Distant scenes

[Otto Giesenfeld <otto.giesenfeld@xxxxxxxxxxxx>]

--On den 10 augusti 1999, 11:22 +0000 Clive Warren
<Clive.Warren@xxxxxxxxxxxxx> wrote:

> The IRFAQ http://www.cocam.co.uk/CoCamWS/Infrared/INFRARED.HTM has a new
> section on filters and recommended exposure times - you may find it useful
> to have a look.
> 

Seems like a good summary to me. One thing that was not mentioned is the
difference between near and far scenes that Kodak writes about in data
sheets for HIE. I am still a beginner when it comes to infrared
photography, and I have yet to use the HIE film that lies in my fridge, but
I have shot a handful of Ilford SFX-200 rolls.

While recently on vacation in Italy, I wanted to see to what extent I could
get haze penetration by using SFX-200 with an 89B filter (actually a B+W
092). I therefore took pictures from various hilltops, featuring the
landscape stretching out below. There would often be many kilometers of
(hazy) sight. 

My problem is that the distant valley pictures were consistently severely
overexposed. My experience from some testing with SFX-200 and the 89B for
normal daytime scenes is that the filter factor is 4 stops (i.e. you could
meter without a filter using ISO 12). I did often bracket a step, but it
was not enough for the distant scenes. Other subjects on the same films,
with or without filter, were correctly exposed.

I assume this is the same effect that Kodak mention (with no explanation)
in the HIE data sheets. Does anyone know the reason for such differences in
infrared illumination? The altitudes of the places from where I took the
overexposed pictures were between 400 and 1200 meters, in case that
matters. The weather was sunny or partly cloudy, with a fair amount of
haze. I do not remember if I measured incident or reflected light in the
particular cases, but in any event I measured without a filter.

Kodak seem to recommend a difference in exposure of two stops between near
and far scenes. Has anyone got anything to add to this advice based on
practical experience?

I might add that processing was done by a commercial lab using their
standard black and white chemistry. (I have not asked what they use.) Since
pictures at close distances (tens of meters or less) usually turned out as
expected (with or without filter), it can hardly be a question of
processing or filter factors.

Best regards,

Otto Giesenfeld

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