shot (since I hate the resolution loss of correcting distortion in post), my 35mm f/1.4 or my 24-70 zoom at 35mm? (Surprisingly, the answer is my zoom.)

This kind of information is easy to find. DxoMark has nice graphs for each lens that show distortion, vignetting, chromatic aberration, and resolution at various focal lengths and apertures for each lens they test. SLRgear.com has a nice pop-up app that shows the resolution across the field of the lens at various apertures and focal lengths.  The Digital Picture has great pop-ups that let you compare two lenses side-by-side for flare, distortion, vignetting and even images of ISO 12233 crops.

A lot of people use those tools when deciding which lens to buy. I use them after I have the lens so I know how to best use it.

 Resolution Maps

One thing that I’ve started using more frequently in post processing is a resolution map of the lens. We all know that every lens has highest resolution in the center and less in the corners. But the pattern of sharpness is different for different lenses.

Some lenses have a high peak of resolution right in the center that quickly drops off. Others maintain significantly high resolution halfway to the corners and then drop like a rock. Others have a rather linear drop-off from the center to the corners.

Just as an example, below are 6 Imatest charts showing MTF50 of 6 different lenses across the field of view. The absolute resolution numbers aren’t important for this demonstration, rather it’s the pattern of how the resolution changes. For each lens, yellow is the highest MTF50, blue is about 1/3 the value of yellow.

Why Does it Matter?

There are a lot of reasons, of course. But one I use a lot is creating sharpening maks for postprocessing. Like a lot of people, I use a masked layer for sharpening, applying less sharpening to the already sharp center of the image, and more sharpening to the softer areas. Instead of just a generic oval, I try to make a mask that mirrors the resolution map of the lens I’m shooting with.

I keep masks as actions for my most commonly used lenses, which speeds up postprocessing considerably. For example, I’d use something like the first mask, below, for images shot with the lens on the upper left above, and the second mask for middle right lens above.

As an example I’ll use two 100% crops from the left edge of this snapshot.

The crop on the left shows what that edge looks like when I sharpened the entire image to give best center sharpness. The crop on the right was when I used a mask to use stronger sharpening, but only at 50% strength in the center of the image. With either technique the center looked the same, but the edges were quite different.

Of course you can simply use an oval mask and adjust it for each image with a bit of trial and error. But I had 500 vacation photos to go through. Since 75% of them were taken with one lens at the same aperture, saving an action with the appropriate sharpening made that quick and easy.

You don’t need Imatest to figure out the sharpness pattern for the lenses you have. A simple photograph of a flat wall or fence with reasonable detail (bricks or unpainted wood are nice) will let you see where each lens starts to soften and by how much. Once you’ve made a good mask for that lens you have it forever. For most lenses, the same mask can be used at different apertures – you simply reduce the strength of the layer if you’ve shot stopped down. For other lenses, though, like my Zeiss 50mm f/1.4, you will need to make masks for different apertures.

Uwe Steinmueller at OutbackPhoto.net and I have been doing a series of articles trying to show how a little gear head knowledge and a little post-processing knowledge compliment each other and help make better images, and this is a great opportunity for that. Uwe’s article and action for corner sharpening, provide a nice photographic demonstration of how sharpening with a mask improves your end result, and a nice script with an adjustable mask.

Roger Cicala

Lensrentals.com

April, 2012