mikew What UV-IR filter can I use that doesn't extend the back focus with the above equipment?
All glass filters that are placed between the exit pupil of your lens and the camera sensor will increase the backfocus. Physics at work.
For a refraction index of optical glass (about 1.5), each mm of filter glass thickness will increase the mechanical backfocus by 0.33mm.
To add zero backfocus, the glass would need to be infinitely thin, or have a refraction index of air.
Just add spacers (0.33mm worth of spacer per 1mm of glass thickness) between your lens adapter and the camera. Look in the box that your adapter came in, and there may be Delrin (or copycat of Delrin) spacers in there. But much better to use metal spacers. You can find them at dealers such as Agena.
Chen
