Can imperfect eyesight cause manual focus errors in an SLR viewfinder?
Asked 2/25/2019
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If my eyesight or glasses/contacts correction isn't perfect, can I still judge manual focus accurately through an SLR viewfinder? I'm mainly using film SLRs in low light, where autofocus is unreliable. Most of these cameras don't have split-prism screens, and only one has diopter adjustment.
What I'm trying to understand is whether imperfect vision simply makes the viewfinder image look generally blurrier, or whether it could actually make an out-of-focus image appear sharp and cause me to focus at the wrong distance. Are there practical ways to compensate besides diopter adjustment, such as focusing technique, focus screens, or stopping down?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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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.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
7y ago
0
Generated from our catalog & community — verify before relying on it.
Imperfect eyesight usually makes manual focusing harder, not misleading. In an SLR, you are viewing the image formed on the focusing screen, which is a fixed plane. If your vision correction is a bit off, that screen may look less clear overall, but your eyes won’t magically make a truly out-of-focus image appear correctly focused.
So in normal circumstances, there is still a “sharpest” point as you turn the focus ring. With mild vision issues, you can focus by rocking past the best point, then reversing until the image is sharpest again. If needed, bracket focus slightly.
If your eyesight problem is severe enough, though, you may not be able to judge focus reliably without better correction.
Also note: if the viewfinder focus and the film plane don’t agree, that’s a camera calibration problem, not an eyesight problem. A technician may need to adjust the focusing screen or mirror alignment.
Helpful options: use diopter correction if available, wear the correction that works best for distance viewing, use a split-prism or other focusing aid if possible, and stop down a bit to gain depth of field when focus is critical.
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AI7y ago
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