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Eye Refraction

Discussion in 'Uncategorized Threads' started by DocJim, Mar 7, 2007.

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  1. DocJim

    DocJim Member

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    Going in for an eye exam and refraction next week. I have a known astigmatism in my right eye (I shoot right-handed) for many years. Is it satisfactory to just be corrected to 20/20 or should I ask for something "special" from the doctor for shooting. Some glasses I have had in the past, although they make the object sharper, seem to make it appear smaller. Any thoughts from you eye professionals out there greatly appreciated.
    Jim G
     
  2. phirel

    phirel TS Member

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    About a year and a half ago I had a pair of glasses that were over corrected. They seemed to be very good for a short time but then funny things started to happen and I did not recognize that something had changed quickly enough to get rid of the glasses. I can not accurately explain what happened, but is seems almost as if the over corrected glasses causes a change in my focusing system that countered the over correction. I realize this is a very poor description but it is the best I can do.

    Pat Ireland
     
  3. Tailbuster

    Tailbuster TS Member

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    Hi DocJim,

    The correct answer is that there is 1 maximally corrective lens that puts the focal point on your retina properly. One step stronger or weaker is not as good. Since target aquisition is greater than 20 feet (optical infinity) you want to be corrected to as good as you can be at distance, but no more. If you are able to see 20/15 or better with a corrective lens, let your doctor know that you want the distance as good as it can be. Your ability to see better than 20/20 is limited by your visual system (some people can, some can't). Some doc's or tech's that refract have a tendency to stop at 20/20. I personally don't, but some do.

    all the best,
    john
     
  4. Tailbuster

    Tailbuster TS Member

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    Jim,

    I should have added that if an increase in the power of the lens makes an object appear smaller it is likely a bit too strong. A proper power will always clear the image and make it appear bigger. When you are doing the exam always look to the smallest acuity line that you can see. That helps the most for you to detect changes. Get that line as clear as it will become. But, if the next step of power pushes the image away or makes it smaller, that is too much.

    all the best,
    john
     
  5. phirel

    phirel TS Member

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    I appreciate your post and I wish you were convenient for me to drop by for an exam.

    Pat Ireland
     
  6. DocJim

    DocJim Member

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    I greatly appreciate your post as well. We have lots of good eye professionals in my area but unfortunatly none are shooters. Thanks again!
    Jim G
     
  7. Neil Winston

    Neil Winston Well-Known Member

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    John, a somewhat different type of question.

    As you say, the eye is shaped the same when it's focused at anything from 20 feet to infinity. It's also a very short focal-length lens, with plenty of depth of field and depth of focus.

    So why are we told to use the lightest lenses possible "to increase depth of field, like stopping down a camera?" Once you close a camera down more than 2 or 3 stops, resolution gets worse, and things that are already in focus don't get any more in focus, only the closeness of in-focus objects increases, and we aren't using that, are we?

    The case for pistol shooting is far different, where a small aperture is such a big help, but here the stretch in depth of field really counts, and can be made, with a small enough hole, to stretch from a yard to almost to the target.

    I wonder what your comments are.

    Neil
     
  8. Tailbuster

    Tailbuster TS Member

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    Neil,

    I'm not qualified to even comment on the camera stuff because I don't really even know what stops are for sure. Regrettably, I have probably forgotten much of the optics that I once knew but rarely use on a daily basis. Now if a stop makes the aperture smaller then probably what is occurring is there is a sacrifice in the amount of light getting in with each step down, so that at some point the depth of field and focus aren't as important as the amount of light.

