Re: Is there a way to determine how many 'degrees' 20/70 vision translates to?

Hi Sharon,

I apologize for taking so long to answer this question as I had lost the email that was sent to me from the administrator. To answer your question, 20/70 vision means you can determine resolution of an object at 20 ft that someone with "perfect" or 20/20 vision can resolve at 70 feet. It is strictly a test to determine visual acuity (sharpness of center vision ), as opposed to other measurements of vision like contrast sensitivity, color, or peripheral vision.

Now, to measure vision in an optometrist's office, we use size of objects (letters usually) that is calibrated by how many degrees (or minutes of arc, which is 1/60 of a degree) they subtend at 20 ft. For example, think of 360 degrees being a pie and so 1 degree of a pie is a very thin slice when you are 20 feet from it. Imagine how thin 1/60 of that slice is. 1/60 of a degree or 1 minute of arc subtended at 20 ft is considered to be 20/20 visual resolution. 1 minute of arc is how much separation there is between the gaps and turns in each 20/20 size letter on the chart. A 20/40 letter subtends 2 minutes of arc, a 20/80 letter subtends 4 minutes of arc, and so forth and so on. Someone who has really poor vision at 20/800 has visual acuity that can only resolve 40 minutes of arc or greater. There is a formula to use to determine how many degrees or minutes of arc one can see based on their visual acuity. To answer your question, 20/70 can be determined simply by dividing the bottom number by the top: 70/20= 3.5 minutes of arc or 0.058333 degrees.

Like I wrote earlier, visual acuity is only one measurement of one's vision but perhaps the most important. The Snellen chart that is most commonly used in eyedoctors' offices determines visual acuity by what I have explained above. I hope this helps and best of luck!

Sincerely,

Kenton L. McWilliams, O.D.

Text referenced: Primary Care Optometry by Theodore Grosvenor, PhD, 2nd edition