Kodak HS ir film and filter factors

[Andrew Davidhazy <ANDPPH@xxxxxxxxxxxxxxxxxx>]

I am not sure if this specifically relates to the current discussion on HIE and
87C but for what it is worth here are some of my observations:

I think that the use of filter factors (or, metering light and then exposing
through an opaque ir filter having made an appropriate adjustment in exposure
index) is not really a bad idea as long as one can reliably count on the ratio
of visible energy to ir energy being somewhat constant in the wavelengths to
which ir film+filter is sensitive. This is not totally unfounded with daylight
or tungsten and when photographing "average" subjects. The problem is when
trying to use sources where the ratio of ir to visible is not predictable, such
as fluorescents or unsusual daylight conditions or when photographing subjects
with very high or very low ir reflectivity (or emissivity or transmittance ...
depending on one's objectives). 

In this case some kind of ir metering system is probably or hopefully better
than none and suggestions for determining an approximate index to use while
metering pure ir through a filter may not be a total waste of time. The process
was explained in an article I published in the Journal of the Photographic 
Society of America sometime in the mid-80's but probably was also described by
others even earlier than that. This article, as well as several others having
to do with the more utilitarian aspects of IR photography are available at the
following website:  http://www.rit.edu/~andpph/articles.html  but you may find
my articles also available at the websites of other people on the web ...
sometimes there with permission, sometimes not.                   

I am not too bothered by that but it is always nice to get some feedback! 

On a related note, since we can not see ir we can not previsualize precisely a
scene that will will photograph with ir film (unless we use the method of
visualizing the scene with a digital camera, an image converter or a camcorder
that has residual ir sensitivity, etc.). The consequence of this is that in ir
photography we are generally not concerned with "proper reproduction of subject
tones" as we concerned with obtaining a negative that has some detail that will
result in a picture, an image, of the subject ... even if proper reprodcution
would call for the subject to be totally black! I mean we "expect" to see an
image. In regular photography we can see that a particular part of the scene is
dar and thus we do not expect to capture detail there. In ir we actually are
never certain of which parts of the scene have a lot of ir present and which do
not. Sure, you might say that a green canpoy of leaves has lots of ir ... but
what if they are stressed and not doing what we think they should?

ok ... i will stop now ... enough!

regards,

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