My ISP has been down for some number of days from the NIMBA virus. I saw
this thread with half-information and mis-information and attempted to post
twice to the list about it.
Here is a definitive answer about what is formally called:
The Additive Photographic Exposure System:
EV = AV + TV, where:
AV is Aperture Value, and TV is Time Value (shutter speed)
EV = SV + BV, where:
SV is Speed Value (film speed), and BV is Brightness Value (subject
luminance)
The following equality is used for metering or otherwise determining an
exposure:
SV + BV = AV + TV
The equations for them, to put the units wherein all these are normally
measured into 1 EV, 1 SV, 1 BV, 1 AV, and 1 TV being the equivalent of 1
stop of exposure, and establishing (somewhat arbitrarily) when they are zero:
AV is Aperture Value in EV units:
AV = log(N^2)/log(2), where N is aperture f-number
TV = Time Value (shutter speed) in EV units:
TV = log(1/t)/log(2), where t is shutter speed in seconds
For f/1 at 1 second, AV = 0, TV = 0, and therefore EV = 0. SV, the film
speed in EV units has nothing to do with an EV calculated from lens
aperture and shutter speed. EV = AV + TV is the camera's exposure setting
in EV.
Since EV = SV + BV
SV is Speed Value (film speed) in EV units:
SV = log(0.32 * S)/log(2), where S is film speed ISO (or the EI being used).
BV is Brightness Value (subject luminance) in EV units:
BV = log(B)/log(2), where B is subject luminance in foot-Lamberts.
For ISO 3.125 film, SV = 0. For 1 foot-Lambert subject luminance, BV = 0,
and therefore EV = 0. EV = SV + BV is what a light meter measures in EV as
the exposure required for a properly exposured image, if you have it set
for the film speed (or EI) being used.
If you rearrange the equality of:
SV + BV = AV + TV
to:
BV = AV + TV - SV,
then you have Brightness Value *only* and *not* EV.
See my comprehensive tutorial on the Additive Photographic Exposure System
here (which has better renditions of the equations than in this posting):
http://johnlind.tripod.com/science/scienceexposure.html
BTW, there is also a discussion of what "luminance" is versus luminous
intensity, luminous flux and illumination, with a *diagram* that shows the
difference. There are also conversions from other units for luminance to
foot-Lamberts, some of which are are actually illumination and improperly
used for luminance.
-- John
< 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 >
|