On the 'bay and elsewhere, there are listings for China-made eyepiece diopters that can be user-adjusted from -1.5 to +3.5 for older Nikon 35mm SLRs (F2, FE, F3, et al)
They're a bit pricey - roughly 32 to 40-something USD.
Does anyone know if they're any good, if they work?
I've never tried them, but you could probably find customer reviews on-line. I use similar eye-piece magnifiers, and they work great.
It's interesting that they only seem to offer them for a few, select cameras. You'd think it would be easy to make them for any camera. That sure would increase their market.
You would think, yet all are offered either on eBay or through AliExpress.
90-something% are mainland China sellers (but for one guy in the US), and prices go from $29 and change to just under $60 for what is seemingly the same product.
*EDIT - couple of eBay customers are more-or-less happy. Gotta do more poking about.
I bought a couple of these a year or so ago for use on my Fuji rangefinders (same threads as Nikon). Unfortunately I didn't get around to trying them out until well after the return window had closed. Neither one had any effect whatsoever. I have a Nikon DG-2 magnifier that actually works.