LensRentals.com

Sensor Stack Thickness: When Does It Matter?

Posted by

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

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

Blame said:

Not sure that the combination of a long exit pupil distance, fast aperture and full frame sensor is no-brainer. I came across this combination while adapting an ISCO cinema lens.

The catch is that the lens mount is going to limit just how wide and close to the sensor the back optical element can get.

Suppose the limit is 40mm optical diameter and distance to the sensor. Close enough to reality. Ok. The active area of that element will be a circle of diameter distance/aperture. For f/1.4 and 40mm that will be about 28mm. 6mm to spare around the edges.

But here is where it gets tricky. The circle will be offset for pixels away from the center by (pixel distance from center)x(exit pupil distance-back element distance)/(exit pupil distance).

Not so good. For an exit pupil distance of 80mm the edge of the back element will add to the total vignetting starting from a 12mm offset from the sensor's center. To reach the sensor's 36mm diameter without loss would need an exit pupil distance of no more than 60mm.

What I can't do so easily is quantify the significance of this issue but offhand I would guess that a seriously wider camera mount standard would offer more options for lens developers.

Mike said:

A couple more:

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

Canon FDn 135mm f/12: 86mm

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:

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.

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!

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.

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). :)

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.

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…

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.

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).

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?

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).

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.

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&

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.

Leave a Reply