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Of Course We Took One Apart

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A Look Inside the Canon 16-35 f/4 IS

This is a Geek Article. Many of you don’t understand the term ‘Geek’ properly, so perhaps this will help. As the graph shows, if you aren’t both intelligent and obsessed with photo gear, you won’t enjoy this article. 

I’ve tried hard to find whom to credit for this, but haven’t been able to. If you know, please let me know so I can credit this brilliant work.

 

We spent most of the last three weeks doing comparison tests with Canon’s new 16-35 f/4 IS. We came away from that comfortable that this is the best wide-angle zoom Canon has ever made. But our day job is fixing broken lenses. I may have mentioned once or maybe three hundred times that trying to keep the 16-35 f/2.8 Mk II lens optically adjusted sucks up our time like a black hole sucks up interstellar dust.

The whole time Aaron and I were testing the new lens, what we really wanted to know was if it was built differently and if adjustments would be either simpler to perform or easier to accomplish, or maybe even both. We weren’t sure, though. According to Canon’s White Paper, the new lens has some new technology.  The one that made us a little queasy was “newly developed four-group zoom arrangement.” When you work on lenses, there are key words you hate. Chief among these is “newly developed,” which we usually translate as “nearly developed.”

On the other hand, the White Paper also said the redesign involved “matching the design of the EF24-70mm f/4L IS USM, for greater zoom durability and to provide the barrel components with better vibration and shock resistance.” That’s been a reliable and sturdy lens, so we did have some glimmers of hope.

If you look at the lens diagrams of the 16-35 f/2.8 and the new 16-35 f/4 IS, they’re rather similar at first glance.

 

Lens diagram of Canon 16-35 f/2.8 Mk II (left) and 16-35mm f/4 IS (right)

 

The Canon White Paper includes a diagram of how the various components move, though, and that made us take notice. Every damn thing inside that lens moves with either zooming or focusing, or both.

From Canon White Paper showing the motion of various elements. Even the aperture is moving.

 

Needless to say, we were itching to take a look inside one. This is not new behavior for us, though, and management is pretty wise to our ways. So, I wasn’t surprised to get a memo that said until we had sufficient backup stock, there was to be no 16-35 f./4 IS disassembly. However, there was no clear definition of “sufficient backup stock,” and after some discussion we decided that having one that didn’t need to ship for another two hours was sufficient backup stock.

Let’s Take Out Some Screws

Those of you working on your lens disassembly merit badges can take out your JIS #00 screwdriver and follow along from home.

Assuming the disassembly position, we take out the 4 bayonet mount screws and the 2 electrical connector screws.

Here’s the first mildly pleasant surprise. I’m no big fan of “weather resistance” because it’s 80% marketing hype and 20% reality. But the rear bayonet came off only with a bit of a tug, because it has a really thick, soft rubber water weal between it and the rear barrel (red line).

Keeping on with the weather resistant theme, the rear barrel cover, which comes off next, has a nice, deep seal with a rubber gasket (red line) where it mounts into the main barrel. (As an aside, the 16-35 f/4 IS requires a front filter for weather sealing; the front group moves and is not sealed.)

With those parts off, the PCB is exposed and ready to have its flexes disconnected.

I always like the nice, clean way Canon’s engineers set up the circuitry on their PCBs.

The zoom/rear barrel assembly comes off next. It’s not often you see a picture of screws in a disassembly post, but there’s a reason for it. The mount on this lens is held on by 7, count ‘em, 7 large screws. The previous record we’ve seen is 6. I love some over-engineering; it’s always a good thing.

In addition to removing the screws, you have to peel up a protective metal cover and remove the zoom key before you can remove the zoom/rear barrel assembly.

After which the assembly slides right off.

If you notice, the zoom ring has a ridged pattern, which is something we’ve not seen before. If you didn’t notice, well, here’s a closer view.

I think this may solve two problems we’ve seen on some recent Canon lens releases, like the 24-70 f/2.8 Mk II. First, it should help the zoom rubber stay in place (stretching zoom rubber is a minor problem on that lens).  Second, it should strengthen the actual zoom barrel. On several recent lenses the barrel has been made so thin that a dent in it will freeze, or at least really stiffen, the zoom ring. This barrel feels a lot stiffer than some previous lenses, which we could easily bend by just squeezing too hard.

The rest of the lens contains all of the optics, the USM system, and the front barrel. It’s not quite as modular as the 24-70 f/2.8 Mk II or 24-70 f/4 IS, where the USM assembly is a separate piece, but it’s more modular than the 16-35 f/2.8 or 17-40 f/4.

While we’re here, though, let’s take a close-up look at that large white adjusting collar you can sort of see near the top of the above picture.

There are several points to make. First, this is a thick, robust collar, similar to what we’ve seen in the 24-70 f/4 IS. Second, the rear group is adjusted with these three collars spaced every 120 degrees. Adjustable collars are nice and precise.  In the 16-35 f/2.8 lens there are no adjustable collars; to center the rear element you loosen three screws and slide it into position by hand, then tighten the screws back down. (see Appendix)

Next we turned our attention to the front, where Canon had a couple of surprises for us. As with most Canon lenses, the name ring peels right off. But underneath the name ring is a second plastic ring. We assumed that would pop off, and ignored the little slots cut in it (red arrow).

Turns out the slots are there for a purpose. The ring doesn’t pop off, as Aaron is ably demonstrating in the above image. Rather, it turns 45 degrees and comes right off because instead of little plastic pop-in pegs (which break a lot), it has a full bayonet-like mount, which holds it very snugly around the front element. It also has a thick rubber foam gasket (red line). It can’t be quite weather proof because the front element slides up and down within the barrel (the reason this lens needs a filter to be weather sealed), but it certainly must help.

With the second makeup ring off, we can see that, again, Canon has used some robust screws to lock the front element in place.

What you can’t see is that the screws holding the front barrel (or filter barrel if you prefer that term) are tucked way down inside.

After which the front barrel comes right off. Notice how obvious the “asphericness” of the front element is. (Yes, I did make that word up. The nice thing about making up words is I don’t misspell them.)

