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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.

Your question is flawed, and you are making connections where there aren't any. Some people are short-sighted, some are long-sighted, and others are neither. This relates to the eye muscles not being able to draw light rays into focus at the retina. One wears glasses to correct for this, but the eye muscles are still used to bring objects into focus. So, you wear the glasses appropriate for your own condition, and with their aid, you use your eyes to focus on the image in the viewfinder.

As 40-something human, I totally understand your question. As you get to about this age, your eye looses flexibility, and it becomes impossible to change the focus distance of your eye like young people can. This is why there are bifocals — by looking through different parts of such glasses, you at least get two options. "Progressive" lenses are basically the same, with continuous change —
but still looking through in a certain way only gives you one focus distance. And, those of us who spend our days mostly looking at a laptop screen about 18 inches away may want a different optimization than people who spend most of their day, say, driving a car.

So, anyway… the article you found is correct: the virtual image in the viewfinder is generally presented at about 1 meter away. (An SLR's viewfinder isn't like binoculars.) But, as also explained in the same article, almost all cameras have a little dial by the viewfinder which lets you adjust this virtual image with a built in corrective lens. This correction is measured in units called "diopters" — in fact, the same unit that (part of) your eyeglass/contact lens prescription is given in.
(Or for that matter, the labeling of reading glasses is too.)

If your prescription is not very strong (closer to 0 than +2 or -2), you may be able to use this instead of your glasses. (Although, again as the article notes, not if you have significant astigmatism.) In any case, this correction should be flexible enough that you can adjust for your comfort no matter what your glasses are optimized for. See How do I calibrate the diopter correction on my DSLR's viewfinder? for how to adjust this correction if it isn't obvious to you.

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.

Your question is based upon a false assumption: that eyeglass prescriptions are measured in focal length rather than in diopters. Eyeglass prescriptions are specifically measured in diopters.

• For myopia the diopters are a negative power.


• For hypermetropia the diopters are a positive power.


• For a combination of both myopia and hypermetropia, the negative and positive diopters a patient needs to see clearly at a specific distance are added together.

For example, my right eye requires about -2.75 diopters to focus at infinity. If I'm wearing corrective lenses (Rigid Gas Permeable contact lenses), I then need reading glasses with a power of about +2.75 to focus at a distance of ten inches. The two corrections cancel one another out. Alternatively, I can focus my right eye at a distance of 10 inches with no corrective lenses of either type. My left eye only requires a correction of -1.5 diopters. Without correction that translates to a best focusing distance of about 24 inches for my unaided left eye. If I'm wearing my contact lenses and a pair of +1.5 diopter reading glasses, I still need about two feet to focus properly. To see at one foot I need +2.5 diopter readers.

When I'm wearing contact lenses prescribed based on distance vision, one click short of the +1 diopter limit of the diopter adjustment of my Canon cameras is what I use to see the viewfinder clearly with either eye. Without any corrective lenses, a click or two short of the -3 diopter limit on the other end of the adjustment wheel works for my right eye. About halfway between centered at '0' and the -3 diopter limit works best for my uncorrected left eye.

When wearing contact lenses and a +1 diopter pair of reading glasses, I must back up to about 1-1.5 meters (3-5 feet) to see text clearly. This leads me to believe the eyepiece of my Canon cameras projects a light field appearing to be at around 1-1.5 meters distant, or appreciably close enough to a distance of between one and one and one-half meters, that correction for my vision at that distance works much better than correction for very short or very long distances.

Keep in mind that even with an optical viewfinder, as opposed to an EVF, the view through the eyepiece is based upon the lens' projection onto the viewscreen/focusing screen, not the actual view through the lens. If any overlays or other graphics are the same distance as the viewscreen, they will be at the same focal distance as the viewscreen and the image projected upon it.

The optometrist needs to needs to know the distance to the desired focal plane which is the apparent image in the viewfinder and he will probably not know this.

Your optometrist might¹ surprise you with what he knows. You're likely not his first patient to request glasses for use with a camera, telescope, microscope, etc. Even if you are, I'm certain there is a substantial amount of literature in the applicable scientific journals read by optometrists to guide him. Your optometrist, or even an optician, is trained specifically to know such things. It's part of their job.

_{might = almost certainly will}

Technically the viewfinder creates a virtual image at infinity, so if you really want to wear glasses, you use driving glasses, ie, glasses that are optimized to see far away.

Or you use your regular glasses, because just after looking in the viewfinder, you will need to look at the screen of your camera to check the result, and this one is going to be close....