[pure-silver] Re: Basic question on light
- From: Ryuji Suzuki <rs@xxxxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx, mark_sauerwald@xxxxxxxxx
- Date: Wed, 30 Mar 2005 13:34:26 -0500 (EST)
From: Sauerwald Mark <mark_sauerwald@xxxxxxxxx>
Subject: [pure-silver] Re: Basic question on light
Date: Wed, 30 Mar 2005 07:30:30 -0800 (PST)
> "What is the intensity of the light being reflected
> from the metered surface in lumens?" - I believe that
> there should be a correlation - my guess is that it
> should look something like:
> A*T/F^2 = K*ln(I)
> where A is the rated speed from the lightmeter (ASA)
> T is the shutter speed (Time that the shutter is open)
> F is the F stop - A being set on the meter, T and F
> being read from the meter.
> K would be some constant, and I the refelected light
> intensity in linear units such as lumens.
Or you can read the meter by the EV value at ISO 100/21 and write the
luminance in terms of k 2^EV. The constant k depends on the
calibration of the light meter. (As far as I know, Sekonic and
Minolta, for example, calibrate their meters to different constants.)
I only know the case of Sekonic L-408, and in this case k is about
0.125 (Sekonic's published value). But this number is only meant for
noncritical measurements.
--
Ryuji Suzuki
"Keep a good head and always carry a light camera."
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