 John Connell |
Trifocal Eyeglasses and Camera Focus I wear trifocals and use a manual focus camera - Pentax Spotmatic. Which area of my trifocal glasses should I use as I focus the camera? Near? Mid? Far?
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doug Nelson |
I wear bifocals. I use the farther distance portion of my glasses, I guess since it's the far distant object I'm focusing on. Would be interesting to know if macro shooting requires that we use the near part of the glasses.
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John A. Lind |
Your viewfinder should show a center spot with a split image and a microprism ring around it. The rest of it should be a matte. Try the "mid" and "far" to see which gives you the sharpest, clearest microprism ring when the lens is slightly out of focus. The microprism ring should look something like sharp points or perhaps a very tight grid that disappears when the lens is focused on the object within its view. Your camera has a focusing screen under the prism. If you've ever seen the ground glass screen on the back of a large format view camera that's used for focusing it, the focusing screen under your SLR prism works exactly the same way. The image you see in the viewfinder is projected onto this screen by the mirror behind the lens. This is what your *eye* focuses on, not the subject material you have the camera aimed at. Because this screen is very close to your eye (about two inches or so), the camera's viewfinder optics adjust the effective focus point for it to about 4-6 feet in front of the camera. This varies some by camera manufacturer. If the viewfinder optics didn't do this, your eye wouldn't be able to focus on the screen, or anything else displayed in the viewfinder (metering, shutter speed, aperture, etc.). It doesn't matter how near or far your subject material is from the camera, or the focus distance set on the lens. Your eye always focuses at the same distance on the image that's projected onto the camera's focusing screen under the prism. -- John
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doug Nelson |
John C Mea culpa, pal. My answer was sloppy and inadequate. I suppose the far works better, but I never knew why. Thanks, John L.
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John Connell |
That was a great answer. Must be a camera manufacturer! In my case, the middle lens will be what I need. That is my 4-6 foot range. Thanks
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John A. Lind |
For John C: If you're buying dioptric correction lenses or special glasses, check out which works best for you first, the middle or far. I'm not an optician, but am guessing it could go either way depending on your camera's viewfinder design and the distance range for your "middle" prescription. For Doug: Getting proper correction lenses for them was an educational experience. I was faced with the same question: near or far? Initially guessed "near" for the first one and that didn't work. Wondered why, inquired with one of the camera manufacturers, and that's when I discovered how the viewfinder optics work. Exchanged it for the one closest to my "far" prescription and that worked fine. I had often been using the cameras without glasses. The corrected eyecups made a dramatic difference in focusing accuracy and speed, especially in lower light levels. -- John
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Emma Doma |
thank you John L for explaining the effective focusing point of the viewfinder image. I now understand why my bifocals set at eighteen inches close focus are quite useless and why my recently aquired prescription glasses set at four to six feet are giving me pinch sharp viewing that I was hardly aware that I had lost. I can now educate my optician! Has anyone had experience with wandering astigmatism or am I on my own with this one?
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John A. Lind |
Emma, No, you're not the only one with astigmatism (although I'm curious about the "wandering" part). Mine is very, very minor at the first increment of correction in one eye. It's so slight that it's not worth having the "prisms" added to the lens in my glasses. Unlike correction for myopia, hyperopia and presbyopia, astigmatism and the "prisms" added to correct for it have an orientation. This creates a problem for placing corrective lenses in nearly all camera viewfinders. Nearly all cameras, 35mm and APS in particular, have a rectangular film format. As soon as the camera is turned "vertical" any astigmatism correction in the viewfinder is now oriented 90 degrees from what it should be. The astigmatism for the user is now twice as bad as it would be without any correction. If you're lucky like me, you can "get by" at least reasonably well without correction. For those who cannot, they must wear glasses and cannot put vision correction into the viewfinder (unless they're using something like I describe in the next paragraph). The notable cameras that do not create this problem are medium format. 6x6 cm cameras have a square film format that eliminates the need to turn the camera. A couple 6x7 cm medium format cameras have rotating film backs; the back is rotated, not the entire camera. -- John
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