Eyeglasses have different potential focal lengths. For example, there are reading glasses, computer glasses, driving glasses etc.
So, if I am shooting photos, what is the right focal length?
The focal plane of the viewfinder is, what, 1 inch away? Do I tell the optometrist to optimize the glasses for a 1-inch focal length?
(PS please don't go into taking off glasses and using the viewfinder focus. This question is about USING GLASSES WITH THE VIEWFINDER. I am not interested in essays on using viewfinder diopters, that is not what this question is about.)
After thinking about this for a little while, I realized that any short focal length lens will have enough depth of field to focus on a nearby object (less than 8 inches. So, according to this idea, any glasses with a focal length optimized for 4-12 inches or less (reading glasses) should bring the viewfinder image into sharp focus. Is this correct?
I did some rough experimentation and it appears that the apparent image of the viewfinder at least on my Olympus OM-D seems to be a little more than 3 inches from the surface of the viewfinder lens. So, it appears to view things in the viewfinder, you want to optimize your focal plane to be about 3-4 inches.
However, after some additional research, I eventually found a long article by Douglas Kerr in which he writes:
...most camera viewfinders are designed to present the virtual image at an apparent distance of about 1 meter.
Now, my tests were using the viewfinder graphics, not the apparent image through the lens, so it is possible that the graphics are in a different focal plane than the image through the lens.
Comment on Human Vision
This question has nothing to do with human vision, mine or anybody else's. Imagine we placed an imager behind the viewfinder instead of a human eye, the question would be the same. For any optical system the first parameter is the distance to the object being viewed. In the case of the viewfinder, this distance is called the apparent distance. This is what I need to know. Asking for the ideal focal length is equivalent to knowing the apparent distance because they are related by the lens equation. If I know the apparent distance I can compute the focal length and vice versa.