ISO

While the terms ISO and film speed may be thought of as more to do with film based photography, they also have an important bearing on digital photography too.

ISO stands for International Standards Organisation, and the ISO speed of film provides an indication of the sensitivity of film to light.

Typically, you can easily purchase films of ISO's ranging from 100-800 (100, 200, 400 and 800).

The higher the ISO number, the more sensitive the film is to light. It follows that when using a high ISO film, the shutter doesn't need to be open for as long to expose the picture correctly. (ISO 200 needs half the light that ISO 100 needs for the same exposure, and equally ISO 400 needs half the light of ISO 200.)

It may seem that it would be beneficial to use higher ISO films all the time, but this is not the case. The way the film is made means that higher ISO's result in a grainier image.

As a general rule, ISO 100 is fine for sunny outdoor shooting, while ISO 200 will give better results if you are shooting in a mixture of indoor and outdoor settings, or you are taking pictures on a dull day. ISO 400 can be used when you want to capture moving objects, or light levels are particularly low.

How does this tie in with digital cameras?

The sensor of a digital camera is effectively its film. The software within the camera can automatically adjust the sensitivity of the sensor to allow a faster shutter speed in low light. The trade off with this is that as the sensitivity increases, the noise level does too. This is displayed as a digital graininess in images particularly in large areas of similar colour.

Most digital cameras have a noise reduction system that can be turned on to as the name suggests, reduce noise. These systems are generally very effective.

Alternatively, the sensitivity in some cameras can be set to a lower ISO value. This will reduce the graininess in the image, but will increase the shutter speed. If the camera is not well supported the resulting picture may display camera shake. To avoid this, the camera should be mounted on a tripod, or sat on top of a wall.

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