Basic back-focus question

[Joel Shepherd]

Suppose I have a (CCD) camera with 25mm of back-focus. With it on the scope, in focus, I can rack inwards maybe another 5mm.

If I put a camera with 15mm of back-focus on the same scope, will I be able to bring it into focus? My guess is no, that I'd need to be able to rack inwards at least 10mm, but some confirmation of that understanding would be very helpful.

Thanks -- Joel.

[kens]

It's the other way around. With less back focus you'll be able to rack inwards another 15mm after reaching focus. That's why people have problems with DSLRs with much bigger back-focus

[Joel Shepherd]

Ah, okay: thanks. The DSLR visual helps keep it straight.

-- Joel.