Now that the front barrel is off, we can see the front optical adjustment assembly.  The screw collar on the left is not adjustable, but there are adjustable concentrics at the other two locations at that element, allowing some tilt of that element also. There’s still a ramp, so rotation of the front element is used to correct spacing. This is definitely an improvement from the 16-35 f/2.8 (see Appendix). Here we have separate brass concentric collars and screws to adjust with. In the older lens there are plastic adjustment collars on a ramp. After they’re adjusted the slot is filled with silicone glue.

We decided not to disassemble the USM because we could see all of the remaining helicoid collars without doing so. All were robust and most were brass.

Now that I’ve seen the insides I’m very optimistic that this lens will be less likely to deteriorate optically over time, and will be more easily corrected when it does. We won’t know for sure until we’ve got a year’s experience with it, of course, but from a design and assembly standpoint it looks really, really good.

I know I’m beginning to sound like a Fanboy, especially considering I hardly ever mount a wide-angle zoom on my camera. But I guess the corny old line from the Most Interesting Man in the World works. “I don’t often shoot Canon wide angle zooms, but when I do, I prefer the 16-35 f/4 IS.”

Roger Cicala and Aaron Closz

Lensrentals.com

July, 2014

Appendix: The Adjustments in the Canon 16-35 f/2.8 mk II

The two questions I get asked constantly about the new 16-35 f/4 IS lens are basically variations on a theme. Will the 16-35 f/4 IS be more likely to stay in good optical adjustment, or be easier to correct if it gets out of adjustment, than the 16-35 f/2.8. So I thought it would be worth showing you the optical adjustment areas on a disassembled 16-35 f/2.8 so you can make the same comparison I’m making.

FIrst, there are three screws like this that hold a round clamp over the rear group. When they are loosened you can slide the rear group around to adjust it’s centering.

The front element has three adjustable collars. These set tilt, and then rotate along a ramp to space the front element. Once it’s in place the slots are filled with silicone gel that hardens locking them in place. Which is great until you need to adjust them (although it’s rarely necessary).

Fionally, way down deep in the lens under the USM motor are a couple of other concentric screws that can be adjusted. Way down deep pretty much means disassemble the entire lens, make your best guess at adjusting them, then reassemble and see how you did. Repeat.

Hopefully this gives you some idea of why we love seeing those big, brass, accessible adjusting screws in the new version. Of course, you aren’t going to ever make optical adjustments, but somebody might be doing it for you some day. He or she will be a person who, 15 times during that adjustment process will decide if things are good enough, or if they could be a bit better. I’m all for everything that makes that more straightforward and simpler.

Canon Wide-Angle Zoom Comparison

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Left to right: Canon 16-35 f/2.8 II, 16-35 f/4 IS, 17-40 f/4. Can you spot the one with the wrong hood? The intern obviously couldn’t. 

 

As is so often the case, I bit off more than I wanted to chew when I came back from vacation. The Canon 16-35mm f/4 IS lens had just been released, a few copies were in stock, and I thought I’d do a nice quick test. But one of the reasons I’d wanted an optical bench was because I don’t trust Imatest results with wide-angle lenses. At 16mm, even with the very largest test charts, we’re testing at about 4 feet shooting distance.

So I after I did our standard Imatest on the 16-35 f/4 IS, I wanted to repeat the results on our optical bench. Of course, I don’t have a big database of optical bench results to compare against like I do with Imatest. So I had to do optical bench tests on some other wide zooms for comparison purposes. Then I had to do some more comparison with other lenses to see if the variations we were seeing on the optical bench were simply a new, higher resolution testing method, or if they were telling me something about variation with wide-angle zoom lenses. (Both things were true.) Anyway, the testing I thought would take a week has taken three.

I realize some of you just want to see the usual Imatest results on a group of these lenses since that’s what you’re used to seeing. Others are also interested in the optical bench results showing how the lenses resolve at infinity, rather than just close up. And of course there are a few of you who want all the gory details of Geekiness that the optical bench reveals. So I’ll try to present this in three parts: the Imatest results first, the optical bench optical test results second, and the geeky stuff third. It’s a buffet; just grab what appeals to you.

Imatest Results

We tested 10 copies each of the Canon 16-35 f/4 IS, 17-40 f/4, and 16-35 f/2.8 Mk II in our Imatest lab. The table below presents the average values of those copies. Center is the MTF50 at center of the lens. Average is the average MTF50 taken at 32 points across the surface of the lens. Corner average is the average of eight measurements, two (one vertical and one horizontal) in each corner.

Those of you who read a lot of our articles might notice the numbers given for the 17-40 f/4 L and 16-35 f/2.8 L lens are slightly higher than those we’ve presented in some older articles. This is because we now use larger, higher resolution test charts and different lighting. (And a good example of why comparing Imatest measurements between testers is an iffy thing to do.)

We tested the 16-35 f/2.8 Mk II at both f/2.8 and f/4 so it would have a level playing field. You can sort the table by clicking on the top row of whatever column you’re interested in.

Center Average Average Corner
17-40 f/4 L 17mm1240875590
16-35 f/2.8 L 16mm at f/2.8890730440
16-35 f/2.8 L 16mm at f/41360970615
16-35 f/4 IS L 16mm14451185795
17-40 f/4 L 40mm1150810460
16-35 f/2.8 L 35mm at f/2.8995820460
16-35 f/2.8 L 35mm at f/413151095795
16-35 f/4 IS L 35mm13601190895

The conclusion is pretty simple: the new 16-35 f/4 IS has the best resolution. At 35mm, the 16-35 f/2.8 shot at f/4 is nearly as good, particularly in the center. The new f/4 IS is slightly better in the center, but it’s close enough that copy variation would have some overlap. The difference is a bit larger away from center. The 17-40, at the long end is clearly not quite up to the other two. (For completeness, the 17-40 f/4 is clearly better than the 16-35mm shot at f/2.8, but that’s a pretty unfair comparison.)

