Re: Glasses for Computer use.

On 3/12/07 6:47 PM, in article
1173750439.154602.172640@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, "grahamew@xxxxxxxxx"
<grahamew@xxxxxxxxx> wrote:

I still have difficulty understanding you.

I think you are saying that your reference is that 1.5D reading glasses work
well at 14inches = 0.356m. You then ask, "What is needed to work well at
0.61m?"

If you had a perfect eye, in the sense that it sees objects at infinity
clearly without any accommodation from your eye, you would need a 1/0.356m =
2.812D lens. Because you are using a 1.25D lens to get that effect, your eye
already supplies the difference of 2.812-1.25 = 1.562D.

To see clearly at 0.61m you need 1/0.61 = 1.639D if your eye were perfect.
Because your eye is not perfect and supplies 1.562D, the lens has to supply
only 1.639-1.562 = 0.077D. That is almost no lens at all.

I would conclude that you need no lens at all to see a screen well at 0.61m.
Alternatively, you have not given an accurate description of what is needed.

Bill

-- Fermez le Bush--about two years to go.


I am saying that 1.25D works for me at the usual reading distance,
which I have assumed to be about 14", or .355m. I want another pair
that works at 24", or .61m.

I don't get your calculations. I do know that I get blurry vision and
headaches while reading the screen at 24"/.61m. So, if 1.25D works for
me at .355m, what will work for me at .61m? That is my question.

I think your last paragraph quoted above says that you want the same thing I
thought you wanted as in my second paragraph quoted above.

I do not think that the presumption that your 1.25D provides clear vision at
0.355m can be justified without actual testing. Certainly 1.5D at 0.355m
would only work if your eye's accommodation provides the additional 1.562D I
calculated above. (The high precision in my calculation is there just to
prevent accumulation of roundoff error. Do not take it too seriously.)

I was suggesting that you do not need glasses to view your screen at 0.61m.
1.25 glasses will only blur your screen.

Bill
-- Fermez le Bush--about two years to go.


.

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