Re: Retinal image size calculation for human eye.

From: Johannes Swartling (johannes.swartling_at_home.se)
Date: 06/21/04 

Date: Mon, 21 Jun 2004 10:04:06 +0200

"Bruce Woodburn" <brucew@dccnet.com> wrote in message
news:10dcr82nec9h888@corp.supernews.com...
> I don't know if this is going to help...
>
> Imagine you start with a normal eye that is in focus at distance. To
convert
> it to a myopic eye, you put an imaginary plus sphere lens inside the eye
at
> the plane of the natural lens, about 5.5 millimeters behind the cornea. To
> compensate, a minus spectacle lens is placed 14mm in front of the cornea.
> You now have a reversed Galilean Telescope (paired plus and minus lenses)
> which makes the image smaller. The opposite situation is true for
hyperopes,
> who see a magnified image (about 2.5% for each diopter)
>
> When you look through a minus lens that is "too strong", your natural lens
> accommodates by becoming more plus. This increases the strength of the
> reversed Galilean Telescope and reduces the image size. It's not so much
the
> minus lens that is making the image smaller as the accommodation of your
> lens. If accommodation is paralyzed with eye drops, the image would go
> blurry, but remain closer to the same size.
>
> I think the "2.5% per diopter" rule is pretty close for both minus and
plus
> lenses.
>
> Bruce
>
> PS: A Galilean Telescope is a 2 lens telescope (plus objective, minus
> eyepiece) arrange so their focal points coincide. Magnification (or
> minification) is the ratio of their strength in diopters (e.g.: a +1
diopter
> objective and a -4 diopter eyepiece gives a 4 power scope.) This power
> calculation doesn't apply to the armchair experiment described here
because
> of the complexity of the optical system.

I'm guessing here, but this is how I thought it was: myopia is caused by the
eye being longer than it should, and thus the retina is too far back from
the "optics". The image that falls on the retina is blurred but also
enlarged due to being projected behind the image plane. When glasses are
used for correction the correcting lens is placed in the front focal plane
of the eye (about 14 mm from the cornea), and the image will shrink back to
the size it would be for the non-myopic eye. This means that in a sense you
can say that correction doesn't change the image size, it's just the same as
it would be for the non-myopic eye.

Johannes

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