Re: Infrared-Digest V #293

[Gary Beasley <beasleyglb@xxxxxxxxxxxxxx>]

>Date: Thu, 8 Jul 1999 20:09:39 -0400
>From: "Don Bryant" <dsbryant@xxxxxxxxxxxxxxxx>
>Subject: Re: Infrared-Digest V #291
>
>Gary,
>
>> Midday gets the lowest EI because of the
>> lower ratio of IR to visible.
>
>Could you explain that statement a little more fully. How did you arrive at
>this conclusion?
>
>Empirically? Just curious.
>
>Thanks,
>
>Don Bryant
It's quite simple really, and pointed out in Laurie Whites book. Early or
late in the day the sunlight is filtered through a significant amount more
atmosphere, which cuts the shorter wavelengths. Witness the redder light of
morning or sunset.. Therefore with more ir content you can use a higher EI
to get the correct ir exposure. Midday the light is about equal and you use
the "correct " EI for your metering. If you don't believe this try a shot
at sunset and see if the exposure is heavier. This only applies with an
unfiltered light meter, usually handheld. If you use a red or IR filter
over your meter you don't have to worry about this phenomenon.
Gary Beasley
http://beasleyglb.home.mindspring.com/

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