Trying to make a telecentric lens

[PaulFurman]

So I did some real shooting with the 85/2.8 PC Micro modified with rear aperture and yes the edges have super funky bokeh. As you step through the frames, the OOF circles go through contortions similar to the green jellyfish bokeh above, in the end adding up to crosses. These are full crops from about 1.2x on bellows. The center is fine, almost as good as the straight lens, the edges are bad. It was nice to not have all that scaling & edge streaking while stacking but impractical for most purposes. It wouldn't look too bad with a DX crop. The front element is about 30mm, the aperture about 10mm at f/5. Moderate extension on bellows, this lens is only designed for 0.5x. I did test a section at f/2.8 and it was slightly sharper but with a hazy look also. I also tested the 90/3.5 APO Lanthar reversed and it's similar, a little better than the 85/2.8 at f/5 but with much cleaner OOF & handling of harsh highlights & no CA. That does better with a huge extension reversed. Well sorry for the tangent, here's the results from the 85 made telecentric:



1200 pixel full scene, not super interesting but for the record.

[rjlittlefield]

Thanks for the illustration of weird bokeh. Yes, things get very strange when both apertures are involved in clipping the ray fans.

It was nice to not have all that scaling & edge streaking while stacking

To avoid edge streaking while stacking, simply start at whichever end of the stack has the narrowest field of view. Even with ordinary lenses, that could be either the front or the back depending on how you adjust focus. If the stack was shot starting at the wide end, then File > Reverse Order before stacking. (The next release of Zerene Stacker will do this automatically by default.)

Commercially, telecentric lenses are used for precision measurements and masking, where it's important that "slightly out of focus" does not also mean "slightly the wrong size".

For normal stacking, I don't see much advantage until one starts trying to stack-and-stitch with deep subjects. Then using telecentric optics can make things notably simpler.

--Rik