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Definition: ISO speed
The international standard measurement of a camera's sensitivity to light, often simply called the "eye-so." For analog film cameras, the ISO is the "film speed," which is based on the chemical composition of the film. Changing ISO means loading a film with a different speed. With digital cameras, it can be changed by selecting a speed from the ISO menu, reconfiguring an electronic circuit that emulates film speed.
If the camera's shutter speed and aperture cannot be altered for certain scenes, changing the ISO is a third variable that can accomplish the desired exposure. In a digital camera, ISO can be set manually or automatically.
High ISO Noise
In a digital camera, higher ISO speeds are accomplished by amplifying the electronic output of the sensor, and some cameras have speeds up to ISO 10,000. However, the more signals are boosted, the more noise is generated, which is equivalent to the grainier image that appears in analog film at ISO 1600 and higher.
Unless a grainy effect is desired, shooting at ISO 64 and ISO 100 has always been the recommended norm. However, each generation of digital cameras is able to use a higher ISO with less noise, and higher and higher speeds are routinely used. Some digitals have ISO speeds up to 6400 and more, and most cameras also have high ISO noise reduction (NR) circuits that can be selected to help smooth the resulting images.
ISO, ASA and DIN
ISO ratings are equivalent to the original American Standards Association (ASA) film ratings; for example, ASA 100 is ISO 100. The ISO speed may also be designated with its DIN equivalent, which is popular in Europe. For example, ISO 100 can be stated as 100/21º and ISO 400 as 400/27º.