Re: [ATM] Exit pulil and brightnees

[Alexis Cousein <al@sgi.com>]

Mel Bartels wrote:
>> For point objects against a black background, the brightness is
>> reduced. This equals a reduction in contrast and hence in detectability.
>>
>> However for extended objects all the brightness is reduced by the same
>> factor. There is no chance in contrast and hence no difference in
>> detectability.
> 
> 
> This last statement is suspect, because while the contrast (difference in
> brightness between object and background) stays constant, the *perceived*
> contrast (the eye's ability to perceive or detect the object) varies with
> changes in the sky background brightness.
> 
You bet it is - "objective" contrast never changes with neither scope nor
eyepiece, so the inference were correct we could all sell our scopes and
view extended objects with the naked eye ;).

The point is valid when expressed in another way, though - perceived
contrast is rarely highest when the exit pupil is very large, so
someone with a pupil of 3mm who is trained to have perfect eye placement
is bound to be able to reach the same perceived contrast on most
objects for which the optimal detection magnification is at eye
pupils of 3mm or less.

What I'd worry about is the quality of the receptors (and
yellowing of the eye lens) and visual acuity. Those affect you even at
small exit pupils.


-- 
Alexis Cousein                                  al@sgi.com
Senior Systems Engineer/Solutions Architect     SGI/Silicon Graphics
--
<If I have seen further, it is by standing on reference manuals>

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