Originally posted by Hattifnatt The way I understand it, ISO100 is the basic iso of the camera, meaning the digital noise of the camera itself when it's turned on. Increasing ISO more than 100 scales up the signal (also scaling up the noise in the process). If ISO100 is the base iso, how is ISO80 achieved then? scaling down maybe?
ISO used to be film speed ! ( sensitivity to light )
The higher the ISO the more sensitive the film was to light allowing for higher shutter speed , thus called faster film ..
So when we got digital , they had to call the sensors sensitivity to light something , so they gave us something familiar ( ISO )
Now whether Digital ISO 100 matches the film speed ISO 100 ???????????????
But , being digital and all , there is no reason they could not have gone , ISO 127 or ISO 96 or what ever .. But why confuse people ... SO we have 100 200 400 800 and so forth ...
But I see no reason , they could not do 100 150 200 300 400 600 800 1200 1600 or something custom defined by the user ...
Say you have a really bright day , like today , having an ISO 50 would be quite useful ... Especially if you are shooting aerial . ( Balloons / Aircraft ) , even an ISO 25 might not go astray ...
As for noise .. All that sensitivity has side effects ...
The higher quality cameras can handle more ISO ( also newer cameras ) .. You will find early digital SLR's such as the istD got noisy very quickly ... ( also because of the lower pixel count which contributed greatly )
ISO is not about noise , that's a side effect of sensitivity to light ..