Glossary: what is ISO Sensitivity?

The sole purpose of adjusting your camera’s ISO is to increase or decrease its sensitivity to light. The higher the ISO setting, the more sensitive the camera is to light. Each setting is twice that of the previous setting, for example my Canon EOS 20D has the following ISO settings: 100, 200, 400, 800, 1600, 3200. So let’s say you are taking a photo of a puppy in your living room… Though you may think the room is plenty bright, compared to natural sunlight available outside your living room is likely relatively dark. When shooting indoors with my Canon 20D, I usually crank the ISO up to about 400 so I can use a fast shutter speed to avoid camera-shake or motion blur. To give you a better idea, typical camera settings for me indoors (in a well-lit room using just the available light) would be ISO 400, 1/80, f/2.8.

ISO Crops

So why not just use ISO3200 with a really fast shutter speed? With film, as you increase film speed (ISO), image quality diminishes and more and more grain is introduced. In the digital world, the same thing happens, but this ‘grain’ is noise – and it’s arguably less attractive than its film counter-part. In the image below, you can clearly see that as the ISO increases, so too does noise.

A trusty guideline here is what is commonly referred to as the Sunny 16 rule. If you’re taking photos outdoors and it is nice and sunny, and you’re not sure what camera settings to use, you can set your aperture to f/16 and then simply match up your shutter speed and ISO. If they won’t match up exactly, just make them as close as possible (i.e. ISO 50, shutter speed 1/60). There’s a little more to the rule than just that, but once again we’ll save that for another time.