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# Eyeglass question

8 replies to this topic

### #1 REC

REC

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Posted 25 January 2021 - 02:29 PM

I had my annual eye exam the other day and when he was checking what lines I could see, most where in the 20/20 range with my current prescription. When he was doing that better or worse flipping, occasionally I could read part of the line under the 20 mark that said 15. Question, is it any better to have glasses that can read 20/15 for observing the sky and stars ect? Would they be deeper and sharper. I do not use glasses when I'm observing through the scope?

Thanks and Happy New Year all!

### #2 chrysalis

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Posted 25 January 2021 - 02:45 PM

There would be some marginal improvement 20/15 versus 20/20 in regards to being able to perceive the very faintest stars.

More meaningful info here - I'll leave the trigonometry to you if you would like to know what height of a letter subtends 5' arc at 20 feet:

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### #3 chrysalis

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Posted 25 January 2021 - 03:34 PM

OK well so here you go:

20 feet =  6096 mm

5' = 0.08333°

Assume your eye is level with center of the letter. So a right triangle can be formed above and below that line, each subtending 2.5' of arc (0.041666°).

tan (0.041666°) = 0.00072722

So 0.00072722 = (x, the height of 1/2 of the letter)/6096. This works out to 4.433 mm. That's 1/2 the height of the letter.

So when you have 20/20 vision, from 20 feet you can clearly see a letter that is 8.87 mm tall.

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### #4 chrysalis

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Posted 25 January 2021 - 03:42 PM

OK well so here you go:

20 feet =  6096 mm

5' = 0.08333°

Assume your eye is level with center of the letter. So a right triangle can be formed above and below that line, each subtending 2.5' of arc (0.041666°).

tan (0.041666°) = 0.00072722

So 0.00072722 = (x, the height of 1/2 of the letter)/6096. This works out to 4.433 mm. That's 1/2 the height of the letter.

So when you have 20/20 vision, from 20 feet you can clearly see a letter that is 8.87 mm tall.

Actually I lucked out, I should have used SINE (opposite/adjacent, where adjacent = 6096 mm).

But the sine of 0.041666° is 0.00072721, so only a tiny bit less; but within the error of the calculation above; and so the result remains 8.87 mm !

Edited by chrysalis, 25 January 2021 - 03:43 PM.

### #5 KBHornblower

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Posted 25 January 2021 - 07:21 PM

If your eyes are capable of resolving the 20/15 line when properly corrected, a good optometrist will be able to find the prescription that will get you there, because you will be able to see the difference in finer flips than can someone who can do no better than 20/20 regardless of the prescription.

The Navy flight surgeons evaluated baseball Hall of Famer Ted Williams at 20/10.  No wonder opposing pitchers thought he could count the stitches on the ball as it approached the plate.

### #6 Jon Isaacs

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Posted 25 January 2021 - 07:39 PM

I had my annual eye exam the other day and when he was checking what lines I could see, most where in the 20/20 range with my current prescription. When he was doing that better or worse flipping, occasionally I could read part of the line under the 20 mark that said 15. Question, is it any better to have glasses that can read 20/15 for observing the sky and stars ect? Would they be deeper and sharper. I do not use glasses when I'm observing through the scope?

Thanks and Happy New Year all!

The question is:  Do you have astigmatism?  When you focus the telescope, that takes care of any nearsightedness/farsightedness you might have.  If you have a significant amount of astigmatism, then wearing glasses can help.

Astigmatism is generally an issue at large exit pupils, resolution is greatest at small exit pupils.

Do you have your prescription?  You should have spherical and cylinder (in some abbreviation) for both eyes. The cylinder is the astigmatism.  This is the TeleVue page on the Dioptrix.  It fits some eyepieces and corrects for astigmatism.  It has some good information about exit pupil versus astigmatism.

https://www.televue....=54&Tab=_Choose

Jon

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### #7 sevenofnine

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Posted 25 January 2021 - 08:05 PM

I've worn glasses/contacts most of my life. I'm legally blind without them. If you see 20/20 without corrective lenses...fantastic for you! And sometimes 20/15? Don't bother getting corrective lenses. They add a layer of bother that you don't want. Trust me. Be thankful you are so lucky

### #8 Tony Flanders

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Posted 26 January 2021 - 06:35 AM

I've worn glasses/contacts most of my life. I'm legally blind without them. If you see 20/20 without corrective lenses...fantastic for you! And sometimes 20/15? Don't bother getting corrective lenses. They add a layer of bother that you don't want. Trust me. Be thankful you are so lucky

I think that for somebody serious about "naked-eye" observing, it might indeed make a lot of sense to get eyeglasses to improve their vision from 20/20 to 20/15. It's startling just how many more faint stars you can see when your night vision is properly corrected.

Note that due to the phenomenon of "night myopia," the optimal prescription for low-light conditions is typically different from the optimal prescription for normal day vision.

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### #9 REC

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Posted 27 January 2021 - 01:24 PM

Thanks for all your reply's. My daily glasses are corrected for astigmatism and are progressive bifocal. I do have contacts that are set for infiity and astigmatism. Just have to wear reading glasses for close up. But it's nice to see the stars with and then go directly to the scope.

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