Back in the day when we used film for photography, ISO (or ASA) indicated how sensitive film was to light. You measured it in numbers (such as – 100, 200, 400, 800, 1600 and so forth). The lower the number meant the lower the film sensitivity and the finer the grain in the shots you took.

Today in Digital Photography ISO measures the sensitivity of the image sensor. The same principles as in film photography apply, the lower the ISO number the less sensitive your camera is to light and the finer the grain. A higher ISO settings is generally used in darker situations to get a faster shutter speed (for example an indoor sports event when you want to freeze the action in lower light) the downside is more noise in the images.

A setting of ISO 100 is generally used as ‘normal’ and the result is sharp, crisp shots with little to no noise or grain. If you keep your camera in ‘Auto Mode’ it will select an appropriate ISO setting based on your shooting environment,meaning it will try to keep it as low as possible. Fancier manual cameras give you the opportunity to select your own ISO setting. When you do set your camera to a specific ISO you will find that it affects the aperture and shutter speed needed for a good exposure. If you bumped your ISO up from 100 to 400 you will see you can shoot at a higher shutter speed and/or smaller apertures.

Here are some general rules when considering an ISO setting:

• Use an ISO of 100 or 200 when taking photographs in well lit conditions.
• If the sky is overcast or it’s evening time, use a range of 400 to 800.
• Night or in low light you might need a setting of 1600 or your photo will appear dark.

When there is plenty of light, desire little grain, using a tripod with a stationary subject use a lower ISO setting. In contrast, if it’s dark, want to add grain, don’t have a tripod and the subject is moving, consider increasing the ISO setting enabling you to shoot with a faster shutter speed and still get a good exposure. There is always a trade off with an increase in ISO as the result is always more noise in the shot. ISO is a very important part of photography and a good grasp of it will give you better control of your photographs.