TTL Exposure of IR

[Andy Finney <andy@xxxxxxxxxxxxxxxx>]

I have read with interest the suggestions to meter TTL when using HIE.  I
myself have done this, but more with the Ektachrome.

I do have a personal preference for using a hand-held meter, and I often
use this for incident light readings rather than reflected, which
'sometimes' gives me better results on foliage when following Kodak's
guidelines on exposure.  (This is HIE I'm talking about.)

Nikon, in their literature for the F90 which I have (but don't use for IR
as I have an FM2 which serves me better for this) point out that the
exposure algorithms in their camera can be fooled by using a dark filter.
They say this about using red with monochrome film but the argument can
also apply to IR, a red filter and TTL metering.  I don't know by how much
this is a problem but it was interesting that Nikon mention it.

The situation becomes clearer if you consider the example of a bright light
source in the centre of frame.  If the camera sees a very high general
light level then it interprets this light source as the sun in shot and so
will overexpose the light source to bring up the surrounding scene (unless
you tell it that you want a silhouette).  If the light level is only
moderately bright then the camera assumes this is an interior scene and the
light is in fact a lamp and so it will not overexpose so much.  By
following this 'logic' you will see that a dark filter can change the way
the camera interprets the scene and so lead to a different exposure.

This kind of issue is only likely to be problematic with a computerised
matrix exposure system such as that in the F90 and now the F5.

Which leads to the question ... has anyone actually run into this 'problem'
in practice or are the errors too small to be significant?

Andy

PS:  I also really like the Laurie White book.  Don't forget the
bibliography on Invisible Light (http://www.demon.co.uk/atsf/ilight).


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