Tech cam lens sample variation

[torger]

I've now got back my SK60XL from Schneider-Kreuznach, without action. They had noted some skew in the lens board, so I changed that.

I've done lots of various testing today with both my lens and the loaner lens, and cross-combining the front and back etc. Somehow it works better now. Actually I couldn't figure out exactly why, which is frustrating. I can speculate that the lens board did help my lens.

There is still some left-right variation, but I think it's reasonable now, I'm pleased with the performance. Also note that the Linhof Techno is a great flexible camera to use in the field, but it's not ideal for lens testing. A pancake camera with HPF ring would be so much easier to do lens testing with.

I'm now pleased with the performance. As you can see in the attached shot there's some variations between the copies (note however that I have refocused between copies), but now the worst case is about the same as the best case in my previous testing.

The crops shown here is of actual shifting (not the lens rotation trick), which means any precision errors in the camera is included too. However I did make lots of lens rotations too and there's some variation there too, ie the real shifting didn't make it much worse.

[mcbroomf]

Another couple of tests to try, similar to your turning the lens upside down, is to rotate the lens 180 degrees on the lensboard, ie loosen the back ring, rotate the lens and tighten it up again.  You should definitely recheck center focus after doing that I think, but if the edge issue follows the rotation it's the lens/shutter not the board.

The 2nd you can do if you still have the loaner; switch the lens elements from your shutter to the loaner and repeat the 180 degree (whole lens/board and also lens rotation)

With the improvement you've seen it may be more difficult to see if the residual decentering is the lens, shutter or the board, but maybe you can do it...

[torger]

Another couple of tests to try, similar to your turning the lens upside down, is to rotate the lens 180 degrees on the lensboard, ie loosen the back ring, rotate the lens and tighten it up again.  You should definitely recheck center focus after doing that I think, but if the edge issue follows the rotation it's the lens/shutter not the board.

The 2nd you can do if you still have the loaner; switch the lens elements from your shutter to the loaner and repeat the 180 degree (whole lens/board and also lens rotation)

With the improvement you've seen it may be more difficult to see if the residual decentering is the lens, shutter or the board, but maybe you can do it...

I have done those tests, with the Linhof Technika lens boards there's no other option as the lens board can be attached in only one way. The edge issue seemed to follow the rotation. I also tried combining front back elements, the best performer seemed to be the loaner lens with my back element, but the difference was not large enough to care.

Anyway, the latest round of testing I got satisfactory results. The old bad results with my own lens could have been due to the skewed lens board, a testing error (I did not do the rotation then), or a bad fastening on the lens board. I could see Schneider-Kreuznach had detached and reattached the lens as they left a scratch mark on the plate from the lens wrench. I never did that on my own, it's only now with the loaner and when I got my own back I've done "advanced" testing.

Why the loaner results improved from my first test run is harder to explain though. In the first run I did the rotation only fasting the ring with my fingers to avoid having to take apart the lens. That could have led to some mistakes (not fastening tight enough). In this second run I used a lench wrench. It could also be a focusing issue, as it's very hard with the ground glass to 100% exactly hit a wall. Normal DoF masks for that, but with curvature effects etc maybe issues get worse in the edges. I never got around to do a focus bracketing test to look at that.

Anyway I now know that my lens performs well when everything's right, and I'm satisfied with that. I've had it with lens testing for a while now.

[mcbroomf]

Ahh yes, I'd forgotten that the mount hole is offset in the Linhoff (I have an ancient III but haven't used it for a long time).

Good that you're happier with the results you're getting now.  It might perhaps be worthwhile playing with shims to see if the decentering could be improved just a tad more.  You can buy very thin self adhesive copper that can be cut into strips and mounted between the shutter and the lens board.  Might be a bit tedious to do but it's cheap enough.

[torger]

Ahh yes, I'd forgotten that the mount hole is offset in the Linhoff (I have an ancient III but haven't used it for a long time).

Good that you're happier with the results you're getting now.  It might perhaps be worthwhile playing with shims to see if the decentering could be improved just a tad more.  You can buy very thin self adhesive copper that can be cut into strips and mounted between the shutter and the lens board.  Might be a bit tedious to do but it's cheap enough.

In the latest test run I did not see any meaningful difference between rotating the lens and doing real shifting, so I'm assuming the camera is parallel enough. My last posted sample might still seem a bit fuzzy and unimpressive, but this is with huge shift, 25mm and I very rarely go farther than 15mm. Looking at other peoples results and indeed the MTF charts the results look reasonable. So it's not like "I give up this will have to do", but rather that I don't think it's reasonable to expect much more from this lens. Sure you can get a little bit better, but searching for that final extra bit of performance is more pain than gain as I see it, and the Techno with it's built-in tilt-swing is not ideal for that, a very rigid pancake camera is then probably better. For my shooting style I prefer the Techno's flexibility over a pancake camera's rigidity though.

Anyway, now I think Schneider-Kreuznach was right in saying that the lens does perform up to spec.