ISO is the digital equivalent to film speeds found with traditional cameras. The ISO number indicates the light sensitivity of the digital camera’s image sensor. The ISO becomes especially important in low-light conditions where more light is needed for exposure.

Normally, a camera will try to compensate in low light conditions by having the shutter remain open longer, thereby exposing the sensor to more light. The more light, the lighter the image. The problem is the longer the shutter stays open, the greater the chance of camera shake (when handheld) resulting in a blurred image. Adjusting the ISO increases the camera’s sensitivity to light making longer shutter speeds unnecessary.

Lower ISO settings are used when the light levels are high. For example, a low ISO setting of say 100 is used when there is enough light to properly expose the image. The lower the ISO, the less sensitive the camera sensor is to light. Examples of high light levels would be a bright sunny day at the beach or snowy slope. The quality of the exposure will be higher with the low ISO settings than it will be at the high end. The lower the ISO, the finer the details and image quality.

Higher ISO settings are typically used for when there is less available light, such as with indoor photography, as they increase the light sensitivity. The higher the ISO, the more sensitive the camera sensor is to light, meaning less light is required to take the picture. Higher ISO settings are often used in low light situations that aren’t appropriate for flash or flash is not available. They are also used in sports photography, along with higher shutter speeds to help reduce the motion blur when there is motion involved.

The lower ISO setting you can use, the better as a high ISO will increase the “grain” and noise levels. (Grain is typically seen with traditional film; noise with digital.) The amount and degree of noise that results from using higher ISOs will vary depending on the camera. (Post processing with noise reduction software may help in this area, although some of the fine detail may be lost.) With smaller images (4×6), the noise is not as noticeable…with enlargements, however, it will be more obvious.