Exposure values vs light values

[xavier]

Light value (LV) can be incident or reflected.
Incident light comes from source and more stable.
Reflected light comes from surfaces that reflects light and depends on more variables.
Exposure value (EV) is a set value (of aperture and shutter speed) to expose film/sensor.
In the viewfinder you have:
The reflected light from scene (LV);
The photo cell that measures this reflected light;
The suggested EV, based on film speed (ISO/ASA/DIN);
So, in the viewfinder you read the camera suggested EV (combination of aperture and shutter speed).

 

[Gumby]

The suggested EV, based on film speed (ISO/ASA/DIN)

... that the camera/meter has been set for.

 

[Freakscene]

One question that may be a little too detailed: the conversion of the LV to the EV needed to properly expose ASA-100 film is based on which function? I'd like to see the function, basically. But you say that the definition isn't absolute, it varies. Basically I'd like to know the empirical relationship between measured light and the film speed. 🙂

You need to buy the standards from ISO. They are:

For B&W film

For colour print film

For colour reversal film

Marty

 

[sevo]

One question that may be a little too detailed: the conversion of the LV to the EV needed to properly expose ASA-100 film is based on which function? I'd like to see the function, basically. But you say that the definition isn't absolute, it varies.

The most common definition of LV is EV@100ISO, nothing more or less - essentially EV without having to determine/negotiate which film is loaded. That is the "light value" or "LV" motion picture lighting staff and cinematographers use to communicate over (when they don't simply talk stops/steps).

There are several other definitions, as the German DIN standard term for exposure value/EV back when the DIN pretty much set photo industry standards was "Lichtwert (LW)". Many competing or corporate systems anglicised that to LV and applied it to their own notion of logarithmic exposure values (which they would more appropriately have abbreviated EV), so that there have been about half a dozen of them around (with different scaling and start values often derived from the electric properties of some early exposure meter).

But for practical purposes, LV as EV at 100ISO is the only one you'll still encounter elsewere than on old hardware. If you are into photometry, you might also run across the APEX light value definition in technical and scientific books - but that standard does not use the abbreviation LV at all.