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Sensor Stack Thickness: When Does It Matter?

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The first post I made on sensor-stack thickness wallowed deeply in PhotoGeekery. This one is meant to be of practical use so I'll try to leave the Geek stuff out. We'll start with the simple facts.

1) There are several pieces of glass right in front of the sensor of every digital camera.

2) The thickness of this layer varies from less than 1mm to slightly more than 4mm depending upon the camera.

3) The thickness of the stack can affect the optics of a lens mounted to that camera.

There is some confusion on when this stuff matters so I'm going to attempt to accomplish two things with this post. First, we'll do a general summary of when it might matter. Second, we'll start a database of information that's not readily available so those who are interested can come back to this page and find out if a certain camera-lens combination might have a problem.

When It Might Matter

Testing

This is pretty straightforward.

1) When testing a lens on an optical bench (right now this is limited to me, apparently) it may be necessary to have glass of the appropriate thickness between the rear element of the lens and the MTF sensor. If it's not done, the measured MTF curves will be falsely low.

2) When testing a lens using Imatest, DxO, or other computerized target analysis, as long as the lens is mounted to the camera it is designed for the proper filter stack is already in place and the measurements are accurate.

3) When testing a lens using Imatest, DxO, or other computerized target analysis on a camera the lens is NOT designed for (testing a Leica M mount lens on a Sony A7r body, for example) the test shows accurately how the lens performs on that body but we can't make any general conclusions about the lens. It might be better on it's native body.

I'll expand on number 3 just a bit. If a tester tests a Leica lens and a Canon-mount lens by mounting both to an A7r, he has created a valid test showing how those lenses perform on that camera. That is good, practical data for any A7r owner to have. But it's completely false to use those test results and say the Leica lens is better than the Canon-mount lens. The results might be totally different on another camera, or if each was shot on the camera it was made for.

Using Lenses on an Adapter

The practical importance comes when we want to use a lens designed for one camera on a different camera. (I'm assuming the adapter contains no optics itself.) Several factors come into play here.

1) The difference in sensor-stack thickness between the camera the lens was designed for and the camera actually being used.

2) The maximum aperture of the lens. Wide-aperture lenses are going to be more sensitive than narrow aperture lenses.

3) How telecentric the lens is. (More specifically, how far forward the exit pupil of the lens is.) A lens with the exit pupil far away from the sensor is not affected by the thickness of the sensor stack very much. A lens with the exit pupil very close to the sensor is affected a lot.

The exit pupil is an optical phenomenon - the exit pupil is not the physical location of the rear aperture or the rear element. It can be measured, but those measurements aren't readily available. In general more telephoto lenses have very forward exit pupils and aren't affected by sensor stack thickness very much. Wide-angle lenses may have very close exit pupil distances. Reverse-telephoto design wide-angle lenses (SLR lenses basically) have the exit pupil more forward than Rangefinder wide-angle lenses, generally.

So in theory, a 135mm f/4 SLR lens isn't going to care much about the sensor stack thickness. A 24mm f/1.4 rangefinder lens can be hugely affected. Here's a theoretical example that Brian Caldwell published on Photo.net almost 7 years ago. This is calculated at 40 line pairs/mm and a 50mm exit pupil distance is fairly close, but you can see even a 0.5 mm difference in filter stack size could have an effect.

Graph courtesy Brian Caldwell

 

Sensor Stack Database

The bottom line is if we want to predict what lens-camera combinations will have problems, we need to know something about sensor-stack thickness and exit pupil distances. Since it's really hard to find that kind of information I've started a database here. Right now it's pretty limited but we'll continue to expand it as we get more information.

The sensor optical measurements are made by Brian Caldwell and measure the optical equivalent as if the sensor stack was made of glass with a 1.52 refractile index. The actual physical measurements may be a little different depending upon what types of glass were actually used.

Thanks to Shane Elen at Beyondvisible.com for all of the Nikon physical measurements. And special thanks to Illija at Kolarivision who provided a lot of data obtained when doing their most excellent IR and other sensor stack conversions. The folks at Kolarivision are people after my own heart - they want to expand the envelope and share their knowledge freely.

For right now, please remember most the physical thickness measurements are single measurements (if there are two confirming measurments, I've made the number bold). Until we get several repeated measurements for each sensor-stack, take them as a suggestion, not an absolute fact.

