From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "waldo kitty" 
---snip---
>
>this positioning of the eye so near the knife edge is likely one of the
driving
forces behind the camera automated testing units...
>its a lot easier and less "scary" to put a camera lens in the
same position as
needed rather than your eye...
>

Hi
 This is one thing that I try to convince people about. The
knife edge doesn't need to be all that sharp. A razor blade right out of
the package is not straight. You are better off dulling the blade with a
fine stone before using it. It will give you a sharper image with less
chance of cutting some part of your body. If the blade is nicely rounded,
it can be about 0.0005 to 0.001 inches thick at the edge and not effect
most of the measurement errors in this test.
 As for how close you need to be to the KE, one can think of
it as an f/ratio problem. For the pupil of your eye being 7 mm, you'd need
to be closer than 1 inch for a f/4 mirror. Since one rarely uses these in
total darkness with fully dark adapted eyes, less than 0.5 inch is a little
more practical. Dwight

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