The final setting of the three main light controls is ISO. (Pronounce it how you like, it’s not important) ISO is the setting that controls the camera sensor’s sensitivity to light. A low ISO value means that the sensor is less sensitive to light, and a higher value means the sensor is more sensitive to light. It’s a bit hard to explain sensitivity in terms of these number. One analogy that I’ve heard is that the ISO value is like the number of worker bees you have flying out of your camera and bringing you back light for the image.
To understand the concept, you can also try to imagine what your eyesight is like when you’ve been in the dark for a long time. You can see better at night after a while because your eyes have become more sensitive to light, and your eyeballs kind of grab onto available light more greedily. This sensitivity level of your eyes would translate as a higher ISO value in your camera. And like the what happened in our title image above, when you suddenly see a bright light at this high sensitivity, it will blind you until your eyes start to lower their “ISO” again.
Unlike Aperture and Shutter speed, the main side effect of ISO is noise. That’s the grainy, pixelated look some photos have. In the days of film cameras, some photographers could use the resulting “film grain” effect artistically, but in modern digital cameras, there really is no artistic side use for digital noise.
Don’t Fear the ISO
This makes ISO usually the last setting to be adjusted by photogs, kind of like a last ditch effort to get a bright enough shot, but there’s no need to avoid the setting like a plague. Today, most DLSR have amazing noise reduction, and the fact is that when you are shooting in a dark area, a little noise is better than not getting a picture at all. Just don’t overdo the it.