The difference is more pronounced at the wide end, where the 16-35 f/4 IS is clearly better than either of the other two lenses. As most of us are aware, the 17-40 does better at the wide end than the telephoto end, but it still can’t keep up with the 16-35 f/4 IS.

I know a lot of you want to see f/8 results, too, and we’ll try to do those in the near future, but for this test I decided I wanted more copies at each aperture rather than fewer copies at more apertures.

Semi-Conclusion

Obviously there’s a lot more to choosing a wide-angle lens than simply MTF50. Flare resistance is critical, off-axis aberrations are very important, etc. But from a purely resolution standpoint, the 16-35 f/4 IS is definitely better than either of the other two Canon wide-angle zooms.

There’s a value difference, too. The 17-40, at $839, is the least expensive of the bunch and is a pretty good lens. Personally, for $250 more, I find the 16-35 f/4 IS the better value both for its resolution and for the IS system, which I love on a wide lens. Many of you never use IS on a wide-angle zoom, though, so you may find the 17-40 a less expensive option. The 16-35 f/2.8 II lens has had a price drop to $1,560 with rebates. If you need f/2.8, you need f/2.8 and that’s your choice. You still benefit, though, because Canon dropped its price after releasing the f/4 IS lens.

I’ll add a bit of editorializing. I think Canon priced this new lens aggressively and I appreciate that. If it had come out at $1,500 it would still have sold. It’s that good.

This ends the quick summary portion of our program. If you just wanted to see the Imatest test results and get my opinion, you may go now. The rest of this post gets into the geeky MTF part of things. If you’re into that, I think you’ll enjoy it, and I’ve left you a couple of fun comparisons at the end. None of the information below is going to change the summary above, but it may add some interesting detail.

Those of you who are forging ahead, let me also warn you that the cool graphics we discussed in the graphics contest post aren’t quite ready yet. There’s way more work involved in that than I realized and it’s going to take some time to implement. So consider this MTF data presentation V1.1.

MTF Bench Results

Basic Methods

Imatest gives us MTF50 results, which are somewhat a reflection of how well the lens resolves.  Imatest is also measured at fairly short focusing distances for a wide-angle zoom: a little over 4 feet for 16mm and just under 9 feet at 35mm using our largest charts. The optical bench tests at infinity focusing distance and gives us some additional information that isn’t easily available with Imatest.

One isn’t better than the other. They give complimentary data.

For bench testing we measured 9 copies of each lens. Each lens was tested 4 times, at 0, 45, 90, and 135 degrees of rotation. We use the distance scale mark on the lens as the reference for “0″ degrees, so if you were facing the lens, the measurements would be made like the diagram below. If we were measuring on a camera, rather than the optical bench, that would give us top-to-bottom, side-to-side, and two roughly corner-to-corner measurements.

At each position we measure 20mm on either side of the center point (the absolute corner is about 21.5mm from the center, the horizontal sides are 18mm) in 2mm increments. I won’t bore all of you with how we handle the measurement data, but for both of those who want to know I’ve included it in an appendix.

MTF Charts

Just for whatever reasons, I should mention actual focal lengths. The 17-40mm measures 17.5mm to 39mm, while both the 16-35s measure 16.75 to 34mm, so there were no focal-length surprises with this group of lenses.

As was suggested after previous posts, we are going to show MTF readings up to 50 line pairs/mm. The graphs below are the average (mean) of all readings at each point.

You don’t need to be an MTF chart wizard to compare the results. Higher MTF is better. Dotted and solid lines of the same color close together is better than when they’re far apart. ”0″ is the center of the lens and 20mm is the far edge.

Legend for MTF graphs
MTF charts for the Canon 16-35 f/4 L IS at 16mm and 35mm

 

MTF graphs for the Canon 17-40 at 17mm and 40mm

 

MTF graphs for the Canon 16/35 f/2.8 test at f/4. 

 

The MTF bench results are slightly different than Imatest, or perhaps just more complete. At the wide end, the three lenses are very similar right at the center, but once you move away from the center the 16-35 f/4 IS pulls away from the other two. In the outer 1/2 of the image (the 10-20mm distance from center) the 16-35 f/4 IS is clearly better and the further out you go the more difference there is. At the edges of the image (18-20mm) the difference is striking.

At the telephoto end of the zoom the 17-40 f/4 just isn’t as good as the other two lenses, even in the center. In the outer half of the image, the 16-35 f/4 IS is again clearly better than the others.

I’ll rearrange the wide-end graphs a bit to make the comparison easier, since most of us are interested in the wide end of wide-angle zooms.

 

A couple of points are worth mentioning here. The first is that the 17-40 is better at the wide end than it is at 40mm, which most of us who shoot with it have always said. The other, which is also often mentioned, is that the 17-40 f/4 is better at the wide end than the 16-35 f/2.8. That is true if both are shot wide open, but if both are shot at f/4, as we did in this test, there’s not really a significant difference. On the other hand, if you’re going to shoot at f/4, no significant difference means the cheaper lens is a better value.

The second point is that the results are a little different when testing these three lenses with Imatest versus the optical bench. How much of that is testing distance (infinity for the bench, 4 to 9 feet for Imatest) and how much is different methods I can’t say — yet. That will have to wait a few weeks until the close focus adapter for our optical bench is available. For now, I just consider it two different ways to look at the same thing, and more data is always better.

Astigmatism

The MTF graphs above show the astigmatism, of course, but it’s sometimes helpful to look at the astigmatism separately. I’ll limit these graphs to 10, 20, and 30 line pairs/mm —  the 40 and 50 line pair astigmatisms are very similar to the 30 line pairs/mm plot for all of these lenses. Again, these are multiple-copy averages. The astigmatism was very consistent between copies, though, with no outliers that were dramatically worse or better than other copies.

As you would expect, the astigmatism is more pronounced at the wide end and generally worsens the further you go from center. The 17-40 and 16-35 f/2.8 have less astigmatism further out, while the 16-35 f/4 IS seems to develop more astigmatism near the edges. But if you look back up at the MTF graphs above, you’ll see that what’s actually happening is that both the sagittal and tangential readings are dropping rapidly with the two older lenses, while with the new 16-35 the sagittal readings stay higher at the edge and only the tangential fall off. This increases the astigmatism number but gives better resolution.

