I had pretty much the same problem. The eyepiece from the N90 (and probably for the F4 and F5 as well) will fit the F3HP prism perfectly, so you might want to try asking for that. Before you order, pay a visit to your local optician. He can probably tell you which power you'll need. The default diopter of some Nikon SLR's is not zero, so that will need to be taken into account. Perhaps he can measure it, or failing that you should be able to dig up that information with a call to Nikon.