Mathematical relationship of retinal image size to amount of corrected nearsightedness

Discussion in 'Optometry Archives' started by andrew Judd, Jun 17, 2004.

  1. andrew Judd

    andrew Judd Guest


    Can somebody help out with a calculation to show the following?

    For your average myope with a pair of glasses sitting an average
    distance from their eyes, what is the exact relationship between
    retinal image size and amount of minus?

    Please take into account the small? amount of image size change that
    is due to myopic elongation (I believe its about 1mm of elongation for
    each 3 diopters)

    So having achieved this calculation, what is then the amount of minus
    required to be present to reduce retinal image size by 100%, so that
    20/40 is read rather than 20/20?


    andrew Judd, Jun 17, 2004
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  2. andrew Judd

    andrew Judd Guest

    After looking thru the following rather comprehensive site it would
    seem that image size in the eye itself for different amounts of myopia
    is ***not*** majorly different ***if** the same part of the eyes focal
    power is altered.

    For example the total focal power of 70D of the - 10D eye creates an
    image which is only 15% different to the 60D normal eye.

    But I know that the effect of strong minus glasses on the perception
    of size is quite dramatic.

    For example if you put on a pair of -6D glasses and get used to
    looking at the person in front of you for about one minute and then
    take off the glasses the increase in size is actually scary! Suddenly
    it seems the person is huge!

    So it seems to me to get size differences due to internal optics the
    cornea would need to be of a significantly different power to the lens
    for each eye.

    Because my question is about a 2 lens calculation maybe i need to look
    at telescope design to get my answer? I guess a
    forum might be where to find out what i want to know?

    Meanwhile i found this stuff interesting and helpful but ultimately
    over my head.

    Image size calculator

    eye schematic

    Image formation concepts
    andrew Judd, Jun 18, 2004
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  3. andrew Judd

    andrew Judd Guest


    I measured the distance between my spectacle lenses to the nearest
    part of my eye as 14mm using a cotton bud of this length held by
    tweezers - so this is accurate. But distance does not seem to majorly
    alter the image size because moving my glasses a small amount further
    away has little noticable effect.

    Hope that helps!

    andrew Judd, Jun 18, 2004
  4. andrew Judd

    Dan Abel Guest

    We see with our brain, not our eyes. Our eyes cannot tell how big things
    are, except in relation to other things. Thus, if *everything* changes
    size, the brain readjusts itself and sees that *nothing* has changed
    size. I used to be very myopic, and I couldn't tell any difference in
    size with my glasses on or off. Of course, a lot of that was because with
    my glasses off everything was so blurry that things *had* no size.
    Dan Abel, Jun 18, 2004
  5. andrew Judd

    Jan Guest


    I know, optometrist do not like to compare the eye with a photographic
    camera but when it comes to optical matters it is not off topic.
    Barrel or pincushion distortions are caused by the following (goes for the
    Otis fixed focused box too):

    Pluslens with aperture behind the lens = pincushion distortion
    Minuslens with aperture behind the lens = barrel distortion.

    The more the aperture (read pupil) and the principal plane of the correcting
    (contact) lens are at the same place the lesser or even no barrel or
    pincushion distortion even if the correcting (contact) lens has a strong

    This goes for every optical system.

    Another thing is how humans are capable to "correct" this kind of distortion
    and here humans differs from photocameras and Otis.

    Jan (normally Dutch spoken)
    Jan, Jun 19, 2004
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