 

There’s far less astigmatism for all 3 lenses at the long end of the zoom range. I’d call the 17-40 and 16-35 f/2.8 (shot at f/4) very good performers, while the 16-35 f/4 IS is really amazing.

 

Field Curvature

The MTF bench gives us a nice printout of the shape of a lens’s field curvature, which is always of interest with a wide-angle lens. As you can see, the 16-35 f/4 IS and 17-40 f/4 have little field curvature at the wide end. At the telephoto end, the 17-40 f/4 has the most curvature and the 16-35 f/4 IS the least.

One thing to notice, though, is the lens designers really do know their stuff. If you look at even the most curved fields, you’ll see that the edges generally still fall in the sharpest area (lightest red color), although sometimes just barely in that range.

 

I also want to point out that these graphs aren’t average of all the lenses of each type tested; they are examples from a single copy. The software that does these calculations doesn’t allow us to average multiple lenses, but we did test all of them and they were all similar. The most common difference was just a bit of tilt of the field one way or another.

When we first started testing these lenses this surprised me a bit, but apparently this is just the nature of wide-angle zooms. The two most tilted copies we found (out of 30) are shown below. The tilt is obvious on the MTF machine, but again, notice that even with these both edges are still within the sharpest range. That’s why we don’t see this on Imatest — the flat field of an Imatest chart would still be in the sharpest focus area at both sides.

You’d be very unlikely to notice it in a photograph with lenses of this focal length and aperture because the depth of field is usually quite large. But if you took a 35mm image focusing, say, 8 feet away you might notice that on one side the sharp area went from 6 to 8 feet and on the other from 8 to 13 feet.

 

Distortion

All wide-angle zooms are going to have some distortion and these are no different. Distortion measurements differ with different testing methods, so I’ll provide both Imatest measurements and optical bench measurements. Remember, Imatest is measured at about 4 feet for the wide end, about 9 feet at the long end, while optical bench results are at infinity.

Wide (Imatest) Tele (Imatest) Wide (Bench) Tele (Bench)
16-35mm f/2.82.65%-1.7%0.6%-0.65%
17-40mm f/42.6%-1.1%0.7-0.6%
16-35mm f/4 IS2.95%-1.05%0.8%-0.75%

There are some differences between the 3 lenses, but not anything significant enough to affect one’s choice of lens, I think.

Sample Variation

When using Imatest, I showed sample variation as a scattergram with each copy represented as a dot placed on a grid showing peak and average MTF50.  We’ve used it for years and it works fairly well, but we’re aware it’s a blunt tool. For example, when we’re testing our own lenses there are a number that pass the ‘peak-average’ resolution test, but are clearly not OK. Usually they have one bad corner or one bad side that while clearly unacceptable, isn’t enough to drop the overall numbers to failing.

This is particularly common with wide-angle zooms, so I was eager to look at the amount of variation we saw on the optical bench.

Within-Copy Variation

First, let’s look at how much each copy varies from one area to another. Remember we test each copy at 4 different angles. For each lens we recorded the difference between the best and worst of the 4 readings at each of 20 locations from edge to edge. The graph below shows the average variance of all readings at each point (sagittal, tangential, at 10, 20, and 30 line pairs /mm).

I should make one point right away. Notice that at “0″, the center, there is a slight bit of variation. In theory, this shouldn’t be, the center should always be the same. In reality, rotating the lens around the center makes a tiny difference in location of the center on the machine’s table; there are slight differences when the same measure is repeated; etc. So this ‘center variation’ really is a good demonstration of the accuracy of the tests.

All three lenses, at both the wide and telephoto end, have roughly the same amount of variation. On average, any of the thirty lenses have one corner that is 10 to 15 MTF points lower than the best corner.

For you ‘best possible copy’ chasers out there, the deviation was pretty low. None of the 30 lenses were less than 9 or more than 20 MTF points difference from best to worst corner.

You Want Some Pictures of That Corner Variation, Don’t You?

I know what you’re thinking. “Roger, take some amazing real life photographs to show us this difference of which you speak!” Well, OK! Here are some amazing real life photographs of a 5-micron pinhole. Let’s face it, no matter what I took a photograph of, you were going to tell me the composition was bad and the lighting wasn’t ideal, either. (Both of which are usually true in my photographs.) So I might as well make them useful. A 5-micron pinhole is about the right size to fill up one pixel on a 5D Mk III (about 6 microns square).

I picked a typical copy (average amount of corner variation) for each lens, put it on OLAF, and took images of the pinhole at a 40 degree angle of view, which is just about in the corner of the lens at 16mm. I should add each corner pinhole image is at best focus for that image, so this isn’t about field curvature or tilt. This is the best possible image the lens can make in that corner.

We’ll start with the 17-40 f/4,

Four corner pinhole images showing typical variation for a 17-40 f/4 lens at 17mm.

then the 16-35 f/2.8,

Four corner pinhole images for a typical Canon 16-35 f/2.8 II at 16mm.

 

and finally the 16-35 f/4 IS.

Four corner pinhole images showing typical variation for a 16-35 f/4 IS lens at 16mm.

Do these pictures mean anything? Well, it does show that what the optical bench sees correlates with reality. For each set of four images above I think you can identify one corner or side that’s a bit better than the other side, just like the optical bench says you could.

But could you actually see a difference in a photograph? Perhaps, but we couldn’t in any of these three lenses, even photographing high-resolution test charts under testing conditions. I’ll add, for what it’s worth, that we use this pinhole view to do most of our optical adjustments and from long experience I wouldn’t try to improve any of these. I’d be far more likely to make them worse than better.

Why then, are some people testing their new 16-35 f/4 IS lenses saying they can see a softer corner or side? Well, a few of them probably have an out-of-spec lens. That happens occasionally with every zoom I’ve ever seen. But I’ll also suggest that some of it has to do with the fact that the 16-35 f/4 IS lens is sharper in the corners than the older lenses.

