Quote Originally Posted by Nicholas Lindan View Post
Nikon uses two thread sizes for the eyepiece - TTBOMK the F3 and the F3HP take different sizes, which may be the cause for some of the confusion. Nikon makes an adapter that goes between the sizes, though the adapter is meant for right-angle finders. There are also two flange diameters - sometimes you can get an eyepiece that fits [screws in] only to find it won't let you open the camera back.

Nikon viewfinders are set so the screen looks like it is 1 meter away with a flat glass eyepiece - or '-1' diopter. A eyepiece marked '0' is actually a +1 lens that with the finder's -1 makes 0 - and so the screen looks to be at infinity. A lens marked +2 will actually be a +3, when mounted to the finder the combination is +2.

I am nearsighted but keep '0' marked eyepieces on my Nikons. Normally this would be a correction for a farsighted person. But with age my eyes have lost their accomodation and with my glasses on I find I am now farsighted - so a '0' lens makes perfect sense.
Yes I have the 0 diopter on my F3HP which is really +1 diopter lens. Now I need the +1 diopter as my eye has gone worse.