Ev is not a measure of light and has no functional relationship
with light levels. It is a measure of exposure only, and is a
function of shutter speed and aperture, and nothing else. Light
meters can be set to give Ev readouts, but the value shown is
a recommended exposure, which will change with the ISO setting,
not a measure of light.
Ev (exposure value) is defined as the sum of time value and
aperture value:
Ev = Tv + Av
where Tv = log2(1s/T), T is shutter speed
and Av = log2(f^2), f is f#, ^ means exponentiation.
log2 is the logarithm base 2, ie x = 2 ^ (log2(x))
The definition of Ev means that 1s and f/1 will always be Ev =0.
Ev = log2(1s/1s) + log2(1) = 0 + 0 = 0
Each halving of the shutter speed, and each standard f# click
increases Ev by 1:
(1/2)s and f/1 is Ev = 1
etc.
(1/125)s and f/16 is Ev = 15:
Ev = log2(1s/(1s/125)) + log2(16*16)
= log2(125) + log2(256)
= 7 + 8
= 15
It will be 15 regardless of light level or ISO, because
by the definition of Ev, 1/125 and f/16 is always Ev = 15.
In sunny conditions a light meter might tell you to use
Ev = 15 as your exposure for ISO 125 film (or Ev = 14 for
ISO 64, etc), but that doesn't make sunny conditions Ev = 15
(or 14, etc). Any use of Ev for anything but an aperature-
shutter speed combination is incorrect.
And meters aren't calibrated to 18 0ray (or any 0ray,
either), but that's a different story.
Paul
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