Scroll back up to the side-by-side MTF charts of the 16-35 f/4 vs. 17-40 and 16-35 f/2.8 II. Look at the edge area (18-20mm from center) and consider the MTF50 line (MTF 0.50 on the graph). For the 17-40 and 16-35 f/2.8 II, only the 10 and 20 lp/mm sagittal lines are at MTF50 or above at 18mm. With the 16-35 f/4 even 40 lp/mm is above the MTF50 line.

When photographing sharpness charts that test resolution (and therefore things like MTF50), you can see a difference in corners much more clearly on the 16-35 f/4 IS. To put it simply, in testing on-camera it’s easy to tell the difference between sharp and not sharp. The difference between not sharp and pretty soft is hard to see. So it’s less likely we would notice this variation in the 17-40 or 16-35 f/2.8 II lens, since they’re already softer in the corners.

Variation Between Copies

Next we’ll look at variation between different copies of the same lens. For this we took the standard deviation of all copies of each lens at every measured point.

First the standard deviation between copies at the wide end.

 

Now the SD between copies at the telephoto end.

I was surprised at how little variation the 16-35 f/4 IS had at the long end. I won’t say I’m surprised by how much variation there was in the 16-35 f/2.8 II. Regular readers of this blog know we have more trouble with the optics on that lens than any other Canon lens. Nevertheless, I wanted to check and recheck these numbers and look at them several ways. Then we even did some optical retesting.

But everything seems to agree – there’s more sample variation in the 16-35 f/2.8 II lenses and less in the 16-35 f/4 IS at 35mm. At the wide end the difference isn’t as striking.

Remember, this isn’t sample variation caused by one or two badly decentered copies; we’d already screened those out using methods we’re pretty comfortable with. This is ‘how much variation do we see between the acceptable copies’. For completeness, though, let’s remember the 16-35 f/2.8 and 17-40 lenses were off the Lensrentals shelves while the 16-35 f/4 IS had been rented once at most. Even if we assume the older two lenses would be better out of the box (I don’t, but I can see where others might) it still is a very nice performance by the 16-35 f/4 IS.

One Last Comparison

I’m completely aware that a lot of people love their 16-35 f/2.8 II and 17-40 f/4 lenses. I’ve never been a huge fan of them, and usually shoot a wide prime lens (Canon 17 TS-E or 14mm f/2.8) on Canon bodies. If I know I’m going to shoot a lot of wide-angle images, I’ll often check out Nikon D800 and 14-24 f/2.8. So for purely selfish reasons I wanted to see how well the new 16-35 f/4 IS compares with these lenses.

In the following graphs the newly introduced lenses are NOT averages of a dozen copies, they’re test results from single copies. I’ll do complete comparisons in the future with multiple copies of them. This was just to give myself a quick estimate as to whether further testing was worthwhile, and I thought I’d share it with you.

Canon 16-35 f/4 IS versus Canon 17mm f/4 TS-E

This is done with both lenses at widest zoom. I don’t know if the results shock you, but they did shock me.  The MTF charts clearly show the TS-E is better at . . . tilting and shifting. That’s huge for me, because I often used it as a straight lens, never intending to use the tilt-shift features, simply because of its resolution. Of course, the TS-E has less distortion than the zoom does, and that can also be important even if you don’t plan to use the tilt-shift features.

 

Canon 16-35 f/4 IS versus Nikon 14-24 f/2.8

I hesitated to put this up, because I know it will start fanboy flame wars. So first and foremost let me state clearly: This compares the Canon at f/4 to the Nikon at f/2.8. (Nikon’s electrically set mechanical aperture is a real pain to set to f/4 for testing off of the camera body. It can be done, but it’s time consuming. It will have to wait until I have time to do a full comparison.)

The Nikon will definitely improve at f/4; probably enough to be as good as, or even slightly better than the 16-35 f/4 IS. But they’ll be hair-splitting close, I expect. So for the people who shoot the Nikon 14-24 f/2.8 on a Canon camera via an adapter, the 16-35 f/4 IS will definitely give them a new option.

Conclusion

I set a high bar for new Canon lenses. I expect them to be excellent and generally their recent releases have been. Since this was a wide-angle zoom, though, my expectations were lowered a bit. Canon has always struggled with wide-angle zooms. The 17-40 is a good, not great lens. The 16-35 f/2.8 II is better than the Mk I replaced, but I’d consider it, at best, adequate considering its price.

The 16-35 f/4 IS changes that. It’s a superb optic — as good as anything else available. Of course, a lot of people want an f/2.8 zoom. But for many, like me, f/4 with IS is just fine for wide-angle shooting.

I have to add that I think the price represents an excellent value, which is not something I’ve said about many Canon lenses lately. The 24-70 f/2.8 Mk II and 70-200 f/2.8 IS II are world-class optics, but I’ve never heard them called bargains. The 16-35 f/4 IS isn’t cheap, but I think it’s an excellent value. If it were the same price as the 16-35 f/2.8 II, I’d still buy one.

 

Roger Cicala and Aaron Closz

Lensrentals.com

July, 2014

Appendix – Data collection

For each lens MTF is measured 4 times, once each at 0, 45, 90, and 135 degrees rotation. At each rotation MTF is measured at 20 points, equivalent to every 2mm across the image surface. At each of those points, 10 measurements are recorded (MTF in sagittal and tangential plane for 10, 20, 30, 40, and 50 line pairs/mm. So a single rotation measurement looks like this:

The average (mean) and standard deviation of the 4 MTF readings at each position and for each line pair frequency is then calculated as a summary table for that copy. The astigmatism (difference between sagittal and tangential readings for each position and frequency) is also calculated for each frequency and position.

A zero variance check of all the original tables (there would be 80 in a 10-copy test) is done to be a reading was not accidently transposed twice.  Additionally, readings are compared to verify some aspects of the testing. For example if all lenses read slightly lower on one side or at one rotation we might have an issue with proper leveling of the rotation table or calibration of the collimator.

