Originally posted by mee However, since the reversed lens is the last on the stack and welll. reversed.. it isn't super difficult to hold the aperture arm open
That's what I meant. A lens without aperture ring will default to smallest possible aperture, which can be f22, f32.. this will cause so much diffraction the photo will be.. not just soft, but fuzzy. I guess you could use tape or something to hold the aperture lever open, but that might leave residue. And holding it open by hand is not practical, as that will cause vibrations. At that magnification, any vibrations are bad.
Btw, I think there was a thread titled "Macro by any means necessary" and another "macro on a budget", which dealt with obtaining the highest possible magnification using really cheap gear and improvisation. There were even some formulas on how to calculate the magnification when reversing lenses and stacking lenses. I don't know much more about that, though. I think RioRico was a good macro contributor:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/136294-ways...han-1-1-a.html
Actually, I think this is the best thread:
https://www.pentaxforums.com/forums/54-pentax-lens-articles/152336-cheap-macro-buying-exploiting-lens-ultraclose-work.html
Those are really old threads, so its probably a bad idea to post in them, but they are a good resource, they have some "further reading" links, and good discussion and experiments