HariesDesign.com - The letters ISO on your digital camera settings refer to the film speed. Even though your camera is most likely not film at all, but rather digital, the ISO setting still does has the same function as older film cameras. ISO determines how sensitive the image sensor is to light.
 

• If you want to ensure the highest quality photograph possible.The lower the ISO the better the quality. An ISO of 100 or 200 will give you a better quality photograph than one set at 1600. If the image is destined for website display, then this may not matter. However, if you wish to print the photograph, then you’ll want to keep the ISO low. Otherwise it will look grainy or noisy (as it’s also refered to).

• If you want to take a photograph in dark situations and not use a tripod.For example, if you were on a tour through a mountain cave or at an aquarium, a tripod may not be feasable. In these cases you could up your ISO to at least 800. This way, your camera will automatically keep to a fast enough shutter speed for you to hand hold your camera. Yet still allow enough light in to your camera’s sensor for a reasonable night shot. As explained in the first section, this would cause a grainy photograph. However, in some circumstances any photo is better than none.

• If you want to take photographs in darker situations other than outdoors, without the use of a flash.For example, if you were taking images in a museum or theatre, camera flash could be prohibited. Or if you were photographing a child blowing out their birthday cake candles. A bright flash could ruin the atmosphere. In this case you would keep the ISO at around 800.

• If you want to take a photograph indoors of a moving subject.For example, if you were taking photographs of a basketball game at an indoors sports center. In these circumstances there isn’t always enough natural light to take a good photo, while at the same time the players aren’t going to stand still long enough for you to shoot at a slower shutter speed either. Therefore, you could up your ISO to around 800. Again, this would allow enough light in to your camera’s sensor for a reasonable shot.