Ever wondered what ISO meant and what all those numbers denote? Simply put, ISO indicates the sensitivity of the film, or in the case of digitals, of the sensor to light energy. It may sound overly technical, but you don't have to be a physicist to understand it.

For starters ISO stands for International Organization for Standardization, which isn't exactly the best acronym, and it doesn't seem to pertain to light and photographs, except it's a slightly arbitrary denotation that simply standardized the figures showing how the fast it takes the sensor to react to light.

There are equations and logarithmic scales for those who really want to get into the nitty gritty, but for photographic purposes; you only need to remember that on your camera, an ISO of a low number means it is less sensitive to light while a high number means the opposite. So an ISO of 100 means the sensor or film needs more light for it to react and the image to register. This is good for sunny bright days where there is plenty of light. A high ISO is good for dim light situations.

The problem with a low ISO is that you often need a longer exposure time, so slower shutter speed or lower aperture to capture a good image. It means your subject will have to remain fairly still otherwise you'll get a bit of a blur. On the other hand, the higher the ISO, the more the sensor will register light energy not from your subject. Light tends to bounce around things, and light energy that is scattered from your surroundings can mess up your picture somewhat. It's called noise and makes the photo look grainy.

As a rule of thumb, low ISO is good for bright situations or when things aren't moving much. High ISO is good for low lights or when things are jumping like Mexican beans. The ISO scale gives photographers more room to be creative, judging when the situation might require an ISO of a 100, 200, 400 or up to 1600 and 3200. ISO works together with shutter speed and aperture and gives plenty of room for experimentation. Some cameras are pretty good at noise reduction and you can leave the ISO on auto, letting your camera judge which ISO would be best, unless you are trying to be creative and want to get a particular effect.