T3D Re: Visual acuity = 1' of angle?

[john bercovitz <bercov@xxxxxxxxxxx>]

OK, I think I have it now.  The one author was basing visual 
acuity on the standard eye chart while the other was basing it on 
lab tests under ideal conditions.  The E which you are supposed to 
be able to see on the standard eye chart has black bars and white 
bars and both are 1' wide.  The cycle is therefore 2'.  Most 
people can see this letter, at least with their eyes corrected.  
So the one author was using this as an optical design figure.  I 
think this is a good idea.  An eye chart gives seeing conditions 
about as good as we are likely to encounter in real life.  

This also works well with my being able to see 1.5' cycles on my 
res chart down in the basement.  My vision in my better eye is 
20/15 which means I can see at 20 feet the letter which I 
shouldn't be able to see unless it is presented at 15 feet.  We 
can deduce that the letter I should be able to see at 15 feet 
would be 3/4ths the height of the letter I should see at 20 feet.  
Therefore, that letter at 20 feet would be 3/4 of 2' or 1.5' from 
the center of one black line to the center of the next.

In future, I'm going to loosen up both my calculations and my 
requirements for my (proposed 8-) designs from 1.5 to 2'.  I'll 
convey this to the lens designer who has offered to design an 
eyepiece of 35 mm focal length (high magnification). 

John B


------------------------------