Can't decide on which DSLR

[peripatetic]

Say you're looking at a 6x4 print at a distance of 3 feet.

If you use a 50mm lens on FF you will have a certain angle of view and DOF.

If you take a 1.6 crop from the centre and enlarge it to 6x4 you will have a narrower angle of view and less DOF. (This is of course exactly equivalent to using a 1.6 crop camera.)

By enlarging you have made some points into circles and therefore they will become fuzzy.

But it's important to remember that although you get the same field of view as you would have with an 80mm lens, your DOF is not going to be as shallow as if you had used an 80mm lens on the FF.

There is a great online calculator at http://www.dofmaster.com where you can play with various values, and see how important hyperfocal distance is in the equation.

If you have a look at my "keepers blog" you can see some examples of "portraits" with a range of different lenses.

http://vanderwooks.blogspot.com