I never had a choice in my other cams.
I see the I can adjust the separately ?
Any help would be appeciated, Frank

They are the same. The choice of which to use is personal preference.

ISO 100 = 0dB
ISO 200 = 6dB
ISO 400 = 12dB
ISO 800 = 18dB
ISO 1600 = 24dB
ISO 3200 = 30dB

In practice similar but not the same technically. As with film, the ISO is a way of describing the sensitivity of a camera to light input. In the old days, we had 25ASA film that needed lots of light, but had very low gain. We had much more sensitive emulsion that might be rated at 400ASA, with more grain. However, if you really had very little light to play with, you used 400ASA film, and exposed it at say 1600ASA, and then the lab let the film develop longer, and you got pictures in very dark conditions. The downside was pushing the film made the grain worse.

This is the chemical equivalent of adding gain to a modern camera. It just gets confusing when the manufacturer can choose how they get the output. The ISO equivalent is the sensitivity of the sensor, and the gain set to 0dB is just what comes from the chip - so the light coming in is hopefully enough, and if not, you get the gain option. Some seem to allow changing the sensitivity of the sensor, but if it's an electronic change, then it's gain - so the two adjustments become the same thing. Although the details are different to establish because the specs don't reveal it, I suspect that the two are just changes in gain applied at different points in the chain, rather like in an audio mixer - where there is gain on a mic input before it passes to the eq and the send section, then a final volume control. Finding the right setting of both controls is essential to get the best noise figures.

While the effective ISO goes up with each dB added, as in the list Matt gave, the sensitivity of the chip seems to show that the point at which the image start to get objectionably noisy happens at different settings on each camera design. 9dB on one of my JVC models is hardly noticeable, but on the other, there's a hike in the grain, so I think the chip sensitivity matters - meaning the left and right columns might move up or down a notch.

thanks to both of you.
Frank

Just remember that you always start with your gain at 0db and this is equal to the base ISO of the camera. However it is important to remember that although 0db = Base ISO your base iso can vary with difference picture profiles so the overall spec of the camera's base ISO is always quoted with no profile.

I have the FS100's baby bothers (EA-50) and the base ISO is 160 which is the same as 0db ... 320 ISO will be 6db, 640 ISO = 12db etc etc However with the camera's PP3 profile the base ISO rises from 160 to 200 but the gain still is 0db ....

Leaving the camera in gain mode is probably less confusing for most of us and you shouldn't really take it too technically ...keep it simple ... when the gain gets up to 21db -24db it's time to either switch to a faster lens or turn on some lights!

Chris

Thanks Paul and Matt. Your explanations were clear, helpful, and, I think, excellent.