Do you know what ISO is? Perhaps not. And, in fact, you don’t need to know what it is – just how to use it. In very simple terms, the ISO setting is a measure of how sensitive your camera sensor is to light. A low ISO (e.g. 100) will not be very sensitive to light whereas a high setting (e.g. 1600) will be very sensitive.

How does this affect you?

It’s simple, and don’t let others tell you otherwise. If you have plenty of light around – say on a sunny day or if it is bright but overcast, use ISO setting 100 or 200. In fact, keep it at that setting for most of your shots. Then just take your pictures as normal.

If however, you are in low light levels, you may wish to use flash (keep the same ISO setting) OR use a higher ISO without flash.

Here is an example of practical use:

You want to take a picture of your friend. Normally, on a normal bright day, you would use programme mode or a semi-automatic mode which will select an aperture of 5.6 and shutter speed of 1/250. No problem.

But if the light is fading and you don’t want to use flash, then the shutter speed for the same aperture might reduce to 1/30 or 1/15. At slow speeds like this you will find your shots are not as sharp due to camera shake. Here, you can switch to a higher ISO setting – for every doubling of the ISO you can halve the shutter speed. Therefore you will be able to use a faster shutter speed in low light conditions with a higher ISO.

That’s it really.

There are two drawbacks. First, you will get a “noisier” image at a higher ISO setting (more grainy – but this can often create a nice effect) and secondly you must remember to switch back to your usual ISO after the shots have been taken (so that your usual images have the optimum ISO setting).

Eric Hartwell runs the photography resource site http://www.theshutter.co.uk and the associated discussion forums as well as the regular weblog at http://thephotographysite.blogspot.com.

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