Originally posted by Zampel Maybe my writing was a bit confusing, I'll try a different approach to my question:
Suppose you're at a certain distance to some object and you want to have it in a certain final magnification. Due to that fact you choose your shutter speed. You've got 300mm/4 and 500mm/6.3 lense, for simplicity keep all other circumstances constant.
Would using 300mm and crop give better results than 500mm and upping the iso (to achieve same shutter speed)?
(Hypothesis: of course one can't answer that question generally. My point is just that you might get the relatively small advantage of 500 vs 300 mm at the expense of larger grain (due to higher ISO) then you'd have if you crop the 300mm image.)
On a K5 or K7, if I was starting at anything below ISO 800 on the 300/4 I would up the ISO and take the shot with the 500,
Somewhere on the forums, is a shot I took at 510mm (300/4 and 1.7x AF TC) with my K7 at 1600 ISO, and 1/40th hand held. Glad I had 500mm, and I can;t fault the image for grain either,
Right now, I shoot with my K7 on Auto ISO maxed to 2000, and my K5 at Max of 6400, and dial up the aperture I want to get DOF, the camera does the rest, keeping shutter reasonable, and upping ISO to suit.