At 12:04 PM 1/20/99 -0500, Warren Kato wrote:
>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?
Warren:
The Focal Encyclopedia of Photography (3rd ed.), p. 292, has this to say about
EV:
"EXPOSURE VALUE (Ev) An integer obtained by adding the light value and the
speed value in the additive system of photographic exposure (APEX). Expressed
in symbol form as Ev = Bv + Sv. Because the exposure value also equals the sum
of the aperture value and the time value, it is used to determine an
appropriate combination of camera aperture and shutter settings. On some older
cameras the controls are sometimes marked in a whole number scale of exposure
values, where each number setting permits twice as much light to pass through
to expose the film as with the next higher number. With these cameras, shutter
and aperture settings can be interlocked so adjustment of one setting
automatically adjusts the other as well. To a large extent Ev settings have
been superseded by new electronic exposure systems, many of which feature
shutter or aperture priority."
The entry for the APEX system (pp. 5, 6) states the following:
"ADDITIVE SYSTEM OF PHOTOGRAPHIC EXPOSURE Also know as the APEX System, the
additive system of photographic exposure is a method of using logarithms for
the lens aperture (Av), exposure time (Tv), light level (Bv), film speed (Sv),
and exposure (Ev), so that all calculations involving these factors are reduced
to addition or subtraction of small integers, as indicated in the camera
exposure equation: Av + Tv = Bv + Sv = Ev. ...
For example, when film speed (Sv) is 6 and light level (Bv) is 8, the
exposure (Ev) is 14. Any combination of lens aperture (Av) and exposure time
(Tv) with a sum of 14 may be used...
This form of expression of the camera exposure equation is of interest in
connection with the exposure value scale marked on many exposure meters and
some older cameras and also in connection with ISO logarithmic speeds, which
correspond with Sv. ..."
***** quotes end here *****
It would appear that there is no absolute reference value in Ev. It doesn't
look like it's based on lumens, lux, or any other known measure of light energy
per se. This makes sense to me -- if you used a known source of light energy
(say, 800 lux) and you could get a "perfect" exposure of an 18 0rayscale with
that illumination using a combination of shutter speed, aperture and film
sensitivity, changing any of those variables (including the average reflectance
of the scene!) would impact all the others in some fashion. Probably the best
you could do is to specify all the above variables, state that you are seeking
the "perfect" exposure, and then do a search for the lux/lumen value which in
fact gives you that "perfect" exposure.
Garth
"A bad day doing photography is better
than a good day doing just about
anything else."
The Unofficial Olympus Web Photo Gallery at:
http://www.taiga.ca/~gallery/
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|