Pysical mm Optical mm Comments
CANON
Canon 10D2.7
Canon 20D2.75http://www.gletscherbruch.de/
Canon 30D2.45
Canon 1Ds Mk II1.0http://www.gletscherbruch.de/
Canon 5D1.52http://www.gletscherbruch.de/
Canon 70D1.2this is a confirmed, new measurement.
Canon 6D2.0
Canon T411.2
FUJI
Fuji X-E12.2
Fuji X-E22.0
Fuji X Pr0-12.2
LEICA
Leica M8 0.5
Leica M9 0.8
NIKON
D1x0.7
D1000.75
D701.4
D2002.2
D401.65
D40x2.2
D3001.8
D30001.1
D7000 ICF1.75
D7001.7
Nikon J12.8
MICRO 4/3
Olympus OM-D EM-53.84
Panasonic GF14.14.2
Panasonic G64.1
Panasonic GH44.15
Panasonic Gx14.1
Black Magic 2.5K1.42.4
PENTAX
Pentax K-10D1.6
SONY
Sony A2002.4
Sony A60002.05
Sony A30000.6
Sony NEX 51.252
Sony A71.85
Sony A7r1.85 to 22.55Very difficult to measure because cemented to sensor

 

Exit Pupil Database

The location of the exit pupil, along with the aperture of the lens, predicts to a large degree how much effect a difference in sensor stack might have. Brian was kind enough to prepare another theoretic graph of how much difference the exit pupil location makes.

Graph courtesy Brian Caldwell

 

We've been able to get exit pupil distances for a few lenses, which I've listed in the database below. The data in this table is largely from Brian Caldwell and Joseph Wisniewski, with a few from manufacturers publications where I could find it.

 

NOTE: THIS INFORMATION WAS UPDATED JULY 5, 2014

Lens exit pupil distance (mm)
CANON
Canon 17 f/4 TS-E90.5
Canon 24mm f/3.5 TS-E86
Canon 50mm f/1.2 L103
Canon 85mm f/1.298
Canon 24-105 f/4 IS L at 105mm102.5
Canon 24-105 f/4 IS L at 24mm106
COASTAL OPTICS
Coastal 4.88mm f/5 Circular Fisheye42.5
Coastal Optics 60mm f/4 UVIR58
Coastal 105mm f/4.5 UVIR94.5
LEICA
Leica-M 28mm f/2.8 ASPH Elmarit29.6
Leica-M 35mm f/1.4 ASPH Summilux40.50
Leica-M 50mm f/2.5 Summarit41.37
Leica-M 50mm f/2.0 Summicron49.93
NIKON
Nikon 15mm f/3.5 Ai62.6
Nikon 16mm f/3.5 Ai51.55
Nikon 12-24 f/4 DX AF-S110
Nikon 17-35mm f/2.8 AF-S@1798.58
Nikon 17-35mm f/2.8 AF-S@3577
Nikon 20mm f/2.8 AF-D54
Nikon 24mm f/1.4 G75
Nikon 24mm f/2.0 AiS69.55
Nikon 24mm f/2.8 AiS60.87
Nikon 28mm f/2.0 AiS61.48
Nikon 28mm f/2.8 AiS55
Nikon 28mm f/3.5 Ai56.55
Nikon 28mm f/3.5 PC63
Nikon 35mm f/1.4 G76
Nikon 35mm f/1.8 DX136
Nikon 35mm f 2.0 AF-D57
Nikon 35mm f/2.8 PC83
Nikon 45mm f/2.852.53
Nikon 50mm f/1.2 AiS103.4
Nikon 50mm f/1.4 AFG81.5
Nikon 50mm f/1.4 AiS71.6
Nikon 50mm f/1.8 A56
Nikon 55mm f/1.2 Ai91.82
Nikon 58mm f/1.2 AiS Noct83
Nikon 58mm f/1.4 G68.5
Nikon 60mm f/2.8 Micro61
Nikon 85mm f/1.4 G89
Nikon 85mm f/1.4 AiS67.23
Nikon 85mm f/1.8 Ai68.3
Nikon 105mm AF-D Micro96
Nikon 135mm f2 AF DC59
Nikon 14-24mm f/2.8 AF G at 14mm81
Nikon 14-24mm f/2.8 AF G at 24mm96.5
Nikon 24-70 f/2.8 AFG at 24mm116
Nikon 24-70 f/2.8 AFG at 70mm92.5
Nikon 70-200 f/2.8 AF-S VR (both)146
Nikon 200 f/4 AF-D Micro147
PENTAX
Pentax 50mm f/1.4 Super Takumar70.5
SAMYANG
Samyang 24mm f/1.476
Samyang 35mm f/1.464
Samyang 85mm f/1.458
SIGMA
Sigma 30mm f/1.4 DX76.4
Sigma 35mm f/1.4 Art77.5
Sigma 50mm f/1.4 97.75
Sigma 85mm f/1.458
Sigma Zoom 18-35mm f/1.8 ART @ 18mm149.35
Sigma Zoom 18-35mm f/1.8 ART @ 28mm91.35
Sigma Zoom 18-35mm f/1.8 ART @ 35mm70.3
Sigma Zoom 50-150mm f/2.8 EX Apo (both ends)189
TOKINA
Tokina 11-16 f/2.8@11100
Tokina 11-16 f/2.8@16110
VOIGTLANDER
Voigtlander 15mm f/4.5 Heliar M24.96
Voigtlander 35mm f/1.2 M44.5
Voigtlander 40mm f/262
Voigtlander 50mm f/1.5 M49.16
Voigtlander 90mm f/3.5 F72.95
ZEISS
ZM 15mm f/2.8 Biogon28.32
ZM 21mm f/2.8 Biogon28.45
ZM 35mm f/2.0 Biogon34
ZM 35mm f/2.8 Biogon29.79
Zeiss 50mm f/1.4 Planar73
Zeiss 55mm f/1.4 Otus78
Zeiss 85mmf /1.467
Zeiss C/Y 15mm f/3.5 Distagon55.7
Zeiss C/Y 18mm f/4 Distagon48.2
Zeiss C/Y 21mm f/2.8 Distagon54.1
Zeiss C/Y 25mm f/1.4 Distagon77.2
Zeiss C/Y 25mm f/2.8 Distagon54.4
Zeiss C/Y 28mm f/2.8 Distagon54.1
Zeiss C/Y 35mm f/1.4 Distagon64.9
Zeiss C/Y 45mm f/2.8 Tessar42.6
Zeiss C/Y 50mm f/1.4 Planar66.4
Zeiss C/Y 55mm f/1.2 Planar80.70
Zeiss C/Y 85mm f/1.2 Planar68.1
Zeiss C/Y 85mm f/1.4 Planar62.4
Zeiss CZ.2 Zoom 28-80mm T/2.9 @ 28mm83.7
Zeiss CZ.2 Zoom 28-80mm T/2.9 @ 80mm156.9
Zeiss CZ.2 Zoom 70-200mm T/2.9 @ 070mm117
Zeiss CZ.2 Zoom 70-200mm T/2.9 @ 200mm133.85

