A Thinner Sensor Stack

A few months ago, before my hiatus from blogging, we did a series of articles showing the effect that a thick sensor stack (the glass above the sensor) had when we use adapted lenses designed for a thin sensor stack. The first one was mostly about theory, the second about when it was likely to actually be noticeable, and the third gave a general summary of when you might expect problems. All of that theory and prediction is good and useful.

The articles generated a fair amount of discussion about removing filter stacks on cameras so that they would perform better with legacy or film lenses designed for little or no sensor stack. In theory, that would make a big difference, but changing the stack has its own set of issues: focus is changed, infrared filtering can become inadequate, etc. Recently, though, our friends at Kolarivision, who had contributed a lot to our database on filter stack thickness, asked us to do some independent resolution testing for them.

They had modified some Sony a7R cameras, removing most of the thick sensor stack and replacing it with thinner Schott BG39 glass (I do not know the exact thickness of the replacement glass, but it is described as ‘significantly thinner’). The replacement glass closely matched the original IR transmission, maintaining accurate colors, but, in theory at least, should improve resolution on wide-angle, wide-aperture, short-backfocus distance lenses. In other words, it should improve the performance of the a7R using wide-angle Leica and other M-mount lenses designed for film.  

What We Did

This was a simple screening test to see if thinning the sensor stack actually made a difference with some better Leica lenses mounted via adapter. We compared the same lenses on a stock a7R with the Kolarivision modified a7R. The same lens-adapter combination was used on each camera, after screening several adapters to get the best lens-adapter combination we could. It’s not an absolutely perfect test, no test using adapters can be, but it should be a practical test, reflecting what a person using the camera would be likely to see.

We compared results with four lenses. To see if the modification improved M-mount lenses designed for thin sensor stacks, we tested the Leica 35mm f/1.4 Summilux M ASPH and Leica 24mm f/1.4 Summilux M ASPH. To see if the thinner stack reduced performance on native Sony lenses, we also tested the Sony 35mm f/2.8 ZA Sonnar and Sony 55mm f1.8 ZA Sonnar lenses. Our standard Imatest lab set up was used with shooting distances of 9 to 18 feet depending on focal length. Best magnified manual focus was found for each combination, then multiple focus-bracketed shots taken, with the best result used for comparison.

Results with Leica Lenses

The $64 question was, of course, if thinning the filter stack made a measurable difference with M-mount lenses designed for a thinner stack. If things worked, we expected center resolution to not change much, but off-axis resolution to improve significantly.

That’s exactly what we saw with the Leica 35mm f/1.4 Summilux M ASPH.

	Sony A7r	Kolarivision A7r
Center MTF 50	1590	1610
Average MTF 50	1120	1200
Average Corner MTF50	550	790

Looking at all of the numbers, the most noticeable difference was in the outer 1/3 of the image area. The modified and unmodified cameras were just about identical in the inner 1/3 of the image with the greatest difference in the corners.

Results with the Leica 24mm f/1.4 Summilux ASPH were slightly different. With this lens the centers are again identical, and the second 1/3 (area from 1/3 to 2/3 of the way to the corners) improved, giving us a higher average MTF50. The absolute corners, however, hadn’t changed a great deal. Whether this is because the sensor-stack change isn’t helping enough at extreme angles, or because of other factors (micro-lens alignment on the sensors for one, perhaps raw data manipulation in-camera, or stuff I haven’t even thought of), I can’t say.

	Sony A7r	Kolarivision A7r
Center MTF 50	1800	1780
Average MTF 50	1065	1130
Average Corner MTF50	660	680

In summary, though, we seem to be seeing a very real improvement. It’s not massive of course, and this is simply a preliminary screening test. Multiple lenses will need to be tested both in the lab and in real-world shooting. Effects on astigmatism, color fringing, and a host of other things will need to be evaluated. But from the point of just resolution using Leica or other M-mount lenses on an a7R, thinning the sensor stack seems to result in a noticeable improvement.

Results with Native-mount Sony Lenses

A lot of people are like me: they would like some improvement with adapted lenses, but don’t want to sacrifice using the native lenses for times when autofocus is important. So we wanted to make certain the thinner stack wasn’t going to mess up performance of some native lenses. (Kolarivision has reshimmed the camera to correct for the change in focus that the new sensor stack creates.)

We didn’t expect to see much change with the 35mm f/2.8 ZA Sonnar, given its narrower maximum aperture, and indeed, there really wasn’t much difference between the two cameras. (Remember, please, these are being tested at f/2.8, not f/1.4 like the Leica lenses, so don’t make yourself look foolish and say the Sonnar is sharper than the Leica, OK?)

	Sony A7r	Kolarivision A7r
Center MTF 50	2100	2080
Average MTF 50	1400	1390
Average Corner MTF50	660	640

As expected, there was no significant change in the resolution of the 35mm f/2.8 lens with or without the sensor stack change. We did some walk-around comparison tests and autofocus, once the camera was properly shimmed, was just as quick and accurate with the modified camera as with the stock version.

The Sony 55mm f/1.8 ZA Sonnar showed similar results. There may have been a slight softening with the modified camera in the extreme corners, but it was very minor if it was real at all.

	Sony A7r	Kolarivision A7r
Center MTF 50	1700	1720
Average MTF 50	1360	1360
Average Corner MTF50	760	720

These aren’t ideal tests for comparison, of course. Ideally we’d be comparing to Sony 24mm f/1.4 and 35mm f/1.4 lenses; but there aren’t any. These are simply the only native-mount primes we had available, so it’s probably the most practical test. The modification doesn’t seem to affect resolution with these native lenses significantly.

I should also point out that there is a slight decrease in corner resolution with the modified camera compared to stock. A difference of 20 lp/IH is almost certainly not significant. The difference of 40 lp/IH might be, that’s about the range where we see a consistent difference. A larger sample size will be necessary to clarify that.

Summary

Don’t take this as more than what it is — a simple feasibility experiment to see if a thinner sensor stack might improve a7R body performance with M-mount lenses designed for thinner or no sensor stack. Theory would suggest it should help improve performance, and this preliminary study suggests that it does, indeed. It’s not a complete cure-all, of course, there are multiple other factors that affect lens performance when shooting on a different body. But this certainly seems worth further investigation.

For those who are are excited about it, I understand Kolarivision is already performing this modification HERE in limited quantities. As these modified cameras get out into the field, we’ll get a much clearer idea of just how much improvement they give to actual photographs.

 

Roger Cicala and Aaron Closz

Lensrentals.com

January, 2015