Originally posted by Frogfish Interesting Falk. Though I may have used the wrong term is saying 'scope'. I was referring to telescopes such as the superb Skywatcher Black Diamond 80ED (which I've seen some stunning shots from). Does the same still apply ?
TELESCOPE SUPPLIERS - SKY-WATCHER TELESCOPE First, let me clarify that while spotting scopes may not fully match a photo tele lens, my argument based on the human eye's capabilities are not accurate. In practice, a Pentax PF-100ED (630mm) may be paired with an eyepiece such as the splendid Pentax XW20 eye piece, yielding a 32x magnification which would translate to ~1500mm focal length on 35mm or 1000mm on APSC (using 50mm as 1x). The XW20 was reported to look good visually. So, at 500mm on an APSC body, it may not be as bad as my initial comment would suggest. Because the eye sees a magnified image of what the sensor sees.
Wrt astronomical telescopes, I already made a comment earlier in the thread. I linked to a source where it looks like the better 4 element telescope designs may actually outresolve a photo tele lens in the center but become worse quickly off center.