Conclusion

I know at this point, most of you want to ask about this camera or that lens. I'm sorry; this is all of the data I can find. We obviously need a lot more. Some of you are skilled enough to make these measurements, or may know sources I haven't been able to find. So if you have access to any of this data, please forward it and I'll add it to the tables we've started. I've written some manufacturers to see if they're willing to share numbers with us (but I'm not holding my breath).

Of course, Brian and I are going to continue to make more measurements and will add them as we do. But it isn't a project we can do in a day or two. For physical measurements to be really accurate we have to destroy the sensor, so I'll be doing that as cameras fail and aren't repairable. (If any of you have a dead camera you want to donate to the measurement cause, send it to me and I'll send you back the pieces :-)

Over time I hope this database becomes quite large. We'll leave it here so it remains available to anyone interested.

 

Roger Cicala, Aaron Closz, and Brian Caldwell

Lensrentals.com

June 2014

47 Responses to “Sensor Stack Thickness: When Does It Matter?”

Andre Y said:

What a great project! I wish I had data or measurements to contribute, but I can only wish you good luck!

Tim Cooper said:

So the size of the sensor stack can cause high variation in the results from a lens, but Canon and Nikon both have wild differences in stack size between cameras designed to use the same lenses? I'm not sure what conclusion to draw from that, but it's weird.

CarVac said:

An interesting note: My Contax Zeiss lenses adapted to my 60D seem to exhibit CA in proportion to how close the exit pupil is to the sensor: my 45/2.8 pancake Tessar has a fair amount, next the 28/2.8, the 85/2.8, and then the 50/1.4.
45/2.8: 42.6mm
28/2.8: 54.1mm
85/2.8: 61.5mm
50/1.4: 67mm (info from the Zeiss datasheets)

When wide open, the CA is weaker.

Wiebe said:

A little contribution of the measurements i have laying around after some conversions.
physical thicknesses
Canon 30D: 2.7mm (40D likely the same)
Canon 450D - 500D - 550D (XSi, T1i, T2i etc) & 50D: 1.2mm (2* 0.6mm), as far as i can tell the whole series uses the same set.
Sony NEX-5N, NEX-6: 1.25mm
all of these cameras have a last bit of glass though which looks to be cemented to the sensor, it is quite thin as far as i can tell (< 1mm ) but i have no details on those.
I think this last bit of glass is present in other brands as well (i know the minolta dynax 7D has it), and Nikons as well. in almost all measurements converters get by measuring the removed filter this last bit will be excluded, so that is something to consider.

LensRentals Employee

Roger Cicala said:

Tim, we'll have to wait until we get more data, but I think I see a tendency that earlier cameras had thinner sensor stacks, finally settling into a current stack. This might be because they had a stable of lenses designed for film cameras. It might be that earlier cameras could get by with less AA filtering or weaker IR / UV filters or something.

