Frank,
You need a correction lens for an optical viewing distance of 1.0 meter (39.37") for a Nikon or Nikkormat from the Nikon F to the Nikon F5 and FM/FE family to the FM3a. The F and F2 and FM/FE to the FM3a all use the same size. The finders on all of these are without a correction lens -1. If you needed a custom made 0.0 diopter correction you would add a +1.0 diopter (not Nikon's +1) as -1 +1 = 0.
If your ophthalmologist or optometrist told you that you need a +1.5 diopter correction to view an object at 1.0 meter you would need a generic +2.5 diopter eyepiece (+2.5 -1 = +1.5) not Nikon's 1.0 or 2.0. Nikon's eyepieces are marked with the combined diopter as in -1 (body) +1 eyepiece is marked 0.
If you call your ophthalmologist or optometrist he/she can calculate the correction you need for 1 meter. Then you factor in the standard Nikon -1 diopter if having a custom eyepiece made or ignore it if buying a Nikon eyepiece. The fly in the ointment is astigmatism. If your persecution includes astigmatism it will include cylinder if I remember correctly. If it's slight it can probably be ignored. If it's strong it's a problem. The astigmatism corrected eyepiece will have to be aligned correctly and it will need to swivel 90 degree for horizontal and vertical. Marty Forscher did a modification for a Nikon F2 or F3 for a swiveling eyepiece for astigmatism. I think there is a link here at NikonGeard.net showing a camera with this modification.
There is a -1 lens in the viewfinder that creates an enlarged virtual image of the ground glass screen which appears to be at 1m. If you can focus at 1m, the virtual image is in focus.
The piece of glass you can take off and replace is
not the -1 lens, although, colloquially, we call it the viewfinder (and Nikon's description is confusing, which will surprise no one who has ever read a Nikon manual). In the case of the FM2/3 that piece you can take off and replace is plain glass. In most Nikons the piece you can take off and replace includes adjustable elements, but that makes no difference in this context.
You
do not subtract -1 from your prescription to get the corrective lens power.
Spectacle prescriptions for "close" focus use reading distance - 0.3m or so. Two diopters long-sightedness means the closest the person can focus is 2m, so a +2 lens enables them to focus at zero distance. A +1 lens enables them to focus at 1m. Nikon's table is correct, and they do not mis-describe their corrective lenses.
Astigmatism should
not be the problem, because Frank is complaining about focusing, which is done in the centre of the image. Astigmatism would affect the sharpness of the metering indicators at the periphery, but reading the +/0/- indicator on the FM2 does not require it to be sharp.
