Yes, the numbers given to aperture size can be a little confusing, so I will explain, because I do not want you ignoring your aperture just because the awkwardness of the numbers.

Understand that those numbers are a fraction: f/4 (sometimes written 1:4). So, just as one half is bigger than one fourth, so f/2 is bigger than f/8. So, a wider, more open aperture will be a smaller number. And note, the aperture is usually called “f-stop”, which is sometimes written ‘f/stop’.

Now, why don’t those numbers follow a more logical sequence, like the film speed does (1/125th of a second, 1/500th, 1/1000th)? A friend and I sat down once to answer that nagging question for ourselves, once and for all. I do not want to dump the math on you (you can look that up yourself, and it is really beyond me anyway), but basically, to halve or double the area of a circle (aperture) you must multiply or divide by the square root of 2 (about 1.4). So, now look at those f-stop numbers and see if they make a little more sense: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and so on. No, I do not expect you to be doing the math in your head as you shoot; just memorize those numbers and life will be easier.

Each number in that list represents a halving or doubling of the light, and each halving or doubling is called a “stop” of light. So, two stops down from f/4—do not think “down” in numbers, but down in the amount of light—is f/8, and that would be cutting the light in half twice. Read that last sentence again, slowly and make sure you have it.

So, if you start at f/2 and you need to “stop your aperture down three stops”, you would be at f/5.6. “Bring it back up a stop” and you are at f/4.

Aperture can be a pain, I know. But it really is one of the most important adjustments you make when taking pictures (and I have not even mentioned the true importance in this entry, for fear of going beyond the original question). We cannot let a little string of numbers get in our way when shooting, so ingrain that data till it is second nature, and especially the idea of stops of light.