Lenses and Optics

The Glass in the Path: Sensor Stacks and Adapted Lenses

Published June 6, 2014
Single glass piece from the sensor stack of a Canon (left) and Micro 4/3 (right) camera. Image credit Aaron Closz.


NOTE: This is a Geek Post. If you aren’t into geeky photo measurements, or into adapting lenses from one brand of camera to another, you’ll not be interested. 

A year or two ago, I wrote a blog post where I basically showed lenses shot on adapters on other cameras aren’t acceptable for testing. If you run them through Imatest the results aren’t accurate. I suggested that reviewers shouldn’t test lenses on adapters, although obviously adapters are a great way to use interesting lenses to take pictures.

More recently, in online discussions about why certain lenses weren’t working well on certain cameras, I brought up the fact that sensor stacks, the various layers of glass in front of the sensor containing AA filters, IR filters, etc. would be contributing to this problem; that there was more to it than just adapter irregularities. Most people thought that really wasn’t having an effect, though, so I forgot about it.

Yesterday I got a dramatic rude awakening that made me return to this train of thought and do some investigation. The way it happened was simple enough. Dr. Brian Caldwell, the guy who designed the Coastal Optics 60mm Macro, the Metabones Speedboosters focal reducers, and a lot of other cool lenses came to visit.  I’ve had the pleasure of knowing Brian for some time, but I will have to admit his visits (like those of several others) have become just a bit more frequent since we got our MTF bench up and running.

Brian had brought a prototype of his latest focal reducer. He told me it was so good that it clearly improved the MTF of full-frame lenses while increasing their aperture when mounting them to m4/3 cameras. He also brought the computer generated MTF graphs showing what it should do, which was pretty spectacular.

Theoretic curves of the prototype “Perfect” focal reducer, showing what the MTF of a diffraction limited f/1.4 lens would look like reduced to f/1.0. Courtesy Dr. Brian Caldwell


Well, we really couldn’t wait to play around with that, so we all gathered by the cheerful glow of the Imagemaster MTF bench and mounted a Zeiss Otus 55mm f/1.4 on it for a test run. As expected, the Otus generated very nice MTF curves.

Then we mounted the focal reducer on the Otus, adjusted the MTF bench for the narrower depth of field and greater aperture, and tested the combination. The results were absolutely awful. We rechecked all our settings and ran it again. Awful. We tweaked some settings. Awful. Here’s an example, of the same side of the same lens with and without the focal reducer straight out of the MTF bench. (I’ve flipped the MTF chart of the lens-with-reducer to make it easy to compare, which is why the numbers are backwards.) We repeated this with several lenses and it’s about the same every time.

Comparison of MTF curves of a single Zeiss Otus 55mm f/1.4 alone (left) and with the ‘Perfect’ focal reducer (right). You don’t need to understand MTF curves to conclude the right side is worse. I know the difference is amazing, but we repeated it with several copies. All were similarly bad.  

The room became really quiet. Then Brian jumped up and said, “Filter stack – the machine doesn’t have a filter stack.” What he meant was that every digital camera has several pieces of glass in front of the sensor. The light leaving the rear of the lens has to pass through this glass before arriving at the sensor. Brian’s design (like that of most lenses) has an optical formula that plans on light rays leaving the lens passing through such a stack before reaching the sensor. Since this adapter is designed for micro 4/3 systems, which have a thick optically stack, the fact that there was no glass in the light path of the optical bench might be causing a problem.

So we found a couple of 2mm pieces of optical glass, mounted them between the lens rear element and the MTF sensor, and ran the tests again. Suddenly the Otus-focal reducer combination was amazingly good. As Brian had promised and predicted, it was a bit better at f/1.0 than the Otus was at f/1.4 (over a smaller angle of view, of course).


OK, But What About Regular Lenses?

The MTF results with Brian’s Perfect focal reducer were ridiculously dramatic, and to be honest I didn’t believe the glass could make that much difference. Brian often speaks to me in English because it’s a common language we both understand. But when he gets excited he lapses into his native Theoretical Optical Physics, which I can barely follow.

Luckily, he had brought along his colleague Wilfried Bittner, who speaks both Theoretical Optical Physics and English (although his native language is German). With Wilfried’s aid as translator, I’m pretty sure I understand that at effective apertures under f/1.4, glass in the optical pathway have a huge effect on spherical aberrations, which are apparent even in the center of the lens’ field. So the fact that we were testing what was, in effect, and f/1.0 lens made the results very dramatic.