Next all of the copies for each lens are summed and compared. The final MTF graphs are the average of all readings at each position, with the positive and negative points also averaged. (In other words, the MTF, standard deviation, and astigmatism readings at position ’20′ on the graph is the average of +20mm and -20mm from the tables.)

Although we don’t show it in these articles, we then check each copy’s averages against the mean of all copies to identify any outliers. In this particular series one lens was removed from testing early in the process (during the Imatest phase) because it was identified in this manner and further testing showed it to be decentered.

Save 15% This Summer

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A New Old Lens

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Like a lot of photo history buffs, I’ve been quite excited about Lomography’s new iteration of the Petzval lens in 85mm focal length. For those of you who don’t know about the Petzval lens, I wrote about it a few years ago. It really has a rather a fascinating story.

Since writing that article, I’ve been rather obsessed by this lens. I own several of them, made in the late 1800s, but I haven’t been able to adapt them to work on a modern camera. Now Lomography has reproduced the Petzval lens in a nice brass housing, for either Canon or Nikon mounts. Our first copies arrived yesterday and I grabbed them for a bit before they headed out the door.

The new Petzval is quite a handsome bit of kit.

 

It truly is the classic design, complete with Waterhouse stops instead of an iris, and rack-and-pinion focusing instead of a rotating ring. None of your fancy focusing helicoids or 16-bladed aperture rings for this lens.

Now most of you, when you get a shiny new lens arriving, run outside and start taking pictures. But that’s not how we roll here at Lensrentals.

What follows is just wrong.

But just wrong seems so right sometimes. We decided some new 175 year-old technology needed to meet some new 6-months old technology.

So the first Petzval got put up on the Trioptics MTF bench.

 While the other one got slapped on OLAF, our 5-micron pinhole tester.

Obviously, this 175 year-old design isn’t supposed to complete with modern lenses for resolution or aperture. It provides a classic dreamy look. But hey, we test lenses, so guess what we’re going to do?

OLAF actually is more fun here, showing what this lens is about. The spherical aberrations should make the out-of-focus areas smooth and pretty.

In case the built-in aberrations aren’t enough for you, Lomography includes, in addition to the standard Waterhouse stops at various apertures, some very fun cut-out aperture stops: a teardrop, hexagon, and a star.

They have some interesting effects on OLAF’s pinhole light. I can’t wait to see what they do with actual photographs.

The hexagonal aperture isn’t all that strange.

 

The star aperture should make nice star points from light sources in night shots.

Star aperture with lens in focus.

 

But it should also make for some interesting effects with out-of-focus highlights.

Star aperture defocused.

 

The teardrop aperture looks much like a decentered lens on OLAF when properly focused.

But becomes more interesting in out-of-focus areas.

Of Course I Tested It

As I’m sure you know, the indispensible Kingslake’s History of the Photographic Lens points out that the Petzval design “uses overcorrected astigmatism to flatten the tangential field . . . giving excellent definition in the center of the image deteriorating rapidly towards the edges.” I was quite pleased that the MTF bench showed that the new version does exactly that. Note that once you get away from the middle 1/3 of the image astigmatism is, well, pretty damn impressive.

Red–10lp/mm; Green–20lp/mm; Blue–30lp/mm – you get the drill.

 

What the MTF charts suggest we’ll get is exactly what Petzval lenses are supposed to deliver; a fairly nice centered portrait with the outer half of the image significantly blurred. Even in the center, though, the lens doesn’t resolve very well by modern standards wide-open. The frequency graph of the center of the lens emphasizes this.

I know that most of you, at this point, are thinking, “Sure, Roger, we expected these MTF results. But can’t you please show us the field curvature, too?” Fear not, my friends. I can and I will. As Kingslake said, the astigmatism of the lens flattens the tangential lines pretty well, but the sagittal lines have some wicked field curvature. Although in this case that’s a good thing, since a major purpose of the lens is to blur everything off-center.

Field curvature. Red shows the sharpest area. “0″ on the vertical axis is the plane of focus. 

 

 

Yes, I Sort of Took Some Pictures

It’s ugly and rainy here, and the two copies we’ve received are on their way out the door today, so I had no chance to exhibit my superb photographic talents. (Which is good, because I really don’t have much in the way of superb photographic talent. I’m a Photo Geek, not a Photo Grapher.)

But I did discover a few useful things. First, you are not going to want to shoot landscapes with this lens. It’s so soft at infinity that at first we thought it wasn’t reaching infinity focus. Not to mention that all the King’s Photoshop Horses can’t make the edges sharp.

But one thing I found while trying this is that the odd shaped apertures really mess up the camera’s autoexposure (on a Nikon D3x at least). The image on top is with the f/4 Waterhouse stop in place, the one on the bottom with the Star Aperture, otherwise they were identical although both autometered.

 

The effect can be kind of cool, though, close up. For the closer shots below, the top image is shot wide-open, the second with the Star aperture (can you tell I like that one?) and exposure bumped up in post about half a stop. I should also mention that I’d shoot raw with this lens. Color seems to change with aperture a bit and almost every shot either needs to be white-balanced individually with the Waterhouse aperture in place, or corrected in post.

Wide open. Even shrunk down for web display, the softening of the image away from center is quite apparent. 

The major use of this lens, of course, is fun. But it originally was called the Petzval Portrait Lens, so a portrait seems in order. Since my usual swimsuit models and studio lighting weren’t available at 7 a.m., I made do with Corey, the only person who managed to be at work on time, lit by the soft, romantic glow of a test chart.

Now, I’ll have to end my little post, as the packers have come and ripped the last copy from my hands. I’ll mention a couple of things that those of you interested in this lens might want to know before we close, though.

The rack-and-pinion focusing is quite accurate, but rather clumsy to do while you’re looking at the LCD to Live-view focus. I’d really recommend using a tripod if at all possible. Or if you want, just set it at the estimated hyperfocal distance and shoot away. It’s not going to be razor sharp no matter how well you focus.

