Re: Some points of ponder

From: Ryuji Suzuki ^lt;[email protected]>
Date: 07/15/04-09:13:23 AM Z
Message-id: <>

There are two issues here.

Usual panchromatic b&w materials are typically sensitized to 650nm or
so and doesn't even cover the longer end of red. By using IR filter of
some sort, you are removing most of the visible spectrum and giving
exposure in the wavelengths that the film is insensitive to. In order
to get decent IR effects, you should use IR films that use sensitizing
dyes that are suitable for 750-850nm range. You can buy 4x5 sheets
made by Maco, or dye films yourself.

Usual light meters are designed to approximate sensitivity of human
eyes (by approximating CIE curve, etc.). Readings from such meters
have nothing to do with the amount of infrared luminance/irradiation
that you should be interested in. Relative amount of infrared to
visible light varies a lot, but some photographers have empirical
factors to estimate infrared exposure based on usual light meters.
Such factors depend on time of day, time of year, weather, etc. and
can't be generalized too much. Indeed, such factors depend on the
particular light meter used. One way to overcome this problem is to
build or modify a light meter to make it sensitive to IR but not to
visible light and do some crude calibration. Such a meter will be
valuable if you are a dedicated IR shooter, especially in LF.

Alternatively, if you have some electrical engineering background, you
can make your own meter. Bare silicon photodiodes have maximum
sensitivity to infrared region (because of the property of silicon)
and with a simple IR filter, such a sensor is ideal for IR exposure
meter. One problem is very wide range of dynamic range is necessary to
make a usable meter. When I checked a few years ago, TI, NS and a
couple of other companies made IC's that incorporated a Si photodiode,
a preamp of programmable gain, an I/V converter and a V/F converter,
outputting the light intensity in terms of frequency.

A simpler approach may be to buy a meter like Sekonic L-208 and remove
the sensitivity compensating filter and glue a piece of IR filter cut
to fit and recalibrate. I have dissected this meter somewhat, and took
pictures inside. It's a very simple construction and I don't think
much electrical engineering knowledge is necessary to modify this
meter. (but needs some dexterity of course) Maybe same thing can be
done with Digisix but I don't have that meter. (For this approach, one
must begin with a straight silicon photodiode based meter, not CdS,
selenium, GaAs, PIN, etc.)

Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)
Received on Thu Jul 15 09:14:06 2004

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