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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.

  • what is your camera? – aaaaa says reinstate Monica Feb 25 at 18:18
  • i look at the file size to determine sharpness; bigger file = sharper image (in general). Also, tack-sharp focuses tent to produce a moire pattern in uniform color areas on my camera's LCD due to aliasing, while a blurry image would be dithered instead. – dandavis Feb 25 at 20:08
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It depends on the nature and severity of your imperfect eyes. Not all eye defects are the same. With severe vision defects, you will never be able to tell whether the camera is in focus without correction.

With mild vision 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, try bracketing. If you keep missing focus, your aperture may be too wide. Try stopping down a bit to increase depth of field. Or your camera may need to be adjusted, as Michael C suggests.

As mattdm states, you are focusing (your eyes) 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, because of the small size. Consider switching to a system with focus peaking, or other focusing aid. Options to consider include:

  • Replacement focusing screens with split prisms are available for some cameras.
  • Diopter attachments are available for some cameras.
  • Viewfinder magnifiers are available for some cameras.

  • A cold-shoe rangefinder attachment could be fun to try if the distance scales on your your lenses are accurate. However, results are unlikely to be better than direct TTL focusing. For best results, you would need to stop down to increase depth of field.

With respect to eye fatigue, 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 and require effort, which having sharp vision alleviates. Consider seeing an optometrist or ophthalmologist (who understands physiology and vision) instead of an optician (who understands optics).

  • No focus peaking, I'm afraid, since I shoot film. For manual focusing I do indeed use the method of tuning the focus by overshooting it, then dialing it a bit back and doing this in a "binary search" kind of way until I find what looks sharpest. Thanks for the advice! – G_H Feb 25 at 15:23
  • Added some options to consider for the missing diopter adjustments on your cameras. – xiota Feb 26 at 2:40
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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.

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    @xiota None of those will make a thing that is blurred appear sharper than a thing which is not blurry. – mattdm Feb 25 at 13:58
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    It is certainly possible for an image to appear sharp in the viewfinder when it is not — there is a limit to the detail you can see that way. And vision problems certainly could make it even harder to tell, but that's not the same thing. Can you image a print — like, a two-dimensional object — that shows more detail to someone with vision problems than to someone with corrected vision? It just doesn't work that way. – mattdm Feb 25 at 14:15
  • Just as I asked this I started to think about it and figured you don't really look "through" the lens but at an image on a focus screen. So in that case it is a matter of finding what looks sharpest, knowing that even if it doesn't look tack-sharp to your eyes it's still the most accurate focus? I'm wondering why viewfinders allow such a range of diopter adjustment then. Isn't there rather limited distance from your eye to what you're actually looking at (the focusing screen)? – G_H Feb 25 at 15:28
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    @G_H Yes, exactly. And, the lens assembly in the viewfinder makes it so the apparent viewing distance of the viewfinder screen is about a meter away. – mattdm Feb 25 at 15:31
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    @Alexandra That sounds more like the focus screen in the camera was slightly out of adjustment and a different optical distance from the lens than the film plane in your camera. That's a real thing that can be adjusted by a competent camera repair technician. – Michael C Feb 25 at 18:44
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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.

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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.

  • I've noticed that on a Canon EOS 33 (Elan 7) I've just acquired the diopter adjustment works great to make an auto-focused image appear tack sharp. Unfortunately my other EOS cameras don't feature such adjustment so I'm left to find other means. – G_H Feb 25 at 14:27
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    @ C_H -- The diopter a unit of lens power favored by opticians. 1d or 1 diopter = focal length 1000mm. 2d = 500mm whereas 4d = 250. This is the stuff of the bifocal add and/or reading eyeglasses. You can experiment at the reading glass counter of your favorite drugstore. You can dawn reading glasses for you photo activity or have your optometrist mount an add to your spectacles, its location will be about optical center. You can find temporary paste-on adds that can be used over and over, made of soft plastic, you can add to your spectacles. Perhaps they can be made to fit your viewfinder. – Alan Marcus Feb 25 at 16:28
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Out of focus eyes will still see the sharpest image when in focus

Other aids Mirrorless - the evf lets you see in the dark - the evf can do MAGNIFIED manual focus, focus peaking

Use a light to SEE subject then focus, turn light off take picture withsmbient light

Use zome focus guessing distance

using smaller aperture for more DOF

Focus bracket (digital) waste a few shots AROUND brst guess

Use faster lens Use more advanced autofocus (faster prism)

Best answer is mirrorless

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