Glass in the Path: Why Using Adapters May Hurt Your Image Quality

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.

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

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

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 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 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

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 of the 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.

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.

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) accounted for this filter stack in the optical formula. Since this adapter is designed for micro 4/3 systems, which have an optically thick stack, no glass in the path might be the issue.

So we found a couple of 2mm pieces of optical glass, mounted them in the optical pathway between the lens 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 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.

But I still wanted to see if this had an effect on even normal lenses. So we put another copy of the Zeiss 55mm f/1.4 on the Imagemaster and tested it. (This is a single lens test so you’ll notice a slight tilt. Every lens has a slight tilt when measured at this way-beyond-pixel-peeping level.) Then we put our 4mm of 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 astigmatic off-axis. (I was surprised at the on-axis effect, but Brian tells me that the glass in the path creates on-axis spherical aberration that could affect center MTF on wide-aperture lenses.) 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.


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 to be at their best, lenses have to be designed to take this into account. Canon lenses supposedly have a medium-thickness sensor stack, and lenses designed for them seem a bit better if 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 (vertical and horizontal focus differences), 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 seeing is believing, so we got a good Leica lens (a 35mm ASPH Summicron) and tried the same tests. Even today, 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 Canon mount lens got a bit better with 2mm of glass in the optical path. 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.


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 purpose of this post is simply to serve as a demonstration that the thickness of the sensor stack does have an effect: if a lens is designed for a certain thickness sensor stack, it may have issues when shot on a different thickness sensor stack. I figured if I was surprised, some of you would be, too, so I wanted to post this now.

Real-World Implications

We don’t know which lenses on which cameras will be most affected. 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

In daylight, it takes nice UV/IR images. Image capture and camera creation credit: Aaron Closz

Under fluorescents it almost looks like a normal camra. 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.

Under fluorescents it almost looks like a normal camra. 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 measure 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

Even testing on our optical bench, corrections may be needed for lenses designed to have a certain thickness of glass between their rear element and the sensor. So 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. Obviously 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.

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.

About the author: Roger Cicala is the founder of LensRentals. This article was originally published here.

  • Amadeusz Leonardo Juskowiak

    Great article! My geeky scientist ego is finally fulfilled. I always wanted to know more about lens designing and physics behind this, so I found the article amazing <3

  • Joe Gunawan

    Very fascinating! Anyone know the optical thickness for the a7 and a7R

  • nehera

    Frankly what a load of bullocks- I’d believe it if their was actual photographic evidence to go with those pretty graphs.

  • mikeaubrey

    Yeah! What has science ever done us anyway???

  • nehera

    Hardly Science when it’s graphs that don’t correlate to anything, no evidence these graphs weren’t just sketched up at random. It looks very much to me like the Daily Fail (Mail)’s take on how to report on Cancer – let’s just pull out some random statistics and a pretty graph but no be willing to show how/why they got the outcome e.g. how many variables did they test what was the control- unless one can provide that isn’t it just a pretty opinion poll (after all none of the lenses tested where tested on their own systems to see if the same happened).

  • Guest

    If the focal reducer needs the 2mm/4mm of glass, why doesn’t it have it ?

  • Mike

    Chill out, man.

  • Jamie De Pould

    The adaptor is designed taking into account the sensor filter stack, which is glass that sits right on top of the sensor. The discrepancy here is because the test bench, unlike a camera, has no filter stack on its sensor, which screws up the test.

  • Jamie De Pould

    Cameras can’t generate MTF graphs, so how could you test “on their own systems?”

  • Mike

    How about using it on different cameras? The different design each manufacturer could use is the problem. This is what this article, and the tests they want to publish in the future, are all about.

  • Lord Minty

    Agreed. There are some many holes in this I don’t know where to start but hey…
    1. These so-called ‘focus reducers’ – they are effectively reverse teleconvertors and at best snake oil. Anyone who has a teleconvertor knows that they have a glass element in them. So why are the effects of that lump of glass not mentioned?
    Also, sticking some lump of glass with undocumented qualities between the lens and the sensor can only reduce the light passing through to form the image. This is simple physics – there is no way a ‘focus reducer’ can create more light or ‘increase’ the aperture of the lens, again this is the simple physics of lens design!
    2. Yeah, where is there any photographic evidence of what is supposedly happening? It might be more beleivable if there was more than just some charts – and charts that are labelled “Theoretic curves” don’t inspire confidence.
    3. “We don’t know which lenses on which cameras will be most affected.” Well no sh*t Sherlock! Perhaps some real evidence might just prove it one way or another, but let’s not let real facts get in the way…
    4. Strangely I though photography was all about the image and the art – who gives a fig if there is some distortion as long as you know what you’re getting. Hell people even stick plastic Diana and Holga lenses on their cameras via adapters – are these guys going to test those as well?
    5. I use adapters on my 4/3 camera (not m4/3) and they work just fine. Should I be throwing away my beautiful legacy Zeiss glass just because some guys produce some clever looking graphs even though they produce gorgeous images that I can’t get with ‘perfect’ modern glass?

