Sensitivity ISO Ratings
ISO is commonly used to refer to film speed. Film speed is the measure of a film's sensitivity to light. Film with lower sensitivity (lower ISO speed) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO speed) can shoot the same scene with a shorter exposure and is called a fast film.
Digital cameras have an ISO rating indicating their level of sensitivity to light. ISO 100 is the "normal" setting for most cameras with a few going as low as ISO 50. ISO ratings can be increased to from 100, 200, 400, 800, 1600, and even 3,200 on high-end digital SLRs.
When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. The only problem occurs when there is undesired noise or graininess on the photo due to increased iso. Think about adjusting iso levels the way you would increase the volume on a stereo system with a bad reception. This will increase the sound of the desired music as well as the static or crackle.
Digital SLRs tend to have much larger sensors with the same sensitivity settings. For example, at ISO 400 a digital SLR will produce a sharper image than a point and shoot at the same sensitivity level. Very few cameras have ISO setting lower than ISO 100 because noise levels are so low at ISO 100 there would be no real advantage in a slower setting. Quite a few digital cameras have an "auto" ISO setting, where the camera will pick from ISO 100, ISO 200 and sometimes ISO 400, depending on the light level and the mode in which the camera is operating.
Is having multiple iso settings important to you? The answer is "no" if most of your shots will be in the daytime or under well lit conditions. Point and shoot cameras would be the way to go.
Since I use my digital camera in many low light situations where I couldn't use a flash or tripod, multiple iso settings became very important. I needed as much control as possible over the exposure (ISO, shutter speed, aperture). I solved this problem by getting a digital SLR.
