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iso is the speed or sensitivity that your film or sensor reacts to light, understanding iso will allowyou, on a good digital camera to adjust the iso shot by shot, a great advantage over the traditional camera where you would have to change the film for one with a different iso rating.
Changing from 100 to 1600 iso will result in having 16 times more light travelling to the sensor or film, making it possible to take photographs in low light without flash when it is unacceptable or unwanted ,as in theatres or churches.
|iso||exposure at f8 for example|
|200 iso||30 th||f8|
|400 iso||60 th||f8|
|800 iso||125 th||f8|
|1600 iso||250 th||f8|
Note, that in these cases the aperture has not changed allowing us to photograph a rock concert or theatre piece with a hand held 200 mm lens at 250 th of a second with a 1600 iso setting, instead of having great difficulty hand holding a 35mm lens at 100 iso.