ISO versus ASA

[kptasteve]

This will seem like a real rookie question. but are ISO and ASA almost relatively the same idea. That of sensitivity to the light of the sensing medium???

[inkista]

They're exactly the same. The only difference is the governing body that maintained "the standard"; ASA stood for "American Standards Association", while ISO is "International Standards Organization".

[PhotoNewt]

Originally Posted by kptasteve

This will seem like a real rookie question. but are ISO and ASA almost relatively the same idea. That of sensitivity to the light of the sensing medium???

At the risk of getting a bit technical here, ISO (ASA in the older days) was a mesurement the film's sensitivity to light, a physical property of the film medium. The lower ISO numbers denotated less sensitivity and the higher more. Therefore, at a fixed aperture, the higher the ISO rating, the higher the shutter speed you could use on a given scene - therefore the ISO number was also referred to as the film's speed, higher ISO meaning faster film.

For digital cameras, the ISO is a representation of the exposure index (EI) required to obtain an image of similar lightness to that produced by a film of equal ISO. The EI is a mesurement, not of the sensitivity of the sensor to light, but rather the signal gain applied before (analog) and/or after (digital) in camera processing to the final image. Defining the EI level that more closely matches the ISO of a film is usually a subjective call of the camera manufaturer.

After all this technical stuff the bottom line is: the digital camera's ISO is designed to provide an equivalent exposure to that of a film camera using the same ISO rated film. As per ISO rules, it can be off by +/- 1/3 EV and still be considered acceptable.