But I still wanted to see if this had an effect on normal lenses. We put another copy of the Zeiss 55mm f/1.4 on the Imagemaster and tested it. Then we put our 4mm optical glass in the pathway. The image below shows the MTF comparison for the Otus when tested with no glass in the optical pathway compared to same lens with 4mm of optical glass in the pathway. Red, green, and blue lines are for 10, 20, and 30 line pairs/mm.


The MTF is better now higher in the center, but there is more astigmatism off-axis. (I was surprised at the on-axis effect, but Brian tells me that the amount of glass in the path creates on-axis spherical aberration that could affect center MTF on wide-aperture lenses. At least that’s what I think he said. Any errors of interpretation are mine.)

But then we realized this is a Canon lens, not an m4/3 lens. Canon cameras, as best we know, have about a 2mm filter stack. So we reduced the glass in the path to 2mm and ran the test again.


The 2mm result does seem a bit better over all, compared to the 4mm. The graph below compares the 2mm and 4mm results to hopefully make them easier to compare.

We repeated this for a couple of other Canon wide aperture lenses and found similar results. The MTF bench results are better when there is a 2mm piece of optical glass in the path between the rear of the lens and the bench’s sensor.

So This Should Work the Other Way, Right?

OK, so if micro 4/3 lenses are expected to have a thick sensor stack and m4/3 lenses have to be designed for them seem much better with a thick piece of optical glass in front of the sensor. Canon lenses supposedly have a medium-thickness sensor stack, and lenses designed for them seem best when we put a thinner piece of glass in their optical path.

What about lenses designed for little or no sensor stack? Actually, it’s already been shown they don’t do well on camera with significant sensor stacks. Panavision has made premium lenses for their film cameras for many years. Recently they’ve released their Primo V series of lenses, which are their Primo lenses modified, according to their website to ” eliminate coma, astigmatism, and other aberrations caused by the extra layers of glass in digital cameras.” U. S. Patent application 14/024,578 describes adding additional optics to the existing lenses to correct for the glass in the imaging pathway, that is between the rear of the lens and the camera sensor.

But we like to see for ourselves, so we grabbed a Leica  35mm ASPH Summicron and tried the same tests. Leica is known to use much thinner filter stacks (1mm or so) than the other camera manufacturers. So putting optical glass in the imaging pathway of an older Leica lens should make it worse.

The Leica 35mm ASPH had an odd reversal of astigmatism with sagittal lines improving a bit, but tangential lines getting much worse. Overall I’d say it wasn’t better or worse, just different.  With 4mm of glass in the optical pathway, though, the Leica clearly gets worse. I would have liked to repeat the test with 1mm of optical glass in the pathway, but we didn’t have any 1mm optical flats.


The things I’ve brought up today aren’t unknown, although they aren’t widely talked about. Bruno Massett had an excellent discussion about the theoretical implications almost a year ago in Mike Johnston’s The Online Photographer. Lens designers plan for the thickness of the sensor stack, and others have made corrective lenses to allow very expensive lenses developed for film to be used on digital cinema cameras.

Obviously this isn’t an exhaustive test using a large series of different lenses. The main purpose of this post is to serve as a demonstration of just how much of an effect the sensor stack has. I figured if I was surprised, then some of you would be, too.

Real-World Implications

People in the real world often shoot a lens designed for one size of sensor stack on a camera with a different size. It seems logical that this kind of issue will affect certain combinations. We don’t know which lenses on which cameras will be most affected, but it would seem logical that lenses designed for film cameras and cameras with very thin sensor stacks won’t do well on cameras with thick sensor stacks.

In order to start making some generalizations, a good database of sensor thickness needs to be made public. I’ve only been able to find references to a few. We know Leica is thinnest and I was told micro 4/3 was the thickest at 4mm. I didn’t believe that, so we took a GX1 apart. As you can see from the first picture, it is, indeed very thick and I can confirm it’s a bit over 4mm.

The extra good news is we now have a micro 4/3 camera with absolutely no glass in front of the sensor at all and a really nice piece of 3mm thick cyan glass for a conversation piece. The camera no longer focuses to infinity, of course, but it takes nice pictures in UV/IR/Visible light spectrum, at least up close. (I know what you’re thinking: but no, we didn’t start this article just so we could make a glass-free GX1.)

