Sometimes things are just easy. We point our camera at our family and get a perfectly exposed picture. In AUTO mode. Why on earth would we want to complicate our lives by using anything else?
Let’s step back a moment… What is exposure anyway? Well, let’s see. We’ve got something at the back of the camera that’s sensitive to light, probably a digital sensor but maybe film if we’re into that: the more light that falls on it, the brighter (whiter) the picture. But we want our faces to be pretty much a mid tone (if we’re from the middle latitudes, or a bit darker if we’re from Africa, or a bit lighter if we’re Nordic) thanks very much, not something as bright as the sun, so the camera’s smarts play around with three things to sort that out for us: how sensitive the sensor is to light, and how much light falls on it by varying the time the shutter is open and how much light passes through the lens. So on a nice bright summer day the camera might set a low sensor sensitivity, set the shutter to be open for 1/100 second, and the aperture — the hole in the lens which controls the amount of light that gets through — to something quite small, known as f 16 (or more correctly f/16, but we’ll get to that later). Or it might choose a larger hole (a larger aperture), which lets more light though, but reduce the amount of time of the shutter is open for so the total amount of light which reaches the sensor is the same as before: big hole, short time or small hole, longer time. It’s the same thing.
So that’s fine. The camera’s got a whole bunch of different options it can choose, and a whole bunch more if we let it vary the sensitivity as well. And it does it all for us in AUTO, so why do we care?
And here’s where it gets interesting. Because the shutter speed, the aperture, and the sensitivity affect other things, things that, assuming you’ve nailed the exposure, make the difference between a great shot and a one that your friends will just flip on through.
And if we care about that then that’s where the those other camera modes start becoming worth the effort.