What is the difference between ISO and Gain?

[Bencuri]

I was told the ISO and Gain is not the same, but I cannot really understand the difference between them. Some say the are identical, som say they are not. What is the truth?

I own 2 Sanyo camcorders, the FH1 and HD1000, both allows setting of ISO, even in video mode. I have other camcorders, too, Canon, Samsung, etc. but none allows this. When I saw tests on youtube, it seemed to me that even more expensive camcorders allow setting of Gain only, too. How can it be?

[Richard_G]

They aren't exactly identical but they are for all practical purposes. ISO goes back to the days of film which had an ISO number as a measure of the sensitivity. Slow film, signified by a low number, needed more light for a correct exposure but gave better quality as the grains that made up the film emulsion were smaller. Fast film, with a higher number, needed less light but ultimate quality wasn't as good due to the larger grains, hence the term grainy. Gain isn't quite the same as the sensitivity of the sensor in the camera will always remain the same, the gain is applied to the output from the sensor. Coincidentally, when you increase the gain you introduce electronic noise into the image which looks very similar to the graininess of high speed film.

Gain is the photographic equivalent to push processing of film where you adjust the processing to increase the sensitivity by pulling stuff out of the underexposed background. ISO is pretty meaningless in digital cameras or camcorders as the sensitivity of the sensor doesn't change, it is the amount of gain that is applied to the output of the sensor that adjusts the sensitivity. It is only used to give some sort of continuity from film cameras. All it means is that for a certain level of lighting if you used a film camera loaded with 400 ISO film, you would need a shutter speed of, say, 1/125th and a lens aperture of f8 for instance. Using a digital camera set at 400 ISO for a correct exposure in the same lighting conditions you would need the same shutter speed and aperture.

[ninjastarstruck]

ISO is pretty meaningless in digital cameras or camcorders as the sensitivity of the sensor doesn't change, it is the amount of gain that is applied to the output of the sensor that adjusts the sensitivity. It is only used to give some sort of continuity from film cameras.

Great info. Thanks!

[Cornucopia]

Not totally meaningless, as the size of the CCD/CMOS pixel sites does affect sensitivity. In fact, some security/scientific cams allow you to gang multiple pixel sensor sites together, for special, high-sensitivity shots (night surveillance).

Scott