Are retinoscopy and visual acuity related?

December 18, 2005 1:30 PM

Is there a relationship between visual acuity (ie. the 20/20 or 20/xx scale) and retinoscopy results (the -6.50 +1.00 that is the prescription for your glasses/contact lenses)?
posted by ajpresto to Science & Nature (9 answers total)

I would say there's a link, but I'm not sure there's a simple correlation.

Visual acuity (20/X) is the ability of your eyes to resolve objects, while the prescription is based on what focal length (in imprecise terms: strength) of lens is required to correct for the imperfect focus of your eye. In that sense, there's a correlation; if your eyes are more out of focus, your visual acuity will be decreased.

The correlation is not straightforward because the lenses in your eyes are not a single focal length. As I understand it, muscles can change the shape somewhat to try and improve the focus. Therefore, your eyes may be able to improve the focus somewhat under certain conditions.

For example, I am (very slightly) far-sighted. Acording to my eye doctor, my prescription (if I ever get around to buying reading glasses) would be +1.25 (I think) for a complete correction. I am able to read small print just fine, but I get a headache if I do it for a while because the muscles are working to maintain that focus.

In other words, I would say visual acuity is linked to prescription plus the ability of your eyes to adjust their focus.
posted by JMOZ at 3:25 PM on December 18, 2005

They are related, but visual acuity is not purely a function of your lens prescription. The visual acuity numbers just represent your ability to discern static images at a distance. Failure to resolve those images could be due to abnormal lens curvature (and thus would be related to the prescription), but could also due to problems with the cornea, retina, ocular fluid, neurological disorders, etc.
posted by Galvatron at 3:30 PM on December 18, 2005

Response by poster: Bah. I was hoping to take a -6.50 and turn it into a 20/400 or whatever it would be. Oh well. I guess I need to call the doctor. Thanks!
posted by ajpresto at 3:30 PM on December 18, 2005

For the record, 20/200 is apparently legally blind. (At least in Massachusetts where my wife's friend lives.) I'm not sure if that's useful information in any way.
posted by JMOZ at 3:41 PM on December 18, 2005

Best answer: If myopia is the only variable (no astigmatism or other problems), you can approximate it. Chart here.
posted by needs more cowbell at 4:01 PM on December 18, 2005

Response by poster: First, I *love* the name. Everything needs more cowbell. Second, that is exactly what I needed. Thanks!
posted by ajpresto at 4:38 PM on December 18, 2005

Response by poster: I obviously mis-Googled... "diopters" is what I should have been searching for. Here's an even better chart: http://com5.runboard.com/bwelcometothevisionandeyeforums.fgeneralvisiondiscussion.t2
posted by ajpresto at 4:54 PM on December 18, 2005

Corrected vision of 20/200 is legally blind. That is, if you're 20/200 without glasses, but can see fine with, you're fine. If you're 20/200 even with glasses, yeah, that's blind.
posted by MrMoonPie at 6:59 PM on December 18, 2005

Thank you cowbell. I didn't even know that I needed that chart, but I do.
posted by smackfu at 7:32 PM on December 18, 2005