Back to the Basics Part III: ISO

Back to the Basics Part III: ISO

What is ISO?

ISO in the photographic world applies to both digital and film photography. For film cameras and photographers, film refers to the sensitivity of the chosen film medium when exposed to light. It is measured in numbers in a stepped sequence, like this: 100, 200, 400, 800. This was also an indication of how much grain would be in an image, the higher the sensitivity, the higher the film grain. When you use a lower ISO film the film grain becomes much finer.

In the digital world ISO applies again to sensitivity, but this time to the sensitivity of the imaging sensor. The same principles for choosing film ISO, applies to the digital world. A change to the ISO system exist though; the grain in your images is known noise.

How to use ISO

If the scene or you wish to photograph is well lit then it is a good rule of thumb to try to shoot as low an ISO as possible, on most modern digital cameras this will be 100, 50 on some higher end models. If it is dark and you do not have a tripod or want a fast shutter speed, a higher ISO speed can be used to capture more information in the chosen shutter speed. This however comes at a price, that prices is your image will have more grain (film) or noise (digital). This will soften your photographs and add seemingly random coloured pixels to your image. I have illustrated this below with two different pictures.
The bus drivers seat, was taken at a high ISO, 640 to be exact, you can see in the shadows the noise. Click on the image to expand.

Now for companion's sake look at this photo of a downtown street. It was taken at 100 ISO, if you click and expand you can see the low appearance of noise.