I've got a square of #87 filter material, which (as I may or may not
have remembered to mention befor) turns out to be a whole lot cheaper
than an 87 filter with a ring and threads and all. I figure one thing
I can do with it is take apart an old UV filter and stick this into
that, but another thing I can do which sounds like fun is to put it
over a flash and not use any filter at all on the lens, so that I can
take flash pictures at night w/o blinding everyone around me. (I did
fire an AF280T through the filter -- it's not _stealthy_, as there's
a noticeable visible red light coming through, but it's nowhere near
as bright as a bare xenon tube. Note that the filter packaging says,
"Visually Opaque, transmission begins above 730nm", but it's not quite
opaque at really bright light levels.)
I'm going to burn a few frames at different apertures (I've got a third
of a roll of HIE left in the Spotmatic at the moment) to see what the
effective guide number will be, but I'm hoping someone here can help me
narrow it down a little so that my experiments start at something
reasonable.
I'm sleepy enough that I don't have confidence in my physics/math right
now, so...
The notes on the inside of the film box say to meter at 50 ASA with a
#25 filter and at 25 ASA with a #67 filter (assuming a meter external
to the camera). I'm trying to suss out whether the net effect of
putting the filter on the flash is about the same as if I put it on the
camera and left the flash bare.
I'm assuming I'll have to put the flash on "H" or "L" and do the math
to set the aperture based on distance because the sensor in the flash
won't be seeing enough IR to know when to quench the flash. Figure
this is gonna suck those AA cells dry pretty fast, and I'll have to
monitor the temperature of the filter if I fire a bunch of shots in
a short time...
The scale on the back of the AF280T says that for 25 ASA film I'm
looking at f/1.4@10m, f/2@7m, f/2.8@5m, [EMAIL PROTECTED], f/5.6@2m -- do those
numbers make sense for putting the filter on the flash? (Fortunately
I'd already figured out that I wasn't going to get away with slow
lenses or long distances doing this trick. Though it looks like I
could cover three flash unites with one square of filter material if
I really need more firepower...)
Note that even after I get the numbers right, this trick will have
some serious limitations: first, it's not useful if the scene/mood
I want to capture will be completely obliterated by changing the
illumination with the flash; second, it's not useful for _stealth_
photography as noted above, so it only makes sense when a full flash
would be rude but an occasonal red blink would be acceptable, and
where letting folks know they're being photographed won't change
what pictures I can get (I like candids -- even if people have said
taking pictures of them is okay, their behaviour and body language
often change a bit while they're noticing the camera); third, well
I'm not going to have much flexibility regardng range -- seven meters
with the 85mm lens, ten meters with a 50/1.4; fourth, the grain of HIE
isn't going to be an improvement over TMZ.
What it'll be good for are situations where a) IR would be a cool
effect but it's dark, or b) I can deal with the above limitations
and don't have enough light to get hand-holdable shutter speeds
using TMZ.
-- Glenn
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