Re: follow up query on what 20/20 means

Hi David,

     In response to your very good follow up question, let me clarify.  
The size of the letter IS NOT what subtends 1 minute of arc, it is the 
angular size of the gaps in the 20/20 size letter.  For example, to 
differentiate a letter O from a letter G or C, we must be able to discern 
the gap which is several times smaller than the letter itself.  Same for a 
letter P or letter F. A 20/20 letter has gaps that subtend 1 minute of arc 
but the letter itself is much bigger.  

Use the following equation:  
tan 0=x/distance

For a target at 20 ft. or 6m: tan 0=z/20 feet or x/6m.  And for a gap 
subtending 1 minute of arc: x=6(tan 0), x=6(0.000291), x= 0.001746m, 
x=1.746mm.  For a letter at 20 feet, the gap height would be 1.746 mm and 
because there are 5 units in the letter E (count the 3 horizontal bars + 2 
spaces), it would be 5(1.746mm)= 8.73 mm or 0.873cm.  

Convert to inches, you get approximately 0.873cm/2.54cm/inch= 0.344 
inches.  To get size of angular gap, 0.344 inches/5= 0.069 inches!

I hope this helps!

Kenton L. McWilliams, O.D.

excerpts from:  Primary Care Optometry
                2nd edition
                TheodoreP. Grosvenor, OD, Phd
                Chapter 1, p.12