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Re: [OM] EV-ignorance

Subject: Re: [OM] EV-ignorance
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Sat, 22 Sep 2001 21:52:01 +0000
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


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