In daylight, it takes nice UV/IR images. Image capture and camera creation credit: Aaron Closz
Under fluorescents the images almost look like those from a normal camera. Image capture and camera creation credit: Aaron Closz. And no, Darryl doesn’t get to use the new machine. When he works really hard, though, we let him touch it. 


I hope to have at least a moderately complete database of sensor thicknesses done and published by early next week. We’re doing some disassembly here to measure sensor glass and have sent some cameras off so the glass can be measured optically. Optical thickness may be somewhat different from measured thickness since different types of glass might be used.  (If you have some knowledge in this area, I’d appreciate an email or comment post. You might save a camera.)

Testing Implications

We may need to make corrections on our optical bench when testing lenses designed to have a certain thickness of glass between their rear element and the sensor. Obviously, we’ll be going back to doing more testing there, too. I suspect, for example, that the numbers I posted in last week’s 50mm article might actually be a tiny bit lower than reality for the Sigma Art and Zeiss Otus lenses.

Or perhaps not. This is a new area and we’ll have to run lots of copies on the bench, and correlate them with Imatest or other complete-systems measurements before we know for sure.

Of course, it’s possible that sensor stack thickness might end up being no big deal. But hey, if it’s important enough for Panavision, it’s important enough for me.


Roger Cicala and Aaron Closz, with the assistance of Brian Caldwell and Wilfried Bittner


June, 2014

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in Lenses and Optics
  • Michael

    Wonder why if this is what causes the sometimes extreme chromatic aberration I get using an old Leitz 560mm R-Mount on Panasonic GH2? Lens designed for very thin stack used on system with extra thick stacks? It doesn’t always occur.

    Thanks for another interesting article!

  • Brian Caldwell

    In terms of aberrations and MTF, only the thickness and refactive index of the rear filters matters, and not the axial position.

  • A question that comes up with me is about the position of the filter stack. Does it matter if the filter is right at the sensor, or somewhere else in the optical path behind the lens?
    I work a lot with IR converted cameras with removed filters, but add filters back in like the astronomik clip filters or just the original in my nex6 with a piece of scotch tape…

  • Anu

    It would be interesting to know if the Sony A7 and A7r have the same optical stack thickness (ie. if A7r uses D800e-style AA-cancellation) or not.

  • Re: Testing with different sensor stacks, obviously the lenses designed for the sensor stacks should be retested and perhaps the testing for repair protocol should include the sensor stacks, but I’d be particularly interested in how differently film era lenses test with and without the stack.

  • Roger Cicala

    Andrew, we want to know about any of them! Please send your data along.

  • Andrew Z

    I suspect there is a difference in longitudal chromatic abberation as well which is why people complain about purple fringing more now than the film days. I recently converted an old samsung nx100 to full spectrum and noticed an improvement there. I’ll measure the filters for your database (not that I expect you care about samsung lenses). I know the IR was 1mm but I’ll have to check the cyan one.

  • Wally

    I always thought of the sensor cover glasses as being pretty thin sheets of glass, but it looks like I was wrong. Nice report on yet another item for endless internet discussions.

    You can use ultrasonic sensors to measure thickness but you may not have enough information for it to work well. You send a pulse through the glass and measure the time for the pulse to return. With luck you could get the thickness of all the sublayers. You would need to know the wave velocity in the glass and I think the density to get accurate values, but it has been many years since I did that kind of measurement so YMMV.

  • Roger Cicala

    Mike, none of this kind of impact. Filters, unless they’re cheap ones with imperfect glass or poor coatings, don’t have much effect and nothing like this.

  • Very interesting, can’t thank you enough for all these posts. Good that Karma is rewarding you by letting you hang out with the cool kids 🙂

    This may explain why I felt my 50mm f/1.4 summilux had a “random performance” problem, where it would sometimes be super nice and sometimes unexpectedly soft.

    I have the a7s on preorder, and my lens set is mainly vintage Leica-R designed for no-glass-stack-whatsoever, so I’d be happy to buy a Metabones E-to-R adapter with glass stack correction. Unless it’s only a problem at very wide apertures: I usually shoot at f/2.8-f/4, and it would seem that this kind of problem would be much smaller away from the f/1.0 of that lovely contraption you tested (it’s already much less of a problem at f/1.4, without the speedbooster).

  • Nqina Dlamini

    Greatly enjoyable articles. Yes there will be talk of this for months to come. I learned something new.

