Two weeks ago we talked about shutter speed. This week it's all about ISO. Most automatic cameras will allow you to play with ISO. Get out your manual (yes really) and see if this option is available for your camera, it's something everyone can use to improve their picture taking skills. My Canon IXUS has an ISO range from 80 - 1600.
ISO stand for "International Organisation for Standardisation" and comes from the greek work isos, meaning equal. I find it easiest to understand ISO if I think about film photography. Film photography is based around the concept of light passing through a material (film) and the film absorbing the light to differing amounts to create a 2D image. Here there are two things interacting with each other - the amount of light reaching the film and how reactive the film is to light. ISO describes the second element - the films potential to absorb light. The higher the ISO, the better the film is at capturing light. Put another way - film with a high ISO allows you to take photographs in situations without much light. If you are using a film camera, as a general rule, you cannot simply "change" the ISO but need to choose different films with different characteristics.
Photo 1934 by Marianne Breslauer
Now, lets convert that knowledge into digital photography. The same concepts can be applied here. The higher the ISO, the more light sensitive the camera is and the better it is at taking pictures in dark situations. A fortnight ago we talked about the relationship between shutter speed and camera shake, and how without a tripod, at slow shutter speeds your photos will be out of focus due to your hands natural shake. One way to get around this in dark situations is to increase the ISO. The camera will now need less light to capture the same image and you can use a faster shutter speed (and therefore avoid a tripod and blurred photos).
ISO 800, f/1.4, 1/160
ISO200, f/1.4, 1/25
The above pictures aren't the greatest of subject material, or quality, but I was a little rushed lastnight trying to finish off this post (oops). But they still illustrate the point I wanted to explain. In the first photo at ISO800 I could use a shutter speed of 1/160, whereas to capture the same image with my ISO set to 200 I needed a shutter speed of 1/25.
The downside of a high ISO is that you sometimes get grainy images. This is commonly referred to as "noise".
Next Week: Capturing Reflections