I’d really avoid any crosshatched backgrounds. Or maybe go find them. The astigmatism makes them look odd, but whether it’s a good-odd or a bad-odd is probably in the eye of the beholder.

I mentioned earlier that the lens is pretty soft at long distances, but it seems to do quite well close up.

You aren’t going to replace any of your current lenses with this one. But for some of you, it might be a fun addition that gives your portraits some really different looks.

Oh, and one last link for the overly serious among you who want to lecture me about how this lens is about taking pictures and not about lab values.

Roger Cicala and Aaron Closz

Lensrentals

July 2014

Sensor Stack Thickness Part III: The Summary

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Well, I have to admit this has been a fun series. I’ve learned a whole lot. That’s what makes this so fun — I get some results I don’t understand, get some help figuring out what is going on, and before I know it, I’ve learned something that explains other things I haven’t been able to understand.

In the second part of this series, we started a database of sensor stack thickness and exit pupil distances, hoping that it would help people decide which lenses would adapt best to which cameras. (And, of course, determine which lenses would not adapt well to which cameras.) A number of people have added information to the database since it was first posted — enough to make it pretty useful.

Since the database is now large enough to be useful, I thought it would be a good idea to make a summary of what we know about lenses and sensor stacks. The best thing about all this, for me at least, is that it lets us make some generalizations about which lenses would be expected to have problems on which cameras.

General Rules

To summarize, there are three main factors that determine if a given lens will have problems when used on a different camera than it was designed for:

  1. Aperture (wider aperture has more problems)
  2. Exit pupil distance (shorter exit pupil distance has more problems)
  3. Difference between sensor stack thickness the lens is designed for, and the sensor stack thickness of the camera it’s being used on.

As always, Brian Caldwell has been kind enough to furnish graphs of the theory, which really helps in visualizing things.

Aperture and Stack Difference On-Axis

Brian’s first graph demonstrates two important points.

Graph courtesy Brian Caldwell

 

The graph demonstrates the on-axis (center) MTF of a theoretically perfect lens designed for no sensor stack filter (orange line), and the effects of adding a sensor stack equivalent to 1,2, and 4mm of glass. The first takeaway message is that for apertures of about f/2.8 and smaller (higher f number) there is really no significant effect on-axis.  (Emphasis on the ‘on-axis’ part. This doesn’t mean the corners will be great, although they may be.)

The second takeaway message is that the effect is proportional to the difference between the sensor stack thickness the lens was designed for and actual thickness of the camera it’s being used on. For example, adding 1mm of extra glass in the path doesn’t really affect things until f/1.4 or so, while a 4mm difference is quite apparent at f/1.8. Once the effect begins, though, MTF decreases exponentially with increasing aperture.

The summary, for on-axis effects, is that shooting a lens at f/1.4 on a camera with a sensor stack different by 2mm or more from what the lens was designed for will probably reduce MTF even in the center. Stopped down, though, you’re unlikely to see any difference on-axis.

Exit Pupil Distance from Sensor

Brian’s second graph shows the effect of a 2mm sensor stack on an f/2.0 lens designed for no sensor stack. If you go back to the graph above, you’ll see that at f/2.0 a 2mm sensor stack difference has really no effect on-axis. This graph shows the off-axis effects out to the edge of the sensor.

Graph courtesy Brian Caldwell

 

As you can see, the off-axis effect shows a mild decrease in MTF and increased astigmatism even with a 100mm exit pupil distance. Would you see this in real-world photography? You might, if you compared an image taken with the adapted system to an image taken with the lens native camera system. But overall, you’d probably be pretty happy with the lens adapted to a different camera.

With a 25mm exit pupil distance, the effects are very severe, even just a short distance from the center. An exit pupil distance of 50mm definitely has some effect, too, although as expected it’s not as bad as 25mm.

Some Generalizations

Sensor Stack Thickness

Our database for sensor-stack thickness remains incomplete (although I’m hoping for more contributions soon). Also remember that physical measurement of thickness and optical thickness may be different. If the glass used has a high index of refraction, it would have an optical effect greater than what is measured physically. For example, we might measure a physical thickness of 2mm on two different cameras, but if one has low-refractile glass and the other high-refractile glass, the optical measurement made might be 2mm and 2.5mm.

The bottom line is all of the sensor stack thickness measurements we have are guesstimates, probably accurate to 0.5mm or perhaps 0.25mm. However, since a difference of 1mm should have only a minimal effect that’s probably accurate enough.

Our optical bench measurements with f/1.4 lenses seem to support this. When we measure lenses with a difference of 1mm of optical glass in the path we see almost no change on-axis and usually just a mild astigmatism change off-axis. A 2mm difference usually causes significant problems, though.

The summary regarding sensor stacks seems pretty clear. Leica cameras (with possible exception of the M240) have less than a 1mm stack. Most SLRs are around 2mm. Micro 4/3 cameras, with the exception of the Black Magic cameras, are about 4mm.

We should expect the following to give problems:

1. Using a lens designed for film on an SLR would give a 1.5 to 2.5mm stack difference, and we should notice a performance drop-off when using wide-aperture lenses with shorter exit pupil distances.

2. Using a lens designed for an SLR on a micro 4/3 camera would give a 2mm stack difference, and we may notice a performance drop-off on wide-aperture lenses with shorter exit pupil distances.

3. Using a lens designed for film on a micro 4/3 camera would give a 4mm difference, and if the other factors (exit pupil distance and wide aperture) are present we will almost certainly notice a performance drop-off.

On the other hand, using a Nikon lens on a Canon camera, or either of those on an NEX or Fuji camera shouldn’t give major problems since all of those sensor stacks are similar.

This should answer one question that several people have asked: Do third-party lens makers have to alter a lens optically when making a Canon versus a Nikon mount? I wouldn’t think so; the sensor stack thickness is very similar. It may (I’m just guessing) also answer an unasked question: Did Zeiss decide to make Tuitt lenses for Sony and Fuji, but not m4/3 because of the sensor stack difference? I don’t know the answer, but Sigma makes the same mirrorless lenses for both m4/3 and NEX. There may be compensating optics in those, but also none have wider aperture than f/2.8, which might minimize the optical difference.

