Chris S. wrote:Robert, I was thinking more about spherical aberration and perhaps other issues. There was a post on the topic of using converging lenses at focus points other than infinity. I can't find it right now, and am pretty sure it was written by Rik. The post referenced this graphic. While the data in the graphic were compiled by varying tube length with finite objectives, my recollection is that we should expect similar results when varying converging lens focus with infinite objectives. The upshot was that with low NA objectives, we can get away with considerable change in tube length (or focus of a converging lens). But as the NA of the objective goes up, tolerance of this change goes down.RobertOToole wrote:When you pull down a lens with short focus you can pull some lenses down quite a bit but all will eventually suffer IQ loss in the corners. I think this might happen sooner than later with a high-NA lens.
I'm interested in this because on my rig, with its Mitutoyo MT-1 tube lens, I've shortened focus by 2-3mm to eliminate a variable-length tube that wasn't as robust as I like. I regularly work with a Mitutoyo 100x/0.70 objective, and have never seen any obvious degradation. An NA 0.75 objective that I tested also behaved splendidly. But I wonder, at those larger NA's, if I'm leaving some smidgeon of quality behind. I don't have an easy way to test. . . .
--Chris S.
This took more time than I expected!
A6300
Century +4 tube lens
(BTW the FL is not really 250mm, more like 230mm, Schneider reported that is normal and within specs)
Infinity focus
Forward/normal mount
75mm tube between TL and Obj.
Mitutoyo M Plan 100x 0.7
Un-cropped and resized to 1500px
Single frame
Flash x 4
Infinity focus
-5mm extension
-10mm extension
-15mm extension
Results? They all look equally bad
So I would say with these results that short focus here did not improve sharpness at least.
Going from 230mm to 215mm did not show much effect (sharpness loss or improvement). Larger % steps going from 200mm to 130mm maybe should have a much more noticeable effect I would expect.
Not much else to say except that this test reminded me that its not easy or fun to shoot at 100x!
Note: Looks like I must have bumped the wafer, the target shifted between first and 2nd frames!
Best,
Robert