  • Mike

    What impact then, do uv or clear filters on the front of the lens have? Might be interesting to find and measure the thickness of some different brands then see what they do to mtf. Old minolta tele lenses almost all had a clear/uv filter at the back of the lens and it was supposed to be kept in the lens when no other filter was being used as it was part of the optical design.


  • Tony

    Roger, you made me wonder if Nikon and Canon have different stack thicknesses, and if so, do Sigma, Zeiss, etc. take that into account in the designs of their 3rd party lenses?

    And if you’re ready for yet another can of worms, the spacing between the lens and the glass might be a significant variable.

  • Richard Gray

    Is there a rational behind the various manufacturers choice of filter stack thickness and in particular why did M4/3 choose such a thick stack?

  • Roger Cicala

    Max, trying to measure the A7r now. It’s difficult because the various pieces are really glued together (not sure if there are air gaps in the stack or not) so we’ve sent one off to be refractively measured.

  • Roger Cicala

    Marc, I think the reason Nikon rotated their OLPA plates rather than removing them in the D800e was to keep the stack the same height. I know there were some changes made early in Digital SLR history, but I assume most manufacturer’s keep the stack a similar thickness for all their cameras now. But that’s an assumption right now, so who knows?

  • Roger Cicala

    Max, that’s not a full Canon stack, just the center of the three pieces of glass. The m4/3 is just the center piece, too, it’s about 3mm by itself.

  • Tilman

    Couldn’t the camera manufacturer compensate the effect of whatever stack he’s applying by placing the sensor a littel deeper in the body? I wonder if this isn’t done. So you could have *all* your lenses with the same focus on sensor-side set to a virtual sensor-plane (where a film would be). The glass-stack on the sensor then refracts this virtual plane to the real sensor position…

  • Max

    By the way, looking at first picture it’s hard to believe left glass is 2 mm assuming right is 4. It’s more like 0.8-1.0 mm. Does it mean left one is not a full canon’s stack?

  • Marc

    Very interesting article. Since I didn’t even know that Theoretical Optical Physics is a language, I may have a stupid question: it’s currently quite fashionable for manufacturers to remove the Optical Low Pass or Anti Aliasing filter. Does this influence the total thickness of the stack discussed here, or does it stay the same because they just replace it with a plain layer of optical glass?

    I vaguely recall when the D800E was introduced Nikon publishing graphics explaining how an extra ‘filter’ was added to neutralize the effect of the OLPF instead of just completely removing it. From other manufactures I haven’t seen such information. So what happens when they decide to remove the OLPF/AA filter?

  • Max

    Great post! Any idea how thick the sensor stack is on the Sony A7r?

  • Rob

    Gary Honis has a good site on astronomic conversion and includes a bit of calculation for replacing the OEM stack to maintain focus calibration.


  • I wonder how will behave the lens that are made in many different mounts as the samyang 12 f2. They seem to have the same glass for the canons, fujis and m43, regardless the sensor glass variations. DXO tests lens in different bodies what may be wise. Olaf lost its superpowers…

  • Roger Cicala

    Joe, those two pictures at the bottom are with a G1x with the entire sensor stack removed. It won’t focus to infinity anymore, and lets in both IR and UV light now, but otherwise works fine. We’ll probably shoot some lenses on adapaters with it to see how they do.

  • Roger Cicala

    Seth, the only thing we’ve done entirely on the optical bench has been the 50mm test you mentioned. All the other tests are on Imatest which are shot on a camera so the sensor stack is already in place.

  • Seth

    Roger, what does this mean about the data you’ve already compiled for lenses not accounting for sensor stacks? You mention going back to the 50mm article, but are there other measurements you’ve made that need to be corrected or do you check everything through Imatest after the optical bench when taking measurements? I imagine your MTFs would be way off just as you describe above?

  • Joe

    Too much fun, if you weren’t already popular with all the camera toys. These new optical benches seem to draw in a whole new level of camera geek. How, how does the thickness of the sensor glass play into it’s durability or ease of getting scratched? I wonder if you were to pull off the glass off a m4/3 sensor, what would the image comparison look like with other system lenses.

  • Brian Drinkwater

    I also wonder if the IR converters understand this.

  • Brian Drinkwater

    I bet the likes of Canikon full well know this which is perhaps why the D800e (for example) is a more convoluted path than than just removing the blur filter.

    Will be interesting to confirm the glass thickness on the newer DX and FX cameras.


  • Roger Cicala

    Ken, not for a bit – it’s still in Prototype phase. But it’s pretty awesome.

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