Film Speed Equivalency (ISO Ratings) for Digital Cameras

From Dummies.com

Pick up a box of film, and you should see an ISO number; film geared to the consumer market typically offers ratings of ISO 100, 200, or 400. This number tells you how sensitive the film is to light. The ISO number is also referred to as the film speed. The higher the number, the more sensitive the film, the less light you need to capture a decent image. The advantage of using a faster film is that you can use a faster shutter speed and shoot in dimmer lighting than you can with a low-speed film.

Most digital camera manufacturers also provide an ISO rating for their cameras. This number tells you the equivalent sensitivity of the chips on the image-sensor array. In other words, the value reflects the speed of film you'd be using if you were using a traditional camera rather than a digital camera. Typically, consumer-model digital cameras have an equivalency of ISO 100.

The practical lesson in all of this is to recognize that your digital camera needs plenty of light to produce a decent image. If you were shooting with ISO 100 film, you would need a wide-open aperture or a slow shutter speed to capture an image in low lighting -- assuming that you weren't aiming for the ghostly-shapes-in-a-dimly-lit-cave effect on purpose. The same is true of digital cameras.

A few new, higher-end consumer digital cameras enable you to choose from a few different ISO settings. Unfortunately, raising the ISO setting on most digital cameras doesn't give you the same advantages that you get when selecting faster film in a traditional camera. When you raise the ISO setting on your digital camera, the camera simply boosts the electronic signal that's produced when you snap a picture to increase the camera's reaction to light. Often, the extra signal power results in electronic "noise" that can create specks and other flaws in your image.