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The Glass in the Path: Sensor Stacks and Adapted Lenses

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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.

Conclusions

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

Lensrentals.com

June, 2014

87 Responses to “The Glass in the Path: Sensor Stacks and Adapted Lenses”

Samuel H said:

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).

LensRentals Employee

Roger Cicala said:

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.

Wally said:

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.

Andrew Z said:

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.

LensRentals Employee

Roger Cicala said:

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

hugh crawford said:

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.

Anu said:

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.

Wiebe said:

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...

Brian Caldwell said:

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

Michael said:

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!

JerryR said:

Thanks for the great information and all the work involved! I don't know if this is the reason some of my older Zuiko lenses don't resolve very well on my Fujis but I'll sleep better having something to blame it on.

I don't always agree with what you present in your blog but I seem hard-pressed to find actual facts to refute any of it. You would make my job easier if you presented more conjecture and less data-supported factual information. ;)

Ramon Santiago said:

If I read this correctly, it means that any new Canon lens I would buy for a Canon 5dm3 would produce inferior results on a film camera such as a Canon EV-1, a film camera. Because the film camera would not have the 2mm thick piece of glass.

Is that correct Roger?

LensRentals Employee

Roger Cicala said:

Wiebe, I've been told by people who know that it doesn't matter much where the position is. I don't have the math to vouch for that myself, but they all agree.

LensRentals Employee

Roger Cicala said:

Ramon, in theory that would be true, but film might be more forgiving that sensors since film actually has some depth. But I don't know for sure.

Wiebe said:

Thanks!
I just read you may be compiling some data on the cover glass thicknesses over the next week. The converters among us would be very happy with that and even more happy if you could include the other dimensions as well!

Phil said:

Thanks for a great article.

I use a speedbooster with my fuji bodies and contax Zeiss cy lenses. I love the results. A question for Dr Caldwell... Does it compensate for the stack or is it just the increased telecentricity that helps keep the problems under control? Thx again.

LensRentals Employee

Roger Cicala said:

Weibe, I just posted that tonight.

Brian Caldwell said:

Phil: All of the Metabones Speed Boosters compensate for the filter stack.

Ivan Muller said:

In other lens test websites I have always noticed that micro 4/3 lenses are worse towards the corners. The Canon lenses for EOS M on the other hand are much better in the corners...perhaps because in order to accommodate EF lenses it had to have the same thickness glass in front of the sensor as the EOS bodies. I wonder exactly why the 4/3 cameras have such a thick glass design?

Helder said:

Roger,removing the optical stack, which made the lens l its infinity focus, it is equivalent of changing flange distance right?
So we would go from 19.25mm to something like 23.25mm... interesting
If you say I'm thinking in changing flange distance from an old EOS rebel to accept FD lenses, it right what I'm thinking.
And it might work if the optical stack from EOS cameras is 2mm thick.

Could this be done? How thick from the optical stack in EOS cameras?

J.Thomsen said:

@Brian Caldwell:
You said all metabones Speed Boosers compansate for the filter stack.
But Rodgher said, he found m43 cameras (which) with no or very thin stack, and was told (?!) that there are m43 Cameras with very thick stacks.

How is it possible for Metabones to compensate that?!?
I Think, it couldn't be compensate then....

Regards
Jörg

J.Thomsen said:

okay, I finaly got it. Roger ripped it of from an G1x. So it can be compensated.

John Leslie said:

So am I to understand from you using a Canon version of the Zeiss and a m43 mount speed-booster that there will be an EF-m43 SpeedBooster along sometime in the not too dim and distant? With electronic control too? Cool...

Lynn Allan said:

Roger: I posted the links to these two articles to DPR's sub-forum.
http://www.dpreview.com/forums/post/53848087

It might be interesting for some of the optical engineers and/or scientists to provide feedback.

CanonRumors.com has already posted ... hope it generates some traffic / revenue for your company.

Nicholas T said:

Very interesting articles and research. Does this mean that it would theoretically be possible to get better performance in the corners when using Leica wide angle lenses on the Sony A7R by replacing the sensor glass with a thinner equivalent? Has anyone tried this ?

