DPR chickens out (part 2)

Started Apr 20, 2013 | Discussions thread
Jack Hogan Veteran Member • Posts: 6,982
Exposure is an Objective Physical Quantity, ISO is a Subjective Suggestion

dinoSnake wrote:

ultimitsu wrote:

Donald B wrote:

so when you use a lightmeter to determine correct exposure ? it uses 3 varibles correct ?

1 Av

2 Sv

3 iso or asa

has anyone got a lightmeter that only uses 2 ? i dont think so.

The light meter has to work on a given ISO basis to determine the right ss/F combination that will result in the correct brightness. It does not mean ISO is part of exposure.


Stop it.  Please.  Just, stop it.

I have PROVEN, by a direct quotation from Merriam-Webster,


that the word "exposure" has several different meanings and contexts and, in one of the contexts, the word "exposure" DOES include ISO as part of the computation.

Donald B is referencing the context that includes ISO computations ("the treating of sensitized material (as film) to controlled amounts of radiant energy"(emphasis mine)) yet you insist on trying to make him change context to your singular interpretation, the one where "exposure" does not factor in ISO ("the amount of such [radiant] energy").

BOTH explanations of "exposure" are, precisely, correct.  But the context of usage is different and I really wish you people would stop switching contexts only to try to prove your correctness over someone else.

In a nutshell, Exposure is an objective, quantitative physical quantity (like the number of photons falling on a certain area during a certain time) independent of sensing medium, whether that be film, silicon or cones in the eye.  Simplifying, it depends on light from the scene, shutter speed and f/number only.

ISO is a subjective, qualitative, sensing medium dependent indication by manufacturers to help you/your camera choose the Exposure that will generate an adequate output level to produce a good picture, whatever that is.  It depends on the physical characteristics of the sensing medium and a multitude of subjective, qualitative factors.

One is a number, the other is a suggestion of what to do with that number.  The number does not change from manufacturer to manufcturer or from sensing medium to sensing medium.  The suggestion does.



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