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    Re: Correcting Night Vision
    From: Frank Reed CT
    Date: 2005 Aug 11, 21:53 EDT

    Jared you wrote:
    "I'm not sure that it is a  "focus" issue per se."
    It's a failure to focus due to spherical  aberration at larger pupil
    aperture, so it qualifies as a "higher order" focus  issue. The point of the article's
    discussion of night eyeglasses is that a  prescription can take better
    account of whatever spherical aberration exists in  the eye and yield sharper star
    images and also a fainter limiting magnitude. I  highly recommend the article:
    pages 28-42 of the September, 2005 issue of Sky  & Telescope. Barnes & Noble
    usually has it available if your local  library does not.
    "I have lasik damage, and during the day when  my pupils are contracted I see
    better than I do at night."
    Most people  see better during the day. Even Maskelyne back in 1789! Lasik
    damage, of course,  is different from the usual spherical aberration. Spherical
    aberration yields  worse focus but not the halos, glare, etc. that people with
    eye damage see (due  to lasik or cataracts other causes).
    You also wrote:
    "Distortions in  the cornea are actually common, and new procedures for
    corneal topography  actually map the front and rear of the cornea separately to
    accomodate for them.  Some of the ray tracing gizmos actually "project" a dot
    into your eye, and you  use a joystick to mark a spot where you see it. That
    position is then compared  to a mathematical position where a perfect eye SHOULD
    have seen the dot. And  almost everyone has irregularities within the eye that
    show up during this kind  of mapping."
    There are lots of details on this in the S&T article.  Well worth reading.
    While I'm here, there's a typo in my original message  in this thread. I
    wrote: "(20/20 is 1.0 arcminute resolution by definition,  10/10 is 0.5 arcminute
    resolution --which is right at the theoretical limit for  ordinary
    resolution)." Of course, that should say '20/10 is 0.5 arcminute  resolution', not
    42.0N 87.7W, or 41.4N  72.1W.

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