I also have a feeling that changes of 1mm or less probably are acceptable for most lenses - especially f/2.8 and higher apertures.

LensRentals Employee

Roger Cicala said:

Weibe, thank you. From experience I know the Sony sensors have to really be cut in half to measure that last bit, which I think is 0.8mm to 1mm thick. We may have to wait for refractive measurements for those. I'm not sure the T4i is exactly the same, but what you measured is what I got for the first two pieces, with the last piece being 0.6 also for 1.8mm total (we did break that one in half), so that may proove to be the whole series.

genotypewriter said:

Also, wider lenses made for digital cameras need the filter stack to be of a certain (minimum) thickness and refractive index or they won't be able to achieve infinity focus. But most lenses can focus beyond infinity so this isn't a field-problem.

Would be interesting to see the difference in performance between film-day Leica lenses and their modern counterparts.

Hope to see more!

Pascal Jappy said:

Roger,

great project, thanks. Is there any way for an amateur to measure the exit pupil distance ? I'm guessing you're interested in 1-2mm accuracy, not micron level. So I'd be happy to help by sending data from my lenses, if at all possible.

Stupid question : given that all manufacturers have "similar" lenses an sensor technology, why is there such as huge variation in sensor stack ?

Pascal

Fabrizio Giudici said:

Many thanks for this very interesting info. A question, about third party makers. I've always thought that a third party, such as Sigma, that produces a lens in different versions - say e.g. Nikon, Canon, Sony - shares the same optical formula, only changing the mount and flange distance. If this is the case, that lens can't be optimised for all the brands, since it doesn't compensate for the sensor stack thickness. Or do they slightly adapt the optical design? Can you please elaborate on this? Thanks.

Henning said:

Maybe you should ask MAXMAX.COM they have removed and replaced for many models. Converting to BW etc etc

AlbertTRAL said:

Here I enclose the exit pupil at infinity and the back focal distance of the Zeiss family for the Contax G. I think there are a few of these around converted to M mount:
Model......Exit Pupil(mm)....BFD(mm)
Hologon 8/16......9,5......6,8
Biogon 2.8/21....10,7......12,1
Biogon 2.8/28....13,5......13,1
Planar 2.0/35....27,7......22,6
Planar 2.0/45....25........28,5
Sonnar 2.8/90....22........43,7

Can anybody bring some light on which is the point of placing such a huge and thick glass in the u4/3 system? Maybe color consistency cutting "big time" the spectrum reaching the sensor?
Even such a heavy glass must stress the actuators of those units with IBIS (In Body Image Stabilization)... in any case, it seems part of the u4/3 system standard...

tjshot said:

Great work!
Interesting info that sheds some light on performance gaps of adapted lenses...
Thank you!

Brian Caldwell said:

Wiebe and Roger: you're correct about that last bit of glass nearest the actual sensor surface. Its generally called the "coverglass", and its function is simply to protect the delicate electronics from abrasion etc.. The air-equivalent thickness is easy to measure with the non-contact optical method (using an autostigmatic microscope), and if I assume a refractive index of 1.52 then it typically has a thickness of 0.7mm. Note that the air-equivalent thickness, T, is given by :

T = t/n,

where t is the actual physical thickness and n is the refractive index.

Below are some measurements in millimeters I have for that last bit of glass (coverglass). Note that I can measure "T" directly with an accuracy of about +/- 5 microns or better. To derive "t" I multiply "T" by an estimated refractive index of 1.523. This is the index of Schott B270 glass, which is commonly used as a coverglass material. Also note that its the air equivalent thickness "T" that actually matters for optical aberrations and so on. Ideally we would measure both T and t directly, and this would allow a fairly accurate calculation of the refractive index. However, as Roger says, this will generally require sawing the sensor in half.

Camera T t Air gap to sensor surface
Canon 5d 0.660 1.005 0.330
Sony Nex-5 0.483 0.736 1.143
Sony a6000 0.467 0.711 1.114
Sony A7r 0.467 0.711 0.896
Panasonic GF1 0.348 0.530 0.627
Panasonic G6 0.300 0.457 0.721
Panasonic GH4 0.463 0.705 0.705
Olympus OMD EM-5 0.464
Blackmagic 2.5k
Nikon J1
Pentax Q

Brian Caldwell said:

Wiebe and Roger: you're correct about that last bit of glass nearest the actual sensor surface. Its generally called the "coverglass", and its function is simply to protect the delicate electronics from abrasion etc.. The air-equivalent thickness is easy to measure with the non-contact optical method (using an autostigmatic microscope), and if I assume a refractive index of 1.52 then it typically has a thickness of 0.7mm. Note that the air-equivalent thickness, T, is given by :

T = t/n,

where t is the actual physical thickness and n is the refractive index.

