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The article has not taken in consideration optometry since. Ask an optometry expert, Why a good sight is phrased in numbers 20:20 or 6:6. What does it mean the numbers in a prescription of eye sight given to you + - No. etc? Answers. A. 20:20 are foot; 6:6 are meters, pay attention to the correlation. Mending the sight, with various numbers of eye glasses is trying to reach an exact calculated point. Now ask the manufacturers of televisions or projectors Why do they express sizes with a diagonal length? Diagonal length is unique to eye sight world. Why? The other world of three dimension objects is measured by Width. Height, Depth. Well, to my poor knowledge. Diagonal X 20 (foot) or 6 (meters) is the "eye sight field". The ultimate point where you are able to see full field without moving one muscle of your eye ball.

Now you can easily calculate picture size vs. distance, or distance vs. picture size.