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Old December 24th 18, 02:29 PM posted to sci.astro.amateur
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Default Let's Photograph Comet 46P Wirtanen

On Monday, December 24, 2018 at 8:09:24 AM UTC-5, Paul Schlyter wrote:
On Sun, 23 Dec 2018 18:38:19 -0600, hleopold
wrote:
On Dec 23, 2018, Paul Schlyter wrote
(in et):
On Sat, 22 Dec 2018 20:09:17 -0500, wrote:
My corrected vision in my left eye is 20/25, i.e., very good.


That's the same as 0.8 right? And full visual acuity would then be
25/25? Or 100/100 etc if you prefer that. Or just 1.0


20/20 is, as I understand it, perfect vision. Mine is 20/400 in

the
right eye uncorrected, corrected to 20/15. left is somewhat worse,

but
corrected to 20/15 also. Or it used to be, these days I am probably

more
20/20 or slightly worse. Plus they have to correct for the bad

astigmatism I
have in both eyes.


If the numerator is always 20, what does this 20 mean? Is 20/20 in
some way different from, say, 25/25 or.30/30?


The '20' in the numerator means a 20 foot distance. If you can read the 20/20 line on the chart from 20 feet then your vision is average/normal.

If you can read the 20/15 line then your acuity is better than average; others would have to be 15 feet away to read it.

If you can read the 20/100 line, and nothing smaller, then your acuity is worse than average; the average person could read that line at 100 feet.