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Eye Care Archive (Expert Forum)
Accuracy of Snellen chart with poor vision
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Accuracy of Snellen chart with poor vision
by MeganW, May 20, 2007 12:00AM
My vision has been pretty poor since I was about 8 (I'm 22 now) and has been progressively worsening over the years. I have an astigmatism and I am near-sighted. I wear either contacts or glasses, with a prescription of 5 and 4.5 (I can't remember if it's + or -).
At my recent eye exam, the optometrist estimated my vision at around 20/1000. She couldn't give me a definitive number, as apparently the Snellen chart does not accurately diagnose visual function at such low acuity. My question is, is there any way to be accurately tested and given a visual acuity score?
At my recent eye exam, the optometrist estimated my vision at around 20/1000. She couldn't give me a definitive number, as apparently the Snellen chart does not accurately diagnose visual function at such low acuity. My question is, is there any way to be accurately tested and given a visual acuity score?
Doctor's Answer
by Forum-OD-MP, May 21, 2007 12:00AM
yes, there is. most eye doctors dont do it b/c it doesnt mean all that much. we usually consider anything over about 20/400 to be not-very-functional vision, so we usually dont bother measuring much worse than that.
but any (and every) acuity can be accurately measured.
the fastest way to do it is by using finger counting:
20/800 CF10' (Count Fingers at ten feet)
20/1000 CF 8'
20/1143 CF 7'
20/1333 CF 6'
20/1600 CF 5'
20/2000 CF 4'
20/2666 CF 3'
20/4000 CF 2'
20/8000 CF 1'
http://www.mdsupport.org/library/acuity.html
there are other ways to do it, but they involve some serious math:
http://www.ascrs.org/publications/jcrs/guestfeb04.html
the question is, why do you need to know? if its a curiosity thing, you can certainly satisfy it using the above methods, but most things that would would need good acuity for (military, etc) require much better vision than 20/400.
but any (and every) acuity can be accurately measured.
the fastest way to do it is by using finger counting:
20/800 CF10' (Count Fingers at ten feet)
20/1000 CF 8'
20/1143 CF 7'
20/1333 CF 6'
20/1600 CF 5'
20/2000 CF 4'
20/2666 CF 3'
20/4000 CF 2'
20/8000 CF 1'
http://www.mdsupport.org/library/acuity.html
there are other ways to do it, but they involve some serious math:
http://www.ascrs.org/publications/jcrs/guestfeb04.html
the question is, why do you need to know? if its a curiosity thing, you can certainly satisfy it using the above methods, but most things that would would need good acuity for (military, etc) require much better vision than 20/400.
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in children really the only way i know of to fairly accurately test an infant or toddler's visual acuity is with a rotating optokinetic nystagmus drum. pretty much only a pediatric optometrist or ophthalmologist will have a set of them, and eye/vision research institutions. i know my optometry school had them...SUNY Optometry in Manhattan, NY
you may already know this but a rotating optokinetic nystagmus drum is a cylinder that turns. the cylinder has vertical black & white "stripes" on it (a "grating") of varying sizes. larger/fatter "stripes" are for lower acuity like 20/400, 20/2000, etc. smaller/skinnier "stripes" are for better acuity like 20/40, etc.
the drum is placed in front of the child and rotated. if the child has good enough acuity to see the size of the grating/"stripes" then the child's eyes will dart back & forth, following the grates. this is called "optokinetic nystagmus" or OKN. the idea is keep getting smaller in size until you find the smallest grating size that produces an OKN response and thats the infant's acuity.
you said you are nearsighted (thats negative or minus) -5.00 in your worst eye. thats an acuity of approximately 20/500 or so
Thank you for your time.