>My guess on the diopter is that you're looking at the wrong >thing to adjust the focus. I know I did that multiple times >until I figured things out. If the son has 20/20 vision, it's >pretty clear that it should be able to adapt properly. The >diopters don't have infinite range of adjustment, so for >example they cannot correct for my eyes (-6.5 diopters in one >of the eyes - I am as blind as a bat) but from what I recall >they can usually adjust from about -2 to +1.5 diopters. If >you are "a bit" farsighted, you should probably be >within the 0-1.5 range (assuming that my memory is correct).
Actually, this is his 3rd Nikon, and I've had a D600 since September, so we are pretty sure we were using the adjustment correctly. It has about 9 "clicks" and for both of us about 2 "clicks" from the center will "focus" the viewfinder to our eyes (without needing our "cheater reader" 1.5 magnification glasses, though they are helpful when looking at live view and LCD image displays).
I recall someone in Italy on a forum with this issue and interestingly my now returned and exchanged first D600 refused to adjust at ALL twice - though I exchanged it for a different reason - it occasionally went black and refused to operate.