Rangefinder Wide-angle Lenses

As the graph above shows, lenses with an exit pupil distances of less than 50mm are significantly affected by a sensor stack difference. Those with exit pupil distances of less than 25mm are greatly affected. Glancing at the database, it’s obvious that most SLR lenses have an exit pupil distance of 50mm or more. Many rangefinder lenses are less than 50mm and some (especially wide-angles) are around 25mm.

The lens database has gotten fairly large, so I’ve changed it to be sortable, which should help you find what you’re looking for. A couple of lenses do stand out that, according to theory at least, should make bad adapter candidates (at least when shot wide open).

The Voigtlander 15mm f/4.5 Heliar M mountZeiss ZM 15mm f/2.8 DistagonZeiss ZM 21mm f/2.8 Biogon, Leica-M 28mm f/2.8 ASPH Elmarit, and Zeiss ZM 35mm f/2.8 Biogon all have exit pupil distances of less than 30mm.

A number of other rangefinder lenses have exit pupil distances of less than 50mm, but particularly notable are the Voigtlander 35mm f/1.2 and 50mm f/1.5, both of which have wide apertures in addition to short exit pupil distances. Obviously, stopping these lenses down will help minimize the effects of sensor stack if you just have to shoot them on an m4/3s camera.

SLR Lenses

Most SLR lenses have exit pupil distances of 50mm to 100mm. One thing that might be lost in the table is that the Sigma 18-35mm f/1.8 Art lens has a nice, long exit pupil distance, especially at the side end where it’s 150mm. This should make it a superb lens to use on cameras with different sensor stack thickness. Other ‘long exit pupil distance’ lenses include the Nikon 35mm f/1.8 DX (136mm), Nikon 24-70 f/2.8 AF-S G (116mm at 24mm), and Tokina 11-16 f/2.8 (100-117mm depending on zoom distance), and Canon 50mm f/1.2 L (103mm). The Canon 17mm f/4 TS-E (91mm) and 24mm f/3.5 TS-E (86mm) also have fairly long exit pupil distances. All of these should give good performance even out to the edges (assuming a good adapter, of course).

Summary

While I don’t have a mathematical formula to predict how well a given lens will work on a given camera, it should be apparent that wide-angle rangefinder lenses designed for film are going to struggle, especially on m4/3 cameras, but to some degree on other cameras. Longer focal length rangefinder lenses tend to have longer exit pupil distances and should do better. Stopping down to f/4 or f/8 should reduce the problem significantly, although it isn’t guaranteed to eliminate it near the edges of the frame.

SLR lenses may also have some troubles on m4/3 cameras, since there’s nearly a 2mm difference in stack thickness. They shouldn’t have as much of a problem as rangefinder lenses, though, since they are designed for a 2mm sensor stack and they tend to have longer exit pupil distances. Older lenses designed for film cameras will tend to have more issues, of course, since they were designed for no sensor stack.

Obviously, these are generalizations and suggestions. There are going to be exceptions. But, if nothing else, hopefully we can help a few people stop trying one adapter after another, hoping to make their 15mm rangefinder lens look great on the m4/3 camera.

 

Roger Cicala and Brian Caldwell

Lensrentals.com

July, 2014

 

Some Holiday Lens Mutilation

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A couple of weeks ago I got an email asking if we would be willing to take some lenses, remove the electronics, fix the aperture wide-open, and permanently lock them at infinity focus. It seems the person who needed this done was having trouble finding a legitimate repair shop or service center that was willing to do it.

Well, illegitimate is our specialty, so I started negotiations about just how exorbitant a fee we would charge for this work. We quickly arrived at a fair price (no money, but we get to take pictures) and yesterday received brand new copies of the Canon 100mm f/2 and Sigma 35mm f/1.4 Art to work on. If you’re the kind of person who slows down to view car wrecks or spent $200 on fireworks for the 4th of July holiday, you might like this.

(For those of you who aren’t American, the 4th of July is when we celebrate our Independence by getting sunburned, making burnt offerings of animal parts in our backyards, and then eating said offerings. During the entire day, we drink massive quantities of American beer and once it gets dark we shoot off massive quantities of Chinese fireworks. All too often, the results of mixing alcohol and explosives prove that Darwin was correct — but hey, that’s what celebrating is all about, right?)

If torn apart camera lenses make you squeamish, then you won’t like this, and I suggest you not read further. You won’t miss learning anything; it’s just for fun. As best I can determine, this post has absolutely no practical use whatsoever. It’s just something to amuse and entertain those of you who are amused and entertained by such things.

Continue reading

Save 15% for July 4th

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Get your July 4th weekend order early, and save 15%. Just set your order to arrive on Monday, June 30th, or Tuesday, July 1st, and get 15% off your total rental fees!

To take advantage, just use the JULY4TH promo code. Already have an order for July 4th weekend? Simply call or email us to change the order to arrive on an eligible date, and apply the discount code.


Geeks Go Wild: Data Processing Contest Suggestions

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A couple of years ago I gave a talk on the history of lens design at the Carnegie-Mellon Robotics Institute. The faculty members were kind enough to spend the day showing me some of their research on computer-enhanced imaging. I’m a fairly bright guy with a doctorate of my own, but I don’t mind telling you by the end of that I was thoroughly intimidated and completely aware of my own limitations. Continue reading

Only You Can Prevent Movi Fires

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When you run a rental house, you basically function as a torture-test lab for equipment. For many years I’ve put out a Repair Data list annually, showing which photography equipment is more likely to fail than others. I get asked to do the same thing for video equipment, but I usually just shrug and say it’s not necessary. I can sum it up simply.

If the product is new and exciting it will probably fail.

If the company is new and cutting-edge, the product will probably fail.

This sounds like a generalization, and it is. It also sounds like an exaggeration, and it’s not. New, cutting-edge video equipment from new, cutting-edge companies has an extremely high failure rate. For a lot of these products, nearly 100% fail within a few months. Continue reading