Re: summer / winter film speeds (was: Presentation)

[Russell Rosener <rrosener@xxxxxxxxxxxxxxx>]

>  > > Why is lower then summer the film ISO in winter?
>>
>>  In the summer the angle the earth is at with respect to the sun allows for a
>>  greater amount of infrared radiation to fall on the earth. 
>>Therefore, you can
>>  generally shoot faster in the summer than in the winter.
>>
>
>Of course you can shoot faster in summer than in winter, but I'm not so
>sure if yours is the right explanation. In summer no doubt the sun rays
>hit the earth in a steeper angle than in winter, which means that the
>TOTAL amount of solar energy (including all wavelenghts from heat over
>IR and visible light to UV), that hits the earth's surface is higher. In
>terms of absolute quantity, we really do get more IR than in winter. But
>we also get more visible light, UV, ....
>
>
>All the best,
>
>Eckhard
>
>
>******************************************************
Well, you are certainly correct from a scientific standpoint. As you 
said, there is more IR reflective material such as foliage in the 
summer than winter, if not more actual IR radiation, it is certainly 
more concentrated. Also, there is generally more daylight to allow 
for shooting.  The sun never gets very far above the horizon in 
winter, and sinks rather quickly at twilight. So that late afternoon 
IR rich shooting time is very short.
We all develop our own methods for shooting IR, particularly since 
there is no real way to gauge how much IR is actually there without 
some kind of viewing device. Bracketing is still the best insurance 
against failure regardless of where and when you're shooting.

Best,

Russ Rosener
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