Digital photography - how ISO works?

[-Tom-]

Can someone explain how digital ISO works? I understand the ASA/ISO in analog photography, it makes sense, different film different sensitivity...but how does the digital ISO work and what makes the noise etc?

[Cassius]

This would be easily googled, but I"ll try to summarize anyway. Noise happens in all electronics. Noise is created by low level interference that gives wrong results; grainy results in our case, staticy results in the case of audio. The level that this becomes noticeable is called the noise floor. ISO essentially cuts out the lowest end of the spectrum, which cuts out a lot of the noise. Higher ISO increases sensitivity by amplification; more amplification of the signal increases everything, noise included.

[Blue Bomber]

^^^ Pretty much. I had this concept in my head the other day: think of it as a volume knob on a stereo (only this one controls brightness instead of sound). When nothing's playing, you hear silence at normal volume levels (ISO 100). Turn it up a bit, and you can start to hear a slight hissing (ISO 800). Crank it to full volume, and the noise is rather apparent and annoying (ISO 6400). Get some NR in there to reduce the hissing, and it will also lose some of the high frequency range. It can only work for so much noise, before even that comes through, with the sound becomes muddy at that level of NR. Same with images and NR in photography and video.

[j1clark@ucsd.edu]

Here's a somewhat readable article on Digital ISO. Bottom line... test for actual noise of the sensor relative to the camera's listed ISO settings... pick the setting with the least 'noise', as the 'normal' for that camera.

If it so happens that noise for settings under a certain ISO value is indistinguishable, use that 'highest least noise ISO' setting as your 'normal'.

You will also have to calibrate your light meter to what the camera delivers... One had to do this for Film Film in the olden days... but for some reason folks in the digital age 1) don't use meters, 2) don't calibrate to their actual camera results...