A digital camera has the ISO rating to indicate the level of sensitivity to light. The normal setting is ISO 100 although the 40D and 5D have ISO 50. The most common ISO settings are ISO 200, ISO 400, ISO 800 and ISO 1600 or even ISO 3200 on dSLRs.

ISO sensitivity worrks pretty much the way a microphone and amp does. If you try to record with a microphone and your voice is loud and clear, you won't need too much amplification to obtain a good recording; but if your voice is extremely thin, you'll have to crank up the volume to register it. Obviously other sounds will be amplified as well, and there will be audible noise.

The same goes for digital cameras. The sensor must use a higher sensitivity under low light conditions. The price is image noise. EOS cameras are extremely good performers in the image noise department, but any compact and ultra-compact camera will have a very bad performance with high ISO sensitivities and even at low ISO numbers in shade areas there will be luminance noise.

Noise at high ISO sensitivities in compact cameras (Canon G9, ISO 1600)	Noise at high ISO sensitivities in dSLR cameras (Canon EOS 5D, ISO 3200)
All the compact and ultra-compact cameras have problems at high ISO sensitivities. Even at ISO 80 and 100 there could be visible noise at shadow areas. Typically a compact camera will have a useful sensitivity at ISO 400.	Canon SLR cameras have an excellent performance at high ISO levels. As you can see in this example, a 5D full frame camera at ISO 3200 has a very acceptable level of noise, comparable to ISO 200 in a compact camera

The ISO rating provides the start point to expose a photograph. Depending on the ISO setting, the aperture and shutter speed should be adjusted.