Archived posting to the Leica Users Group, 1997/09/24

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Subject: Re: 90/2.8--internal reflections
From: "Henning J. Wulff" <henningw@archiphoto.com>
Date: Wed, 24 Sep 1997 09:01:33 -0700

Peter Jon White said:

>> That is my gut feeling until I begin thinking about light being reduced
>> by the inverse square of the distance.
>
>That's true, but the distance from the sun to the moon at full moon
>is only a small percentage greater than the distance of the sun to
>the earth. So the difference is a small fraction of a stop.
>>
>> I'll bracket liberally! Don't want to wash out the old man's facial
>> detail. :-)
>
>Bracketing never hurts, except in sports!

The inverse square rule does not apply because it deals with point sources
of light. In this case the source of the light is an effective point, but
that point is the sun. If the moon were twice as far from the sun as the
earth is, the moon would receive 1/4 the intensity of the light. However,
the moon is effectively the same distance away from the sun as the earth
is, for these purposes.

If you had a true line source of light, the intensity would diminish
directly in poportion to the distance from that line. For surfaces, the
intensity is not dependant on the distance, but constant. As a 'light
source', the moon is similar to a point source, but as a surface to be
photographed, it behaves as a surface. Therefore the 'sunny 16' rule
applies. On the other hand, the reflectance of the moon is less than most
surfaces and scenes on earth. Also, since we do a lot of moon viewing when
the moon is not far above the horizon, a lot of atmosphere often gets in
the way, which reduces the amount of light.

So the end result is that...
Bracketing never hurts! :-).


   *           Henning J. Wulff
  /|\     Wulff Photography & Design
 /###\      henningw@archiphoto.com
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