Understanding ISO

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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 the iso is simple but please refer to your instruction book for how to do it as cameras vary from make to make. 

When the iso is a low value the lens must be set to pass more light by increasing the aperture or the exposure time. A high value enables the aperture or exposure time to be reduced.

When a short exposure time is needed, to cope with camera shake or to shoot a moving subject a higher iso setting will help.

One must think of iso as a way of increasing or decreasing light intesity. However as the iso rises a small loss of quality will be found for each increase.

The normal iso range on a good digital camera will be 100 ,200,400,800, and 1600 representing a change of one stop of aperture each time.

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.

This table demonstrates the relationship between iso ,aperture and exposure. This is imeritive to understanding iso and it's possible uses

iso	exposure at  f8 for example
100 iso	15th	f8
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.

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