Perfect Vision
We all call perfect vision 20/20 but in actuality 20/20 is only one aspect of vision called visual acuity. Visual acuity is a test of fine detail central, vision. Considering 20/20 to be perfect visual acuity is incorrect, actually it is average vision and many people can see 20/15, one step better than 20/20, some people can actually see 20/10.
Breaking visual acuity into pieces the numerator (top number) is a simply a reference of 20 feet, even if the reading is actually taken at a different distance we adjust it and always reference to 20 feet. The denominator (bottom number) is how well they actually see relative to a normal person.
For instance assume Johnny can see 20/30 and Jane sees 20/20. They both start walking towards a street sign from 50 feet away. Johnny can first make it out at 20 feet away. Jane stops at 30 feet and she can just make it out there. Said another way, Jane can stand 10 feet further away than Johnny and read the exact same thing. In this instance Jane sees better than Johnny.
But visual acuity is not a comprehensive measurement of vision. For instance which one of them can spot something in the periphery of their vision better? There is no way to tell, visual acuity only tests central, fine detail vision. Assume for a moment that Jane has glaucoma, and has lost significant portions of her side vision. She has a significantly reduced visual field. Now despite having the reduced visual acuity you may say that Johnny actually sees better than Jane. In actuality, it probably is not a fair comparison. The only true statement is in certain instance Jane sees better than Johnny and in other instances Johnny sees better than Jane.
Other aspects of vision beyond visual acuity and visual field include depth perception, color vision, night vision, tracking, and focusing. When your child goes to a pediatrician or a school screening they generaly only test visual acuity and the other aspects of vision overlooked
In other words: Don’t believe just because you child passed a school screening or read 20/20 at the pediatricians their vision is fine.
“Dodge”
William Dodge Perry, OD
Quick Links
All Eye
Care Services
Keep
In Touch
MAP
Powered by: 
