Quote:
Originally Posted by ericwbenson
Hi Paul,
FYI beware that the mechanical backfocus will not be the same as the optical backfocus due to the thick glass plate. I imagine it will be explained in the user manual.
For the curious, the exact difference actually depends slightly on the focal ratio and wavelength. Years ago there used to be a webpage that had a relatively clear explanation of the derivation but it disappeared so a while back I copied the gist of it, fixed it up, and posted it on my website.
http://www.faintgalaxy.com/focusshift.htm
Best,
EB |
Hi Eric, I am using the Optical backfocus of the unit. It is stated in the paperwork. It is actually a bit shorter than the physical backfocus, by about 5mm from what I can tell.
Thanks for the link. Interesting reading.