Below are some measurements in millimeters I have for that last bit of glass (coverglass). Note that I can measure "T" directly with an accuracy of about +/- 5 microns or better. To derive "t" I multiply "T" by an estimated refractive index of 1.523. This is the index of Schott B270 glass, which is commonly used as a coverglass material. Also note that its the air equivalent thickness "T" that actually matters for optical aberrations and so on. Ideally we would measure both T and t directly, and this would allow a fairly accurate calculation of the true refractive index. However, as Roger says, this will generally require sawing the sensor in half.

Camera T t Air gap to actual sensor surface
Canon 5d 0.660 1.005 0.330
Sony Nex-5 0.483 0.736 1.143
Sony a6000 0.467 0.711 1.114
Sony A7r 0.467 0.711 0.896
Panasonic GF1 0.348 0.530 0.627
Panasonic G6 0.300 0.457 0.721
Panasonic GH4 0.463 0.705 0.705
Olympus OMD EM-5 0.464 0.707 0.664
Blackmagic 2.5k 0.627 0.956 1.290
Nikon J1 0.467 0.712 0.793
Pentax Q 0.325 0.495 0.521

Brian Caldwell said:

Sorry for the illegible tables above. Let me try again:

Camera .............T ... t ... Air gap to actual sensor surface
Canon 5d .......... 0.660 1.005 0.330
Sony Nex-5 ........ 0.483 0.736 1.143
Sony a6000 ........ 0.467 0.711 1.114
Sony A7r .......... 0.467 0.711 0.896
Panasonic GF1 ..... 0.348 0.530 0.627
Panasonic G6 ...... 0.300 0.457 0.721
Panasonic GH4 ..... 0.463 0.705 0.705
Olympus OMD EM-5 .. 0.464 0.707 0.664
Blackmagic 2.5k ... 0.627 0.956 1.290
Nikon J1 .......... 0.467 0.712 0.793
Pentax Q .......... 0.325 0.495 0.521

Brian Caldwell said:

Pascal: 1-2mm accuracy is good enough for SLR lenses, although for rangefinder lenses it would be better to get 0.5-1mm accuracy.

The exit pupil is simply the image of the iris diaphragm as seen from the rear of the lens. The exit pupil distance is the distance from the exit pupil to the image plane. I normally measure the exit pupil distance using an optical bench equipped with a microscope that travels along the optical axis with a position readout accurate to 0.01mm. However, a simpler arrangement can suffice.

Try setting up the lens under test (LUT) so that it points out toward a distant subject - at least 1000 focal lengths away. Now stop the LUT all the way to f/16 or f/22 or however far it goes. Then examine the back of the lens with a macro lens attached to a camera. Adjust the macro lens so that the iris diaphragm of the LUT appears to be in sharp focus. You may have to illuminate the back of the LUT so that you can see the texture of the iris blades. Next, move the camera and macro lens away from the LUT until the distant subject comes into focus. Its important to move the camera and macro lens as a unit, and *not* re-focus the macro lens. The exit pupil distance is then simply the distance you have moved the camera. Alternatively, you could move the LUT instead of the camera/macro lens.

Jon Gilchrist said:

You might look to the Panorama Photography world for some entrance pupil numbers. You won't get them to 3 decimal places, but knowing the approximate entrance pupil is important for setting up a panoramic head to avoid parallax problems.

Note that in the Panorama world, the entrance pupil is usually measured to the lens mount on the lens.

At the very least, any data you generate will be useful to the Panorama people as well.

Brian Caldwell said:

Jon: I do have a bunch of entrance pupil data as well as exit pupil data, since I normally measure both at the same test session, and the measurement technique is basically the same (just turn the lens around to view it from the front). However, I use the more typical optical standard of measuring it relative to the front lens element.

Nicholas said:

Wow, Roger !

This looks like it’s going to be a quite interesting thread for people who use mount adapters to use e.g. classical rangefinder lenses on modern mirrorless cameras !

It’s also a nice surprise that Dr. Brian Caldwell is reading your blog, and contributing his valuable insights !

AlbertTRAL, I assume the data you supplied comes from Zeiss’ datasheets, and thus uses Zeiss’ convention, whereby the exit pupil distance is measured relative to the optical system’s last vertex (the glass surface that’s closest to the imaging plane).

For assessing the aberrations involving the cover glass / optical stack in front of the sensor, it’s easier to use the exit pupil to imaging plane distance.

The back-focus distance (BFD) in Zeiss’ datasheets is the distance between the last vertex to the imaging plane.

Thus, the imaging plane to exit pupil distance of Zeiss’ lenses can be calculated by just adding the Zeiss-convention Exit Pupil distance to the BFD.

For the Zeiss Hologon 16mm F/8, for example, the exit pupil distance, in the meaning used on this thread, would thus be 9.5 + 6.8 = 16.3mm

AlbertTRAL said:

Nicholas, you are spot on, this is why I included the back focal distance in the table.

