The ISO number refers to the sensitivity of your recording media to light. An ISO of 100 requires more light to expose properly than an ISO of 3200. The higher the ISO number, the greater the sensitivity or in other words, the less light required for a proper exposure.
There is a tradeoff however. While lower ISO settings will require more light, image colors will appear much cleaner and have less digital noise. Sensors using higher ISO settings will be more sensitive to light, but there is also a greater amount of noise.
Take a look at these two pictures of a dark shadow taking on different ISO setting. The first is at an ISO of 100. The second is at an ISO of 3200. What is supposed to be black is greenie and unpleasing to the eye. This is the result of high ISO settings.
Ideally, try to shot at ISO ratings of 400 or less. However, some camera sensors such as Canon’s produce acceptable images at higher ratings, depending on what you are shooting. A number of different ISO films are available for film cameras. However, with digital sensors, the ISO really refers to the amount of artificial boost the sensor has. What these means is that the sensor becomes more sensitive to light, but only does so because of an artificial electrical gain added to the incoming signal. It is not clean light. It is amplified light. This is why noise artifacts become visible at higher levels.
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