ISO (also referred to as film speed) refers to how sensitive film, or a digital sensor, is to light.  The lower the ISO, the less sensitive to light the film or sensor is.  The higher the ISO, the more sensitive it is.

When film was the capture medium of choice, you were limited to the film speed the film was manufactured to be.  So, every image shot on a roll of film would be shot as the same ISO as every other.  With the shift to digital, the ISO can be set for each photograph individually.

When the ISO is set to a low number, like 100, the film or sensor will require more light to achieve a proper exposure.  This can be achieved through changing the combination of shutter speed and lens aperture to allow more light to reach the film or sensor.  A low ISO is termed to be “slow.”

When the ISO is set to a high number, like 1600, the film or sensor will require less light to achieve a proper exposure.  This can be achieved through changing the combination of shutter speed and lens aperture to allow less light to reach the film or sensor.  A high ISO is termed to be “fast.”

With film and digital, there is a trade off to be made between image quality and ISO.  With film, higher ISO films are typically exhibit a more grainy appearance.  With digital sensors, a higher ISO will exhibit a higher amount of digital noise in the image.

Tips:

• To achieve the lowest amount of noise in an image, use the lowest ISO that will allow you a shutter speed that you can still hand hold.
• In low light situations, such as indoors, or outdoors at night, it may be necessary to increase your ISO so that you are able to shoot with a shutter speed that you can hand hold.
• In a well lit shooting situation, outside during day, setting a low ISO would be more appropriate.