Peter said:

Call me an idiot, but could you post a few photos of how much effect this has? Or specify this in some kind of real-world terms (for the 24mm f/1.4 case, are we talking effective resolution goes down to 640x480, to 2MP, to 6MP, or to 36MP)?

I'm just wondering if this is an optical-bench-measurable-effect or an actual real-world effect.

Anthony Burokas said:

I have written a few articles about using B4 lenses on M43 cameras and never thought of the sensor glass thickness as a variable, but, like water, it can certainly affect the image the sensor captures.

I wonder if this plays in to how the B4 lenses, when sampled in "Extreme TeleConversion (ETC)" mode on the GH-series cameras get all milky and soft at wide apertures. I had felt it was that the lenses were designed to compensate for additional glass and prisms not found in single sensor cameras, so this is somewhat related- in the reverse way: that removing all that glass from in front of the sensors detrimentally affects the usability of the B4 lenses on M43 cameras.

That said, I'm actually using my B4 lens to shoot 4K on my GH4 and, in direct comparison with a M43 lens, the results are much closer than I thought they'd be.

https://www.youtube.com/watch?v=h3LooNHBbXo

I also run the B4onM43 group on Facebook and welcome discussion there as well.

LensRentals Employee

Roger Cicala said:

Peter, it's not a simple resolution change and is very lens specific. Some are affected little if at all. More severe effects are significant off axis astigmatism (that would be visible in a photo if you compared to the lens on another camera - but it might still be acceptable for use). Severe effects would soften resolution even on axis and aberrations would increase markedly, easily visible on a photo.

I'm a bench guy, my job is to quantitate the problem and identify where it might have visible effects to photographers. Even when you can clearly see it in a photo, though, it will depend on the photographer and subject as to what it means to them. An architectural photographer would find these things entirely unacceptable. A portrait photographer or abstract photographer might love the look.

Joachim / CH said:

Just a question: Each bayonet type has a flange distance. Is this the distance to the real sensor's surface or the distance to the first surface of the glass layer in front of the sensor? If it's given to the real sensor / film surface, one just could measure the depth between bayonet and first surface. If the air gap is important for the whole calculation, this measuring wouldn't be enough. But it could easily be done without destroying anything.

At the same time I wonder how the manufacturers avoid humidity in this air-gap - in the case the glasses are sealed to the sensor it's easy, but if they are loose? Anyway a couple million sold DSLRs prove the question irrelevant :)

Dmitry Anisimov said:

>Is there any way for an amateur to measure the exit pupil distance ?

because exit pupil location is the apparent location of aperture diaphragm, you can use any rangefinder which does triangulation (not the laser ones which measure time). close the lens' aperture and unmount lens from the camera.

second method:
steep down the lens slightly so aperture opening will be completely visible from the back. measure the apparent diameter of lens' aperture opening (in millimeters, you can do that with shooting with a tele lens) and multiply by f-stop you used.

third method:
take a large aperture tele lens, place in on the optical axis of teh lens being measured so both outer perimeter would be visible too and focus on the lens opening. move a mark outside the lens in such position so the mark and the opening will be in focus at the same time.

Oskar Ojala said:

Thanks for the info! The numbers are useful, however they have not been easily available. Incidentally, I needed some exit pupil locations for one application, but Zeiss no longer lists them in ther data sheets, so I just asked. What I got was 34.0mm from the image plane for the 2/35 ZM and 59.0mm for the 2/50 ZM, HTH. I asked the same for one Leica lens, but never heard back.

As anecdotal evidence, the 2/35 ZM performed very well when stopped down on a Nex-7, but the edges were consistently poor on an OM-D E-M5, so the report of a thick filter for m4/3 is expected. Also worth to note that the effect is obvious when comparing a number of real life shots, a test bench is not needed to spot the difference.

I have the removable filter packs for the Nex-3 and Nex-7 around, could measure those if that's useful. I recall they both have different thickness.

One request: the thickness for the filter pack of the Sony A7; both because I'm considering buying and want to use rangefinder lenses for compactness and it would be interesting to compare the A7 and A7r in terms of filter arrangement.

Ben Goren said:

Roger,

This is a bit off-topic, but your inclusion of the Coastal Optics 60mm lens in the list reminded me of something I've been wondering about, and you're likely the perfect (and perhaps only) person to be able to answer.

Specifically...especially for copy work / fine art reproduction, how does the new Sigma 50mm lens compare to the Coastal Optics lens? The hype over the Sigma makes it seem like, for at least the visible spectrum, it should be within shouting distance of the Coastal Optics...but is that really the case?