    Now using a small aperture or hole to look through, I understand. When you use a pinhole to look through, it clears the image because it reduces the size of the blur. You do sacrifice brightness this way. Pick up a piece of paper put a tiny hole in it, without your glasses on and with one eye, look at something. It will appear clearer. Not much value unless you break your glasses in the woods. You can put a pinhole in a leaf to see where you are and get out. That effect is used in peep sites etc. Don't know if this helped but it's all I've got.

    all the best,
    john
     
  9. phirel

    phirel TS Member

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    Lighter lenses will result in a reduction of the pupil size (the pupil is not a structure but simply a hole). This will decrease light rays coming into the lens at angles that cannot be bent enough to be focused on the fovea of the retina. Light that comes into the eye but not focused on the fovea cannot be seen clearly. We have a wide field of vision but most of the field is not seen clearly. With a smaller pupil, some of the light rays that hit the eye at sharp angles will be blocked resulting in a smaller, but sharper field of vision. When shooting, we only need a very small field of vision.

    However, I prefer lenses that are comfortable and for me this means a little darker. The theoretical advantage of a smaller pupil is not even close to the advantage of focusing your mind on only the target.

    Pat Ireland
     
  10. Texshooter

    Texshooter Member

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    Telling him you want the focus at infinity helps, otherwise they will set the focus just short of infinity. The new glass material is helpful. Trivex is stronger, clearer, and lighter than polycarbonate. Most importantly for sports applications, Trivex minimizes distortion called "chromatic aberration", which is distortion that occurs when objects are viewed away from the optical center. I am unusually sensitive to the chromatic aberration. AJ
     
  11. Neil Winston

    Neil Winston Well-Known Member

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    I wonder about that explanation, Pat. In your version, the closing-down of the pupil will cut off the light rays landing outside the fovea.

    Why, I have to ask you then do you to stop-down a camera to increase the (film-side) coverage of a lens with a view camera when using lens offsets? In other words, the image circle gets bigger, not smaller.

    Neil
     
  12. Tailbuster

    Tailbuster TS Member

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    Think of the pupil as a light regulator. When it's smaller, less light comes in minimizing glare and preventing the retinal pigments from being bleached out (think about what you see after you catch a camera flash- your retina pigments were bleached and needed time to get back to normal). In the dark when it is large more light is let in to stimulate the receptors in the retina. A tiny pupil will increase depth of field and focus but at the expense of the amount of light entering the eye.

    The fovea is the only area of the retina that is responsible for fine precise vision. It is tiny and centered behind the center of the pupil. The rest of the retina is peripheral and good only to pick up movement etc in the periphery. Bad for detail. When you follow a target you follow with the fovea. You pick it up with the peripheral retina but follow with the fovea.

    Tim,

    Phil Kiner, Pat, and Rollin have a lot of knowledge about using the tapes etc and I will defer to them. Dominance is a complex issue in shooting with more than one thing occurring. A week or so back Phil laid out a nice piece on the subject. Do a search.

    all the best,
    john
     
  13. idoc

    idoc Member

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    Hi John, It's Rich. Nice job. I wrote an article with Bender on the light transmission stuff. If anyone is interested they can e-mail me and I'll send it along or if someone can tell me how to do it I'll post it here. Drop the xxx an the end.

    Texshooter- You are right on about the new Tivex material. I think it's the future in shooting lenses..........................Rich
     
  14. idoc

    idoc Member

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    Pat what the lighter lens does is shifts the vision more toward cone function. Cone stimulation (Photopsia) is much better for pursuit and saccadic movements. Pursuit would be following a bird. Saccadic would be moving from one bird to the other in doubles. The less light you let in the more you shift toward Rods (scotopic). Scotopic is lousy for the pursuit and saccadic moves. Also lousy for depth perception. Have you ever tried to dock a boat in the dark? You can't tell how far away the doc is. That's why the lighter the better. Under the environments were talking about pupil size is not that big a factor as long as the patient is corrected for any significant optical error at infinity. Hope this helps....................Rich
     
  15. Tailbuster

    Tailbuster TS Member

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    Hi Rich,

    Nice to hear from you. Did you get a skeet lesson from Bender while you were writing or did you write the article via correspondence? Yes, I'm more than a little envious.

    all the best,
    john
     
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