  • Lord Minty

    Oh and
    6. If they are targetting m4/3 then they should only be testing the centre of the lens – the smaller m4/3 sensor only uses the centre of the lens so much of the supposed performance ‘data’ about the outer edges and distortion is irrelevant.

  • Mike

    What do you know about optics, as a designer and engineer, rather than a “point and shoot” end user?
    Say that you at least studied it, and I will believe you and not the article.

  • Mike

    “If they are targetting m4/3 then they should only be testing the centre
    of the lens – the smaller m4/3 only uses the centre of the lens so much
    of the supposed performance ‘data’ about the outer egdes and distortion
    is irrelevant.”

    And thus you prove that you have no idea at all what a focal reducer does.

  • OtterMatt

    I love how the commenters on here are uncannily sure that they know more or better than the people with letters after their names. I mean, they only have doctorates in physics, what would they possibly know about light?

  • Lord Minty

    ‘”point and shoot” end user?’
    How very adult.
    Show me the science behind a ‘focus reducer’ then – if these were so good all the camera manufacturers would be making them surely?
    I’ll repeat what I said earlier – if you put any additional glass, in this case a ‘focus reducer’ in between the lens an the sensor you will *reduce* the light reaching the sensor and most probably introduce additional distortions/aberrations etc. etc.
    It is the law of *very* simple optical physics.
    It is telling to note that there is no mention of the optical qualities of the so-called ‘focus reducer’ and its effect of the light transfer.
    Anyway I’m off to create some images and art rather than just geek out on a kit fetish.

  • Lord Minty

    As I said the writer clearly has no idea about the 4/3 or m4/3 systems, if he did he would just select more appropriate lenses to use on adapters. There are better native lenses available for the price of this toy.

  • Mike

    “What do you know about optics, as a designer and engineer”?
    You offer criticism of an engineer’s work, are you one in the same field?

    Please answer this simple question this time.

  • Lord Minty

    I don’t have to prove anything Mike – I see no proof from anyone else, just some unsubstantiated nonsense.
    When an advertorial like this uses words like “Theoretical” and “Perfect focus reducer” you know it’s the photographic equivalent of penis enlargement.
    Why on earth would you want to put a lump of glass of unknown quality between the lens and the sensor?
    But if you want to spend $$$s on one don’t let me stop you.

  • Mike

    “Why on earth would you want to put a lump of glass of unknown quality between the lens and the sensor?”
    Exactly. No one in their right mind would do that.
    This is why Cicala made that post. He’s sharing a problem, giving us end users a look into the world of optics design and QA.

    Now when some super cheap Chinese knockoff comes out with bad optics, not taking into account the filter stack, we will know what the problem is and what to avoid buying.

  • Lord Minty

    Sheesh. I mean the reverse teleconvertor!!
    The article makes you doubt the camera that the maker spent millions developing, yet you believe a adapter made by some third party in some unknown factory will have no effect on the image/light transfer?
    I’ll say it again – you are stuffing some chunk of unknown quality glass between the lens and the sensor.
    Would you use a no-name teleconvertor?

  • Mike

    “I mean the reverse teleconvertor!!”
    This is what I mean, either.

    “The article makes you doubt the camera that the maker spent millions developing”
    Maybe it makes YOU think this. It only means you didn’t understand the article.
    When I finished reading it, I didn’t think anything about the cameras, only the challenges of designing a focal reducer that fits all.

  • Joe Gunawan

    Armchair weekend physicist who stipulates their finding based on their personal opinions and what they read in a forum, lol.

    I’ll take David Caldwell and Roger Cicala’s opinions and expertise any day over those commenters, thank you very much.

  • Trevor Dennis

    Yes. Those comments have to be the most extraordinarily brave examples of people sticking their empty heads above the parapet, ready to have them shot off, I have _ever_ seen.

    The article was interesting, but went way beyond pixel peeping for me. I’m happy to have people like Roger tell me which kit will take the nicest pictures, and while I barely understood the article, I could clearly see the passion with which Roger and his friends applied to the subject matter. So I’ll keep on reading as long as Roger keeps publishing, and maybe absorb just a tiny bit of real world information while doing so.

  • OtterMatt

    FWIW, he did admit in the article that it was in that way-beyond level you mentioned, but overall, the differences between lens configurations are dramatic. To the point where I wouldn’t dare to drop money on a specialty lens and a converter without being able to see the product firsthand. Especially if you’re buying a MFT lens that depends on all that glass being there.
    As usual, the TL;DR here is “caveat emptor.”

  • Brian

    Hi Minty:
    Not only are the optical qualities of the focal reducer “mentioned”, they are shown in great detail in the form of both calculated and measured MTF curves. And guess what – the focal reducer actually improves a Zeiss Otus when used on m43 format, speeding it up to f/1.0 and making it sharper over the entire m43 image circle. What’s not to like?