Thanks,

b&

Eric said:

Does that mean that new lenses designed for digital cameras will not perform well on film cameras that share the same lens mount?
What about the other way around (eg: older Canon or Nikon AF lenses on DSLRs)?

As different camera models within the same brand have different sensor glass thickness how can the lenses be really optimized?

Nicholas T said:

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

LensRentals Employee

Roger Cicala said:

Eric, the problem isn't insurmountable. In most cases a difference of 1mm isn't huge, 2mm becomes significant (several factors including exit pupil distance and aperture are also important). Most cameras seem to have about a 1.5-2.5mm glass thickness so this problem wouldn't be too big of a deal. It's when you go from film (0mm) or Leica M (0.8mm) to other cameras, or from other cameras to m4/3 (4mm) you commonly notice something. Leica or film lens to m4/3 camera would be the worst combination.

Roger

LensRentals Employee

Roger Cicala said:

Ben, that's probably true, I think the Sigma would do well unless you needed IR or UV spectrum shots too, like people working with older documents often do.

Ben Goren said:

Thanks, Roger. I've been contemplating saving up for the Coastal Optics lens, but that seems hard to justify now unless the art reproduction / giclee thing *really* takes off...and, even then, I'd likely rent it from you the first few times just for jobs that really demand it....

Cheers,

b&

Scott Kirkpatrick said:

On amateur optical benchmarking -- since the accuracy with which you can determine the focal plane with a mechanical rangefinder is about the depth of an eyesocket (5-10 mm), it would be hardto get the exit pupil distance accurately by getting the aperture in focus, then shifting the camera to get the view at infinity into focus. But how about setting up a physics lab grade optical bench with a piece of paper grazing the last cm or so before the image plane and doing some actual ray tracing, with a laser pointer source. The exit pupil is also the point at which the rays of light from objects at different angles to the optical axis appear to come from, as they strike the image plane.

Dr Croubie said:

Hi Roger,
The question I've got concerns the 3rd party lens designers, ie RokiSigRonBowYangZeiss (it's the best portmanteau I could come up with).

Consider from your table above, the 5D has 1.45mm and 30D has 2.45mm. In the Yellow corner the D1X has 0.7mm and D200 has 2.2mm. No data for Pentaxen (Pentices?) yet, maybe similar to Nikon?

So if I'm designing a lens to use on all major DSLR models (with just a rear-mount change like RokiBowYang seem to do), should I be calculating optical formulae based on the 0.7mm thin D1X glass, or the 2.45mm thick 30D?
Even if I'm going to calculate a different formula per brand/mount like Zeiss might (I'd hope they would, with their extra cost and all), or indeed even if I'm Nikon making a new Nikon lens for my Nikon DSLR series, should I calculate based on the 0.7mm of the D1X or the 2.2mm of the D200? The D1X may be the more 'flagship' of the models, but there'd be a hell of a lot more D200s out there. Or stab somewhere in between and have decent-at-both but best-at-neither?

Furthermore, if all the m4/3 lenses are currently calculated for 4mm, are they locked into that forever, and can't go down to 1mm? (I can see the whingers already, "my new m4/3 body isn't as sharp as my old body, using the exact same lens!")

And extending one final time (following on from a recent thread on this topic over at APUG.org), a film-era lens would have been calculated to have no glass thickness at all, obviously. Taking a new lens made today which expects, say, 2mm of glass, its results on film wouldn't be as 'good' on film as on the digital sensor (lets just ignore grain/pixel resolution for a minute).
But with the new calculations and 20-years of tech advances, would not the performance of the 'digital' lens on film would still be better than the older lens on the same film? (my speculation).

Dr Croubie said:

ps, I've very interestingly noted that a m4/3 BlackMagic has a lot less glass than your regular Olympus / Panasonic bodies.
An oversight on the part of BM?
They didn't measure the glass thickness of regular m4/3 bodies?
There's a patent around that doesn't let them copy it exactly?
Maybe it's the same 'optically' but not 'physically'?

Or marketing genius that figures most people are going to adapt DSLR and/or Film-era manual-focus-manual-aperture lenses (even short-pupil Leica RF lenses) onto that body, and so would praise the camera more for the 'better' resolution that comes with having thinner glass when using these lenses?

Brian Caldwell said:

Dr Croubie: The Blackmagic BMCC (2.5k) and BMPCC (Pocket)don't have the same amount of filter glass as normal m4/3 cameras, whether you measure it physically or optically. I think what happened is that the original BMCC model came with a Canon EF mount, and they used the filter stack more or less appropriate for that mount. Why they didn't use a standard filter stack in the BMPCC is a mystery to me. When we developed 0.64x and 0.58x Speed Boosters for the BMCC and BMPCC, respectively, we had to take the actual filter thickness into account due to the extreme lens speeds made possible by those reduction factors (f/0.80 and f/0.74, respectively).

