Lenses and Optics
Sensor Stack Thickness: When Does It Matter?
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
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.
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 10D 2.7 Canon 20D 2.75 http://www.gletscherbruch.de/
Canon 30D 2.45 Canon 1Ds Mk II 1.0 http://www.gletscherbruch.de/
Canon 5D 1.5 2 http://www.gletscherbruch.de/
Canon 70D 1.2 this is a confirmed, new measurement.
Canon 6D 2.0 Canon T41 1.2 FUJI Fuji X-E1 2.2 Fuji X-E2 2.0 Fuji X Pr0-1 2.2 LEICA Leica M8 0.5 Leica M9 0.8 NIKON D1x 0.7 D100 0.75 D70 1.4 D200 2.2 D40 1.65 D40x 2.2 D300 1.8 D3000 1.1 D7000 ICF 1.75 D700 1.7 Nikon J1 2.8 MICRO 4/3 Olympus OM-D EM-5 3.8 4 Panasonic GF1 4.1 4.2 Panasonic G6 4.1 Panasonic GH4 4.15 Panasonic Gx1 4.1 Black Magic 2.5K 1.4 2.4 PENTAX Pentax K-10D 1.6 SONY Sony A200 2.4 Sony A6000 2.05 Sony A3000 0.6 Sony NEX 5 1.25 2 Sony A7 1.85 Sony A7r 1.85 to 2 2.55 Very 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.
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-E 90.5
Canon 24mm f/3.5 TS-E 86
Canon 50mm f/1.2 L 103
Canon 85mm f/1.2 98
Canon 24-105 f/4 IS L at 105mm 102.5
Canon 24-105 f/4 IS L at 24mm 106
COASTAL OPTICS Coastal 4.88mm f/5 Circular Fisheye 42.5
Coastal Optics 60mm f/4 UVIR 58
Coastal 105mm f/4.5 UVIR 94.5
LEICA Leica-M 28mm f/2.8 ASPH Elmarit 29.6
Leica-M 35mm f/1.4 ASPH Summilux 40.50
Leica-M 50mm f/2.5 Summarit 41.37
Leica-M 50mm f/2.0 Summicron 49.93
NIKON Nikon 15mm f/3.5 Ai 62.6
Nikon 16mm f/3.5 Ai 51.55
Nikon 12-24 f/4 DX AF-S 110
Nikon 17-35mm f/2.8 AF-S@17 98.58
Nikon 17-35mm f/2.8 AF-S@35 77
Nikon 20mm f/2.8 AF-D 54
Nikon 24mm f/1.4 G 75
Nikon 24mm f/2.0 AiS 69.55
Nikon 24mm f/2.8 AiS 60.87
Nikon 28mm f/2.0 AiS 61.48
Nikon 28mm f/2.8 AiS 55
Nikon 28mm f/3.5 Ai 56.55
Nikon 28mm f/3.5 PC 63
Nikon 35mm f/1.4 G 76
Nikon 35mm f/1.8 DX 136
Nikon 35mm f 2.0 AF-D 57
Nikon 35mm f/2.8 PC 83
Nikon 45mm f/2.8 52.53
Nikon 50mm f/1.2 AiS 103.4
Nikon 50mm f/1.4 AFG 81.5
Nikon 50mm f/1.4 AiS 71.6
Nikon 50mm f/1.8 A 56
Nikon 55mm f/1.2 Ai 91.82
Nikon 58mm f/1.2 AiS Noct 83
Nikon 58mm f/1.4 G 68.5
Nikon 60mm f/2.8 Micro 61
Nikon 85mm f/1.4 G 89
Nikon 85mm f/1.4 AiS 67.23
Nikon 85mm f/1.8 Ai 68.3
Nikon 105mm AF-D Micro 96
Nikon 135mm f2 AF DC 59
Nikon 14-24mm f/2.8 AF G at 14mm 81
Nikon 14-24mm f/2.8 AF G at 24mm 96.5
Nikon 24-70 f/2.8 AFG at 24mm 116
Nikon 24-70 f/2.8 AFG at 70mm 92.5
Nikon 70-200 f/2.8 AF-S VR (both) 146
Nikon 200 f/4 AF-D Micro 147
PENTAX Pentax 50mm f/1.4 Super Takumar 70.5
SAMYANG Samyang 24mm f/1.4 76
Samyang 35mm f/1.4 64
Samyang 85mm f/1.4 58
SIGMA Sigma 30mm f/1.4 DX 76.4
Sigma 35mm f/1.4 Art 77.5
Sigma 50mm f/1.4 97.75
Sigma 85mm f/1.4 58
Sigma Zoom 18-35mm f/1.8 ART @ 18mm 149.35
Sigma Zoom 18-35mm f/1.8 ART @ 28mm 91.35
Sigma Zoom 18-35mm f/1.8 ART @ 35mm 70.3
Sigma Zoom 50-150mm f/2.8 EX Apo (both ends) 189
TOKINA Tokina 11-16 f/2.8@11 100
Tokina 11-16 f/2.8@16 110
VOIGTLANDER Voigtlander 15mm f/4.5 Heliar M 24.96
Voigtlander 35mm f/1.2 M 44.5
Voigtlander 40mm f/2 62
Voigtlander 50mm f/1.5 M 49.16
Voigtlander 90mm f/3.5 F 72.95
ZEISS ZM 15mm f/2.8 Biogon 28.32
ZM 21mm f/2.8 Biogon 28.45
ZM 35mm f/2.0 Biogon 34
ZM 35mm f/2.8 Biogon 29.79
Zeiss 50mm f/1.4 Planar 73
Zeiss 55mm f/1.4 Otus 78
Zeiss 85mmf /1.4 67
Zeiss C/Y 15mm f/3.5 Distagon 55.7
Zeiss C/Y 18mm f/4 Distagon 48.2
Zeiss C/Y 21mm f/2.8 Distagon 54.1
Zeiss C/Y 25mm f/1.4 Distagon 77.2
Zeiss C/Y 25mm f/2.8 Distagon 54.4
Zeiss C/Y 28mm f/2.8 Distagon 54.1
Zeiss C/Y 35mm f/1.4 Distagon 64.9
Zeiss C/Y 45mm f/2.8 Tessar 42.6
Zeiss C/Y 50mm f/1.4 Planar 66.4
Zeiss C/Y 55mm f/1.2 Planar 80.70
Zeiss C/Y 85mm f/1.2 Planar 68.1
Zeiss C/Y 85mm f/1.4 Planar 62.4
Zeiss CZ.2 Zoom 28-80mm T/2.9 @ 28mm 83.7
Zeiss CZ.2 Zoom 28-80mm T/2.9 @ 80mm 156.9
Zeiss CZ.2 Zoom 70-200mm T/2.9 @ 070mm 117
Zeiss CZ.2 Zoom 70-200mm T/2.9 @ 200mm 133.85
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
Author: Roger Cicala
I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.
Jason T. Miller