[Startrek (Martin)]

Originally Posted by JA

Hi Martin,

I think that focus is simply more critical at lower f-ratios, probably because of the increased field curvature that the flat sensor has to deal with, resulting in a reduced (acceptable) range of the plane of focus. As a result external influences like temperature, which change the optical path length will be even more apparent.

Best
JA

Thanks for the optical explanation for focus shift
Glad I’m not imaging at f2
My f5 Newts up to 1250mm fl are obviously more forgiving

Cheers
Martin