Steve Hendrix
Well-known member
Steve,
Nice to see your tests. I presume, it's a given that you adjusted the camera to subject distance in order to equalize the image scales between the 80mm and 110mm lenses?
Regarding lens sharpness performance, I am mostly interested in wide open, and 1 and 2 stops closed down. Beyond that, lenses pretty much all converge towards similar performance, unless you get a real dog. So when I want to test a lens, or compare lenses, I take shortish exposures of the starry sky. Point sources at infinity, scattered all over the image, are the best test of sharpness and aberrations (as well as infinity point accuracy). It's no test of bokeh though!Although it can be a great test of foreground bokeh, if you defocus deliberately.
I just wonder why more people don't do this. I know there's the "I take photos of people/landscapes, not stars" attitude, but that is completely missing the point. It's not (necessarily) about astrophotos as the end usage goal; it's about the starry sky as "Nature's free optics testing lab" :thumbup:
Ray
Correct on your presumption. I agree about lenses - generally - that you'll find the differences at wide open (and also, to a lesser degree, stopped down). But normally the difference in lens prices is largely attributable to the wide open performance. Most of the time - that's what you're paying for.
My question about your tests are - you say "shortish" exposures of a starry sky. Can you define shortish? Objects at that distance would - to me - seem prone to introduce all manner of intrusive phenomena, shifting environmental matter and reflections, etc. I've never been a sky shooter, so completely at the mercy of your experience.
Steve Hendrix