Quote:
Originally Posted by cometcatcher
I did, but if you're shooting extended objects, the exposure is determined by f ratio not aperture. For instance a 3" f6 or a 20" f6 have the same exposure time since they're both f6.
But I've got a funny feeling I'm missing something.  Someone explain it to me. |
You're correct. The difference in exposure time (ignoring central obstruction) only depends on the f/ratios: 10*10/8*8 = 1.5625
However, if you're an astro imager you'll also want to think about image scale as well as exposure time. This will be very different when comparing a 3" f/6 and a 20" f/6.