Brian Caldwell said:

Scott Kirkpatrick: I'm not too sure what you mean by a mechanical rangefinder. The method I use involves a long working distance microscope equipped with a reticle at the object plane of the eyepiece. Having the reticle prevents errors due to eye accommodation. Alternately, you can just have an old guy like me do the measurement, since I can hardly accommodate any more! The microscope objective can be a simple cemented doublet, and if you encounter too much aberration just stop it down with a Waterhouse stop (i.e., a piece of cardboard or black tape with a hole cut into it).

For most lenses I can use a 60mm or 80mm focal length objective. However, some lenses with a very long exit pupil distance will require 100mm or more.

Note that in lieu of a microscope you can just use a camera with a good macro lens. Just remember to focus by moving the entire camera/lens combo rather than focusing the lens.

Ilya Zakharevich said:

@Brian Caldwell:

AFAIU, the air-equivalent thickness is irrelevant. Remember that it includes all the air gaps in between the components.

Thickness is ∑ component_thickness/component_n. Abberrations are given by (IIRC) ∑ component_thickness*(component_n - 1). A very significant difference…

Vivek said:

Both the "physical" and "optical" data are far off. (I have not gone through all the comments here)

Case in point: Sony A7r.

[Hint: including the cover glass, the total thickness is <2mm- and only the cover glass is cemented to the sensor and not the UV-IRcut/dustshaker (hence it is easy to measure!) and anything in m43rds is close 5mm!]

You do not need several repeated measurements from different sources. All you need is one that is correct. :)

Brian's approximation for his measurements is a no go (ref indices of actual materials used are way different). However, I know the context of those measurements. Not sure if they can be used directly here for this story.

Vivek said:

One does not have to "saw a sensor in half" to get to the cover glass and measure its thickness. There are far easier ways to do it and even I could do it (so do Dan at MaxMax or several astrophotogs who make monochrome sensors themselves). :)

cpc said:

Zeiss list the both exit pupil position in front of the last lens vertex and the back focal distance (last lens vertex to image plane) in their documentation for most of their older lenses (ZM and ZE/ZF series are notable exception).

Lens docu PDFs can be found here, for example:
http://zeissimages.com/mtf.php

Here are the numbers for the Contax G lenses. Summing the two numbers gives the following results for exit pupil to image plane distances:

16/8 G Hologon -> 16.3mm
21/2.8 G Biogon -> 22.8mm
28/2.8 G Biogon -> 26.6mm
35/2 G Planar -> 50.3mm
45/2 G Planar -> 53.5mm
90/2.8 G Sonnar -> 65.7mm
35-70/3.5-5.6 G Vario-Sonnar -> 25.2mm (wide end), 46.5mm (long end)

Similar info is available for their C/Y, Hasselblad, Contax 645, Contax N lenses. Hope this helps.

Assaf said:

Very interesting article!
I have two questions: I'd like to use an old Yashica 50 ML f1.7 on an Olympux E-M10. The sensor stack thickness is probably 4mm while the lens is probably designed for 2mm. Is that means that wide apertures are doomed to suffer from low performance? or does a smart adapter capable of somehow adapting the lens to expect a 4mm sensor stack? This is particularly important for focal reducers that make it an f1.2 lens. I'm afraid that such wide apertures won't be usable.

I searched for the exit pupil distance and couldn't find it for this lens. However, it probably shares the same formula with the Zeiss C/Y 50mm f1.7 which is described here: http://www.zeissimages.com/mtf/cy/Planar1.7_50mm_e.pdf. Summing 38.2+28.9=67.1mm. Is that means that The article warns of 50mm or less, but I wonder if 67mm is high enough to mitigate the 2mm vs. 4mm difference I mentioned before.
Thanks!

Mike said:

The Canon FDn 85mm f/1.2L lens has an exit pupil distance of ~73mm.

I'll be testing my other two Canon FD lenses today: the 20-35mm f/3.5L and the 135mm f/2. This next week, I'm looking at the Voigtlander 58mm f/1.4 SL and the Minolta 58mm f/1.2 MC, and the Nikon 200mm f/2 AI-S.

Uwe Kulick said:

0.6mm sensor stack thickness for the Sony A3000 (ILCE-3000) ????

Must be an ideal shooter for adaptions! I can confirm that for lenses which support ideally the focus-peaking function of the NEXes and ILCEs since monitor and finder resolutions of that cheap early "ILCE" are often too weak w/o that focusing aid. My most focus-peaking-sensible lens seems to be the Sigma macro 90mm f2.8.

So, please, Mr. Caldwell, is this filter thickness really as measured by You? 0.6 mm ??

Mike said:

A couple more:

Canon FDn 20-35mm f/3.5:
20mm = 65mm
35mm = 78mm

Canon FDn 135mm f/12: 86mm

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