Re: IR and Flash Photography

[boblong@xxxxxxxxxxx (Robert Long)]

On Tue, 20 Aug 1996 15:45:16 +0100, you wrote:

|problem: AFAIK, Schwarschildt discovered that long exposures need 
|additional time to get the same results as was calculated. Caused by 
|slower acting film when few photons hit the film. 

This is what is generally meant by reciprocity departure, or sometimes
as reciprocity failure or reciprocity effect or even, simply if
inexactly, reciprocity.  Image density is proportional to exposure
(brightness x time) until the brighness gets very low, when the time
must be increased to achieve comparable density.

|I am not 
|sure if this phenomena equals the requirement of *less* exposure at 
|very short shutter times, I somehow feel it should also require more 
|exposure than calculated.

My memory--based on using, I believe, Ektachrome sheet film many years
ago--was that the short-exposure emulsion was meant to prevent *loss*
of density in extremely brief exposures (as in electronic flash), just
as the long-exposure version was meant to avoid reciprocity departure
in low-light situations.

I suppose that if "reciprocity departure" has any meaning, it should
apply equally to extremely short and extremely long exposures, but
I've never encountered it in the context of short exposures.

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Topic No. 23