Originally posted by filorp normhead i am not physicist - but i can use my imagination without wasting - as i can see now - about 15 minuts of my time to create the examples, and get to the simple conclusion that competition betw. 70mm vs 105mm will always result in 105mm finest details despite that the angle of the optical whole is the same - that's because compacts are so inferior not because "less megapixels". The difference are almost insignificant in some cases especially in order to create images up to 5x7 or even A4 - the six pack for those of you who can determine where is actually the border line between usability apsc vs FF!!!
I think that full frame is better at the extremes. High iso situations, printing really big, really shallow depth of field. The question in my mind is how many folks are really pushing the envelope of what is possible with APS-C. As to seeing a difference in "real world" situations between say, a D600 and K3, I doubt you would. This is assuming relatively low iso (below iso 800), a lens that is stopped down a little and a good quality lens (even a DA 50 is more than adequate on a K3).
People for whatever tend to focus on the extremes and yes, at iso 6400 a full frame camera is better. If you print 36 inches on a side, you'll see a difference. I just don't know many people who do those things a whole lot.
Griping about not seeing a difference between photos at web resolution is the point. That's what most people are doing with their photos -- printing them at 8 by 10 and smaller and posting at 2000 pixels and smaller on the web. So, no, most of them won't see a difference.