In the days of analog photography, ISO meant how sensitive film was to light. You would choose a roll of film (100, 200, 400 ISO) and had to shoot all images with it. The higher the number, the more sensitive to light the film was.
In digital photography, we also have to take this setting (the sensitivity of the image sensor to light) into account. In the same way, the higher the ISO, the more sensitive to light the sensor is.
You typically want to increase the ISO when you have poor light conditions. If you make the sensor more sensitive to light, it will correspondingly capture more light in the photo and allow you to use faster shutter speeds. The great advantage with digital is that you can change the ISO from one photo to the next, and experiment.
There is a price to pay when you use a higher ISO setting and that is noise (grain in the analog days). When this happens, you will start to notice little specks in the image as you can see in the image below. This can be a stylistic option for some, but in most cases you will want to avoid it.
Bumping up the ISO can be a real help in some situations where there is nothing you can do about the existing light. That is the case of a live show, for example. Imagine your lens is already at maximum aperture, say f2.8, and your shutter speed is at the minimum to freeze the action, say at a 125th of a second. The only option left is to increase the ISO, if you want better exposed images. You must find a compromise between correct exposure and amount of visual noise. If the alternative is not having any images, I don’t hesitate to increase the ISO. At least I’ll have something to show for.
As with all things related to photography, experimenting is key to achieving better results. If you can keep the ISO low, all the better. But remember, it can save you in those low light situations, so don’t be afraid of using it.
Has ISO saved a photo of yours? Share in the comments. I’d love to hear your thoughts.