Tjyylli

I've already encountered this question about wearing glasses as a photographer. A more specific question I have is as follows. If your eyesight is not perfect, or your glasses and/or contact lenses aren't entirely on point anymore, can you actually determine if focus is sharp, or will you be thrown off target? I was wondering if imperfect eyesight will cause the image in a viewfinder to appear sharp when the lens is actually out of focus, or if there will still be some point of "sharpest focus" when adjusting manually that will appear somewhat blurry to the photographer but is in fact sharp. In other words, is it simply some offset or will bad eyesight never see a sharply focused image?

One thing that I suppose would help greatly is a split prism focus screen, which should make focus clear through alignment, whether it appears perfectly sharp to your eyes or not. Are there other ways?

I often shoot in low light (low enough to make auto-focus unreliable), using film SLRs. None of them has a split prism focus screen and only one has a diopter adjustment. Most of the time I'll be wearing contacts, occasionally glasses. I've noticed my eyes seemed to have deteriorated very slightly the past couple of years, but on the other hand my optician didn't recommend getting my vision as sharp as possible because I work with a computer all day and this would apparently be fatiguing for the eyes. So either I get my prescription changed, learn to compensate focus slightly or find some good trick for this.

The image in the viewfinder is focused on the ground glass screen. The focused image converges on a 2D plane. (It's not like binoculars.) When you look through the viewfinder, your eyes focus on the image on the screen, which is at a fixed (virtual) distance. You can use the diopter adjustment to change your perception of the fixed screen. Glasses or contacts affect this. But the image on the screen is what's coming through the lens.

Just as poor eyesight can't make a blurry print be sharp, you will never "accidentally" see an in-focus image when the image in the viewfinder is actually out of focus.

It might still be hard — without focusing aids it is hard with perfect (or perfectly-corrected) vision. But it will just be harder to see, not misleading. If your vision can't be corrected in a way that works nicely with the viewfinder, it may be time to look at a camera with an EVF (e.g., a mirrorless camera), which can show focus peaking or zoom in to show focus more accurately.

It depends on the nature and severity of your imperfect eyes. Not all eye defects are the same. With severe defects, you will never be able to tell whether the camera is in focus.

With mild defects that result in slight blurriness, you can focus by observing the image improve as you turn the focusing ring. At the point that it begins to worsen, reverse the direction you are turning the ring. The image will again improve, then worsen. Back off slightly and take the picture. If in doubt, you can bracket. If you keep missing focus, your aperture may be too wide, stop down a bit to increase depth of field.

As mattdm states, you are focusing on ground glass, not through the lens optical system, so you cannot compensate for a blurry image in the viewfinder that is caused by misfocus. Ground glass in DSLRs is difficult to focus even with good vision. Consider switching to a system with focus peaking, or other focusing aid.

The eye has muscles to focus the lens that are under partial conscious control. Also, squinting both deforms the eye (which can improve focus) and narrows the aperture (which improves depth of field). These compensatory actions are fatiguing. Having sharp vision is not. Consider seeing an optometrist or ophthalmologist (who understands physiology and vision) instead of an optician (who understands optics).

It is possible for the image in the viewfinder to appear sharper at a different focus distance than when it appears sharpest in an image captured by the film (or digital sensor) in your camera. But it has nothing to do with your vision. If the focusing screen in the roof of the mirror box is a slightly different optical distance, via the mirror, from the lens than the optical distance from the lens to the film (or sensor) plane is when the mirror is flipped up, it can cause manual focusing errors. A competent camera technician can adjust the mirror's resting position or shim the focusing screen (it depends on the specific camera model and the exact nature of the misalignment - are both top and bottom of the frame off by the same or differing amounts, etc.) to correct the issue.

But assuming the focusing screen is the same optical distance from the lens as the film (or sensor) plane is, you'll always see the image in the viewfinder as "least blurry" at the same focus position as when the lens is focused as "least blurry" for the film (or sensor) plane.

You adjust the diopter setting based on your ability to make out numbers, reticle (horizontal and vertical alignment lines) etc. In other words, you adjust as best you can for the displayed data.