Date: Thu, 20 Mar 1997 22:47:11 +0100 From: Clarke Brunt <clarke@brunt.demon.co.uk> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg1031$foo@default> Subject: Re: Lights
On 19 Mar 97 at 12:24, Chris Teichreb wrote:
> > Want to vastly simplify things for yourself? Purchase
> > a good light meter...
> >
> Isn't there a way to do this using a 35mm camera
> and calculating it with the built in light meter?
I've sort of done this. My method, which I can't see any problems
with, is to place a sheet of white paper where the plants are going
to be, then aim the camera at it making sure that the paper fills
the whole frame (or at least the portion that the light meter is
sensitive to, if you know it). Take your reading - probably easiest
if you can fix the aperture and read off the corresponding exposure.
You'll need a camera which actually yields a numerical reading, so
an 'automatic everything' won't do.
Some comments on this: I only intend to get a comparative measure of
light intensity - take a reading with the sheet of paper in
sunlight, and compare with it under your fluorescents. Try under a
cloudy sky as well. I guess people will realise that an exposure
reading of 1/60 sec would mean than the light was twice as bright as
1/30 sec etc. If you can't help the aperture varying as well, then
each 'stop' is twice as much light, therefore compared to F4, then
F5.6 is twice as bright, and F8 is 4 times as bright. So long as you
ensure that your sheet of paper is large enough to fill the frame,
then the distance of the camera from the paper won't affect the
reading (for the technically minded, as you move away, the light will
reduce according to an inverse square law, but you will be seeing
correspondingly more of the paper, so the overall light getting into
the camera will be the same). Obviously the distance of the paper
from your fluorescents *does* matter, so put it at the same distance
as the plants will be. Aiming the camera at the light source itself
won't give a proper reading - it would be OK if both sources looked
the same, but one is a bright sun in the sky, and the other is a
long thin tube.
-- Clarke Brunt (clarke@brunt.demon.co.uk) http://www.brunt.demon.co.uk/ Cacti in Mexico, Frogspawn, etc.
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