>
> Sort of related to the OM4 and 2s manual spot metering discussion, what does
> EV18 mean related to the real world. Is EV0 (zero) the lowest threshhold of
> human vision? Does EV18 relate to f 16 @ 1/ISO, the sunny 16 rule?
>
> Always been wondering.
>
> Warren Kato
EV (exposure value) is a measurement of the exposure, not the light. EV0
is 1s at f/1, and it increases one unit per stop reduction in exposure.
1/2 s at f/1 is EV1, etc. EV18 is "sunny f/16" for ASA 1000 film, that is
1/1024 s at f/16, or equivalent.
You can use the formula 2**EV = A**2/T
where A is f number, and T is shutter speed: 2**18 = 16**2/(1/1024)
Light is measured in a confusing welter of units. For the US, at least,
traditional photographic units are footcandles for incident light
(illuminance) and footlamberts for scene luminance. The metering range
of an SLR SHOULD (but generally isn't) be specified in footlamberts for
a given metering aperture. (An f/1.4 lens will collect twice as much light
for the camera's meter as an f/2, and will halve the minimum and maximum
scene luminances that the camera will meter.) Usually ranges are given as
EV ranges for a given ASA and lens aperture. You can convert to footlamberts
using
L = K * 2**EV / S
where S is ASA, and K = 3.33
"sunny f/16" is about 1000 footlamberts. Interior scenes run about 1-10.
Paul Farrar
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