Re: Change in lens power with distance
- From: Salmon Egg <SalmonEgg@xxxxxxxxxxxxx>
- Date: Wed, 20 Feb 2008 19:43:19 GMT
In article <34mnr3ddvg9ir7jgidr1g3tnqpds570bgc@xxxxxxx>,
John O'Flaherty <quiasmox@xxxxxxxxx> wrote:
I wonder if someone could help me understand something about glasses.
I am 62, myopic, and don't think I have any accommodation to speak of.
My prescription is about -2,-2.25, averaging in the astigmatism.
When I got my eyes checked, I mentioned that I wanted a special pair
of glasses to watch TV, at a distance of 11 ft. (3.35 m), because I
found my old glasses blurry at that distance. I was told that the
distance of the eye chart was 11 feet, and that was taken as
equivalent to infinity. My new glasses seem ok at 11 ft. for watching
TV, but not as good outdoors at infinity.
Is it true that going from infinity to 3.5 m, to maintain perfect
focus, the negative power of the glasses would have to be reduced by
1/3.5 m, or about 0.3 diopters? If true, wouldn't this be a
significant difference in sharpness?
The short answer is that your calculation is correct. It should not
depend upon your astigmatism. I am not a vision professional, but I do
understand optics.
I believe that lens power ordinarily is available in 0.25D steps. There
may be finer steps available.
Bill
.
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