Originally posted by clackers "The print graph shows us that when the 810 uses the full frame it will outperform the K5. Not because it has bigger pixels (which it doesn't), but because it has a bigger total sensor area."
Ask yourself: Why would this not be true in the screen graph as well?
You keep denying you have a shaky grasp of the basics, but your posts are repeatedly saying otherwise. ☺
Because you consider only Snr per photosite as the precision you go for the light level but you ignore the loss in quality in spatial frequency that you have in a K5 vs say a D810. If you both print small from a K5 an a D810 the D810 shoot will look better because the downsampling done either by yourself, by the printer or by your eyes will average the noise present in small pixels.
If you both print very large, the K5 will show bigger "blobs", less refined details than the D810 degrading picture quality. In both case the K5 picture is not as good.
In many case it doesn't really mater say iso 100-400, no heavy crop, no 30"+ prints. In others like 30"+ prints, iso 800+ the D800 would get the edge.
Of course normhead is right, it depend if you still get enough deph of field with the FF vs APSC. Using 200mm f/2.8 instead of 135mm f/2.8 may mean the apperture is too thin and 200mm f/4 doesn't make you loose the high iso benefit.
On the APSC side, it would depend if you will find a lens that can be 1 stop faster than with the FF with the right crop factor and that will perform as well as the FF lens... It may be difficult to find a 50mm at f/2 that would match the quality an FA77 provide at f/2.8 on an FF for example.
Still basically the FF has more margin and if you master it you will be able to use this margin for your own benefit: conveniance to use a pair of f/2.8 zoom each on its own FF body instead of f/2 prime to get the same high iso performance is one example. Astrophotography or a distant landscape is an interresting case as you always focus on the infinite. A narrow deph of field portrait taken at f/4 instead of the f/2.8 where lenses are typically not as good is one another.
Overall an FF is better. The question is to know when it is worth it and if you also accept the downsides: more expensive, bigger/heavier gear.
To me if you are not after very narrow deph of field or very high iso shooting, it is not.