Re: Workable speed

[Andrew Davidhazy <ANDPPH@xxxxxxxxxxxxxxxxxx>]

> >How does rating the film at EI 1600 translate to an INCREASE in exposure of
> >3.5 to 4 stops?   It seems to me that you would be severely underexposing
> >the film.
> >Um...do you meter through the 87C filter?   This would make sense then.  I
> >typically meter through a 29B (H+W 92) at EI of about 1000.

When you measure through the filter you are dropping the sensitivity of the
meter a significant amount. Much more so than the film is affected by having to
look at a scene through the filter. So, to "compensate" for the loss of
sensitvity of the meter you "artificially" jack up the film speed ratings to
end up with an exposure recommendation that is consistent with what the
photographer decides (by way of testing and personal use practices) producesa
good negative.
                                                                           
This is described in an article I published in the PSA Journal sometime in the
mid 80's or maybe earlier. It is called Action Infrared Photography. It is
available on the web at: http://www.rit.edu/~andpph/text-infrared-action.html
                                
regards,

Andy  o o  0 0 o . o  Davidhazy, Imaging and Photo Tech
       \/\/\/\/\/\/          http://www.rit.edu/~andpph
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