08. Illumination - Photometry with a Photographic Light Meter
Description
This article is from the Photographic Lenses Tutorial, by David Jacobson with contributions by others.
08. Illumination - Photometry with a Photographic Light Meter
The first caveat to keep in mind is that the average unfiltered light meter doesn't have the same spectral sensitivity curve that the human eye does. Each type of sensor used has its own curve. Silicon blue cells aren't too bad. The overall sensitivity of a cell is usually measured with a 2856K or 2870K incandescent lamp. Less commonly it is measured with 6000K sunlight.
The basis of using a light meter is the fact that a light meter uses the Additive Photographic Exposure System, the system which uses Exposure Values:
Ev = Av + Tv = Sv + Bv
where:
Ev = Exposure Value
Av = Aperture Value = lg2 N^2 where N = f-number
Tv = Time Value = lg2 (1/t) where t = time in sec.s
Sv = Speed Value = lg2 (0.3 S) where S = ASA speed
Bv = Brightness Value = lg2 Bfl
lg2 is logarithm base 2
from which, for example:
Av(N=f/1) = 0 Tv(t=1 sec) = 0 Sv(S=ASA 3.125) E Bv( Bfl = 1 foot-Lambert) = 0
and therefore:
Bfl = 2^Bv Ev (Sv = 0) = Bv
From the preceeding two equations you can see that if you set the meter dial to an ASA speed of approximately 3.1 (same as Sv = 0), when you read a scene luminance level the Ev reading will be Bv from which you can calculate Bfl. If you don't have an ASA setting of 3.1 on your dial, just use ASA 100 and subtract 5 from the Ev reading to get Bv. (Sv@ASA100=5)
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