JasonT said:

Thanks for your article.
Fascinating. Makes me wish I finished high school etc.
But since I didn't....
what - if any - is the implication for third party lenses?
E.g. Tamron or Sigma.
I've always assumed they use basically the same design for a given model change the base and load it with appropriate firmware. Sigma's relatively new convert your mount service had re-inforced this idea in my head.
But now this.
Presumably there is an adjustable group to tune each design for the different mounts in 3rd party lenses?
Or are they more customised than that?

LensRentals Employee

Roger Cicala said:

Jason, I can't say specifically whether there is a lens difference, but Canon and Nikon filter stacks are similar thickness so it's doubtful any change is necessary.

Roger

Mad Hungarian said:

The amateur astrophotographer types often use Canon DSLR's, and when they do they usually remove that IR filter from the sensor stack, because it also cuts off much of the red end of the visual spectrum (i guess CCD's are overly sensitive to red or something). Red is where a lot of the interesting things with nebulas happen, so you don't want to cut it off. Causes problems with daylight photography now, but you either use a separate IR filter or buy another DSLR.

Danny Chau said:

I wondered there is any correlation between pixel density, angle of micro lenses on the sensor in combination with thickness of sensor stack? For example I found the Sony A7s do not show the same purple fringing as the A7R, and maybe because of the lower pixel count in the A7s which maximises the original design of wide angle M mount lenses such as Voigtlander/Zeiss 15mm and 12mm lenses?

Danny Chau said:

One more thing come to mind is the DXO actually test the lenses on the relevant camera body before they give out the figures, surly this is the best way to test out the combine lens to body/sensor effect than just purely reading from the MTF rig?

LensRentals Employee

Roger Cicala said:

Danny, so do all Imatest based sites. It has certain advantages and certain disadvantages. Chief among the disadvantages are the short distances at which the tests are performed (2-3 feet for wide angle lenses). The other disadvantage are you add the variable of the camera if you are interested in the lens. For example, a test of a Sigma lens on a Canon camera can be very different than the test of a Sigma lens on a Nikon camera.

roberto said:

A few months ago I took apart my old Panasonic Lumix G1 and swapped the low-pass filter glass (again, 4 mm thick) with some regular, see-through glass. I know it is far from perfect, but it emulates the original one good enough so that my Olympus 12-50 Zooom lens works perfectly and focuses to infinity throughout the whole range. I did it for the reason of having an infrared camera of course, and with the addition of screw-on IR filters (680 upwards) I can make pretty pictures now with 1/200 or even better.

So yes, the thickness (and layer-ness?) of these pressed or single glass elements in front of the sensor do make a difference. Ask Sony - they even sell their RX cameras as special Mark II or III editions with removed (or swapped like mine?) low-pass filters.

Jeff Livacich said:

This gets me thinking about cameras like the Nikon D800E, with its extra AA filter to recombine the diffusion of the first, and the Pentax K3, which just doesn't have an AA filter at all.

Jeff Allen said:

The AA / OPLF is not just some piece of optically flat glass it is prescriptive to the sensor / optical design of the camera and specialist companies make these. Badly designed OPLF filters rob resolution and in themselves can add to abbérations or induce flares or color casts particularly at high ISOs and point light sources coming straight down the lens.

Regarding film you can adjust the focal depth into the layers of the film in motion picture we did this sometimes for various reasons plus grain depending on emulsion played a part. Film is more forgiving than digital regarding sharpness.

The other aspect of sensors that could also play a part are the micro lenses on the photo sites especially out to the corners of the frame that is why telecentric lens designs are preferable.

Then their is nyquist and matching pixel size the list goes on.

Max Rockbin said:

Is the Canon APS-C sensor stack the same thickness as the full frame?
If not - that'd have consequences for using EF lenses on the smaller sensor bodies. You'd expect EF-S lenses to do better...

Eric Hiss said:

Anyone know what the sensor stack thickness, optical thickness of MFDB's are?

Eric Hiss said:

Ps. from my crude measurements based on a Leaf AFi to graflok film adapter that the optical distance must be about 2.7mm on the 80mp Dalsa chips

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