  • AMVR

    This is all interesting but what does it have to do with photography ? Seriously, all of this is a complete waste of time for anyone who actually cares about the craft. This is taking things too far, pixel peeping has been detrimental enough to photography and now we’ll have to endure people arguing about this level of geekery ? what difference does this make to improving your photo’s composition? how does this relate to the practical quality of your them ? Is any of this going to translate in anyone being a better photographer or is all this just another set of excuses for camera collectors to justify their camera of choice (less or more glass stacks). I appreciate the technical value of this findings but beyond some scientific curiosity, what does it bring to the medium ? All I see is another set of potential nomenclature for gear geeks to get obsessed about on gear forums just like sensor size, ISO, DR, IQ, DOF, MTF charts and all the other inconsequential technical/marketing jargon that people use to hide their lack of expertise as a photographer.

  • eric westpheling

    I purchased one of the “focal reducers” from Metabones. It came in a nice cardboard box. Any lens I put on it was 1 stop brighter, and .71x wider. My USAF resolution charts came up sharper in the center. It does what they say it does.

    Please stop trolling.

  • Gerbert Floor

    Funny, I was wondering about this yesterday. I’m only interested in real world tests, what does it do to center sharpness, and corners. Does it give more distortion, does it give more CA? It’s nice to play around with a f1.0 lens on mft (although you always lose some depth of field compared to full frame, unless the adapter is 0.5.)

    What are the downsides, there must be some very big ones, why wouldn’t lens manufactures make native f1 or lower lenses for mft :)

  • Theo Lubbe

    Based on what I’ve seen, Canon’s EF 75-300mm III (USM) is one of those lenses which performs abysmally on APS-C sensors, while it actually does quite well on full-frame sensors; so it may serve as an example?

    I know that in terms of test charts etc, I’ve encountered numerous Canon lenses which you can see a clear improvement in image quality at the same focal length+aperture combinations on FF vs APS-C, but I last went on a mission to track these down in 2011 and so can’t remember which they all were…

  • Tiktian C

    Caldwell is a fairly renowned optical scientist/designer these days, he’s designed a number of very high quality optics. You can assume quality in the same way you can from Canon or Nikon based on his reputation and previous outputs.

    Also, though more glass does result in less light reaching the sensor, f stops are a ratio of focal length to aperture diameter, thus if you decrease the focal length (what the focal length reducers do) while maintaining the same aperture diameter you will get a larger f stop. There will be some extra loss of light from the glass, modern coating technology reduces that to a minimum.

  • Kurnia Lim

    How about normal adapter, I mean not speed booster or focal reducer? Thanks

  • Jamie De Pould

    Angle of view is directly related to producing the image you want. Trying to go wide on a crop sensor is still frustrating years into the digital revolution. This is one way around that.

  • Jamie De Pould

    Size. All of the lenses we’re talking about are for larger cameras. There’s nothing technically stopping a lens designer from including these kinds of optics on the back of a native-mount lens, but it’s going to be big.

  • Jamie De Pould

    Depends on the adaptor. If there no glass in it (e.g. Nikon F to Sony E), then there’s no change optically (assuming everything is lined up properly).

  • Mike

    We like to learn what makes cameras tick. Get off our backs.

  • randsgrown

    Casual dating could
    be defined as having relationships without the ties, but however we define
    it, one thing is for sure: casual dating is becoming more and more popular.
    But why


  • ryanhoffman456

    my neighbor’s
    step-aunt makes $62 /hour on the computer . She has been laid off for 5
    months but last month her paycheck was $15066 just working on the computer
    for a few hours. linked here w­w­w.j­o­b­s­9­9.b­i­z

  • Brian

    The problem with glassless adapters is they cannot match the attached lens to the filter stack of the camera. This is a big advantage of properly designed focal reducers: they actually cancel the aberrations induced by the filter stack.

  • Brian

    One of the main reasons I designed separate Speed Boosters for Sony NEX and m43 is that the filter stacks are very different, so the optics must be corrected differently. Similarly, the 0.58x and 0.64x Blackmagic-specific Speed Boosters introduced last fall took into account the slightly thinner filter stack found in Blackmagic cameras compared to normal m43 cameras. In this case the difference was only 1.6mm, but with speeds as fast as f/0.74 you have to be very careful with filter stack thickness.

  • Brian

    It was only a momentary screwup – due more to a lack of morning caffeine than anything else. As soon as we rigged the proper amount of glass between the test lens and the MTF detector everything worked exactly as expected from the design.

  • Mark Brown

    Huh, neat. So that’s why my old Pentax 50/1.4 looks great at 1.4 on my Pentax 35mm cameras, but has bad purple fringing visible even from the LCD on my Olympus E-P3 (when at 1.4, anyways).

  • Chito

    Leica users since the M8 have known that the filter stack had to be really thin for the M lenses to perform well.. This is why all the digital M’s have a bit of infrared contamination (M8 a lot, M240 less).

    And this is also why M lenses underperform on the A7 / A7R.. Guess the lesson is, use the lenses on their intended systems.. bummer