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What does it mean the ISO number in digital photography?

Well, I know that in film cameras this number indicates the film sensitivity to light, but in digicams I've never been quite sure of what is the 'analogy' that takes this name that doesn't belong to to the digital world...
Thank you for solving my eternal doubt!

I just bought a nice digital SLR as an upgrade to my Film SLR. That same question perplexed me. In a film camera, the iso was determined by the film that you purchased and loaded into the camera. You had to go to the store and buy the iso film that you needed. The lower ISO was good for portraits or pictures you wanted to enlarge a lot. If the lighting was low or the action was fast, you had to have a higher iso film so that you could capture the low light or freeze the action at higher shutter speeds. The crystals on the film that reacted to light had to be bigger, so when you enlarged the photo, it would be more grainy. The iso was set by what you loaded the camera with and it was no going to change until you changed the film.

Digital cameras don't have film, so ISO means something slightly different. The light sensor inside the camera has a light sensor. The light sensor is hooked up to an amplifier. The more juice you give the amplifier, (gain, or volume control as on stereos) the more sensitive it is to weak signals. If you have ever listened to a cheap stereo or guitar amplifier with nothing playing and then turn up the volume. You can hear a hum. This is noise. The more hum you hear, the worse the signal to noise ratio is. In a digital camera, what would be hum on a speaker shows up as off color pixels in the photo, which gives your photos a grainy look as if it were film. The process is different the end results are the same.

In a camera, it is all about controling the amount of light to get a picture. Too much light is over exposed, to little is under exposed. With an f-stop, each stop changes the amount of light by 2. It either dubbles it or halfs it. The same with the shutter speed, each change either doubles it or halves it. With the iso on a film camera you are stuck with it, but with a digital it is adjustable so each iso setting either doubles it or halves it. This flexability in changing the iso allow you to be able to adjust the depth of field in you picture in a wider range of lighting situations, or give you faster shutter speeds when you need them when photographing sports.

What is ISO in photography / digiscoping? Lions in Kruger National Park, South Africa

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