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


Hi Tom
 As I've stated before, I find that my eye is more sensitive
to time differences of zones, caused by the lateral motion of the edge.
This effect is enhanced by masking off inner zones and doesn't depend on
intensity matching. When at the right distance, both edges seem to snap in
at the same time. If I'm not at the right distance, there is an apparent
motion, as though someone was moving the light across the back of the mask.
The direction of this motion tells me which way to move the edge, in or
out.
 I understand that other do this as well.
Dwight

>From: "Tom Krajci" 
>
>The Foucalt knife edge test is often used with a Couder mask to help
>isolate the zones being tested.
>
>Unfortunately for the outer zones...the tester must decide if the two
>widely spaced zones are equal in brightness.  This is playing against an
>inherent weakness of the eye...accurate comparison of brightness of
>widely separated objects.  (This is one reason some folks favor wire and
>caustic tests.  The eye measures positions of shadows...which the eye
>can do pretty well.)
>
>OK, plenty of folks still insist on testing with a Couder mask.  For
>those folks I have a question:
>
>If one were to set up a light source with adjustable brightness to shine
>on the Couder mask...and adjust it so that it were equal in brightness
>(or close to it) to the outer zones being tested...would this help
>increase accuracy/sensitivity of the human eye in our effort to match
>the brightness of the two widely separate zones?  (And perhaps this is a
>worthwhile technique for testing all zones...not just the outer zones?)
>
>It does allow another variable to enter into the testing procedure...is
>this light that shines on the Couder mask vignetted in any way...or is
>part of the Couder mask dirty/smudged in one part?  If this variable is
>not controlled it would put a systematic error in the test results.
>However, if controlled carefully, maybe this technique can help work
>around one problem with Foucalt/Couder zonal testing.
>
>I'm sure others have tried this before.  What are their findings?  Any
>formal studies been done?  Where can I find them?
>
>Thanks in advance,
>Tom Krajci
>Tashkent, Uzbekistan
>
>
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