Light measurement

From: Chris Teichreb (teichrch@MEENA.CC.UREGINA.CA)
Date: Fri Mar 21 1997 - 12:17:05 PST 

Date: Fri, 21 Mar 1997 14:17:05 -0600 (CST)
From: Chris Teichreb <teichrch@MEENA.CC.UREGINA.CA>
To: cp@opus.hpl.hp.com
Message-Id: <aabcdefg1034$foo@default>
Subject: Light measurement

Hi all,

        Thanks for all the responses so far about measuring
light intensities. I got the Ortho book out of the
library last night. I forget who mentioned that it was in
this book, but thanks for refreshing my memory! Anyways,
following is the guide they give for converting your
camera's reading to approximate foot candles (f.c.) of
light. As Clarke pointed out, you can only do this
with manual camera's, not the automatics.

Method 1

        Set ASA to 100
        Aim at white sheet in area where you plan on putting plants
        Set f-stop to f4
        Adjust shutter speed until you get the correct exposure. The
inverse of the shutter speed=f.c. of light eg: an speed of
1/250th second is equal to approximately 250 f.c. of light.

Method 2

        Set ASA to 20 and shutter speed to 1/125th second.
        Focus on white paper as above.
        Adjust f-stop until you get the correct exposure.
        Use table below for conversion to f.c.

        f2.8 32 f.c.
        f4 64 f.c.
        f5.6 125 f.c.
        f8 250 f.c.
        f11 500 f.c.
        f16 1000 f.c.
        f22 2000 f.c.

        As they mention, using the camera is not as accurate, but
is good for the hobbyist. At least now I don't have to worry about
buying a light meter (even though I should!).

Regards,

Chris Teichreb
Department of Biology
University of Regina
Regina, Saskatchewan, CANADA

teichrch@Meena.CC.URegina.CA

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