How does ISO setting work?

David Arnstein wrote:
The digital cameras that I have seen have a feature that allows the
user to choose an ISO setting. I would like to know what this feature
does, since I suspect that it is useless.

My concern is that setting a high ISO number simply
1. Causes the camera to take pictures that are quite dark
2. Post processes the picture by increasing the brightness, in
software.

If this is the case, then I prefer to increase the brightness myself,
in Photoshop.

On the other hand, if the ISO adjustment on the camera actually
changes the physical properties of the photosensor, then it's a
different story.

I'd like some feedback on this before I spend my time experimenting
with different ISO settings.

The ISO setting is one parameter the exposure computer uses to determine
exposure (shutter speed and aperture [lens opening]). The default value
should give you the best balanced exposure. There IS now a definition
of the ISO exposure for digital cameras, but I can't find it right now.

You can push a digicam just as you can push a film camera by dialing in
a higher ISO value, which causes a smaller exposure (less energy) than
for the nominal value. This either shortens shutter speed, reduces
aperture, or both, depending on the exposure 'mode' you are using.