Diopter correction v Reading glass stength

Discussion in 'Casual Photo Conversations' started by Ludmilla, Sep 20, 2021.

  1. SCL

    SCL

    Basically same as regular glasses. However, you need to be sure that the portion of the glasses you are looking thru is a constant, otherwise you'll be adjusting the variable diopter each time you use the camera. I've been wearing progressives for over 20 years and it took a little decision making and adjustment to get things in proper alignment.
     
  2. Simpler than that. It's the reciprocal of a lens' focal length in metres. E.g. A positive lens of 333.33mm focal length has a dioptre power of +3, while a negative lens of 250mm focal length has a dioptre power of - 4.

    When used as an opthalmic prescription it just describes the corrective lens focal-length and polarity needed to give a person 'normal' vision.

    The advantage of using dioptric strengths is that dioptres can simply be added to give the resulting reciprocal focal length. Rather than using the cumbersome formula: 1/F1 + 1/F2 = 1/F(total).
     
    Last edited: Sep 23, 2021
  3. I think I've learned to ignore being off when looking through my progressive eyeglasses. In other words, the image can be out of focus that I see but be in focus for the film or sensor plane. But because I'm adjusting the focus by going in and out of it, I just stop when it looks best even though it is still out of focus to my eye.
     
  4. That's what makes adjusting exposure easier by using "stops" among Iso, shutter and aperture rather than cumbersome formulas.
     
  5. Oh Man, and just when i thought i was starting to understand what you were saying. :)
    I realize this is a great post, but it has gone over my head.

    My vision is "Good".
    My only problem is probably familiar to a lot of people as they age. I need "Reading Glasses".
    The cheap ones from "The Dollar Store" help a lot.

    I see pretty well in low light, pretty good at a distance, but trying to read the label on a cereal box at the grocery store is almost impossible.
    My cameras are all 1970-1980 SLR.
    Do i need a Minus or a Plus diopter.?
    Thank You
     
  6. Plus, I think.
     
  7. No offense Steve......................but i need somebody That Knows.! :)
     
  8. Plus corrects poor near vision, minus is to correct for poor distance vision.
     
  9. Oops, guess I read Ed's post backwards, sorry. I have to admit, I just twiddle it until it looks good, only one of my cameras is actually marked +/-
     
  10. Absolutely.!
    But i am dealing with circa 1975 cameras. I have to buy and screw in a single diopter.
    I don't have any of these, Slick, Exotic, Options that you kids of today have. :)
     
  11. My 1955 Zorki 3m has dioptre correction ;)

    I've re-read the thread and rechecked my own camera and I still think you need a plus correction for far sightedness (I'm near sighted), but best wait untill a few more people have replied! Sorry to not be overly helpful...
     
  12. Again, plus is for close up, minus for distance.

    To complicate matters, i need minus glasses for distance viewing, am good without glasses at distances around 1 - 2 meters, and could use plus glasses for reading.
    I used to see well extremely close without glasses. Goes with being nearsighted; extra extension in the eyeballs. With age, nearsightedness improves a bit. But also with age, the lens in our eyes becomes less elastic, so close focus becomes an issue.
    So even when nearsighted, needing minus glasses, you may need plus correction in your camera eyepiece.

    So try.
     
    Last edited: Sep 25, 2021
  13. Mine too. Rodeo Joe explained it clearly. Diopter is another way of expressing focal length so that the strength of lenses is additive. Diopter is the reciprocal of the focal length in meters. A focal length of 1/3rd meter is 3.0 diopters.

    I'm not sure how negative diopters are measured except as a compliment of a positive lens, adding to zero (infinite focal length), like a flat piece of glass).
     
  14. Negative lenses have a focal length you can measure directly.
     
  15. But not easily. Not like imaging the sun with a positive lens.

    Easiest way is probably to find the positive dioptric value that cancels out the negative. Or use a strong positive lens and find out how much its focal length is extended by the addition of the unknown negative element. Either way, just the sun and a yardstick aren't enough.
     

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