Originally posted by Andi Lo When DOF is taken out of the equation, cropping is exactly the same as zooming. Try out the example posted in that thread, shoot at 200mm then at 10mm, but dont move your camera, then crop the 10mm image to look like 200mm. Resolution and DOF aside the images will look exactly the same.
When you change FL, perspective only changes because you have moved your camera in order to keep the same framing, not because the FL changed
I agree with you, except you neglected to mention that cropping decreases the signal to noise ratio.
To JP's original question, it depends on how you feel about the K-5's AF. Is it doing the job for you? I like having 24mp for sure, but the K-3's fast AF-C and tracking were even more alluring.
(Note: I am coming for a K20D and K-x. I did not have a K-5 for long. The one I bought had AF issues and I returned it).
My intended solution for longer glass is the Pentax HD TC for my DA*300. It is coming, that's why they updated its name yesterday. In the meantime, I'm making due with a Kenko 1.5X, which has great IQ but slow and random AF.