How aperture is measured

Shutter speed is a fairly easy thing to understand, because we are all familiar with fractions of time. Minutes are fractions of an hour, seconds are fractions of minutes, the line at the DMV is a fraction of your life you will never get back, and so on. Aperture is a little more complicated. You have seen how the iris in your lens closes down to block out more light. What makes aperture a little less easy to understand than shutter speed is that when it comes time to specify the size of the aperture's opening, we are talking about measuring the area of the circle, and that's something most of us don't have much experience with.

Over time, you will simply memorize what we are about to see here. Aperture sizes are denoted with an f- s top number, and the bigger the number, the smaller the aperture. What we are seeing here is a list of standard aperture sizes, ranging from a fairly wide f/2.8 to a fairly small f/16. What you can't tell from looking at the numbers, or at the size of the openings for that matter, is that these apertures are each one stop apart--that is, each smaller aperture lets in half as much light as the previous aperture.

Now, here is the depth of field bit: wider apertures yield shallower depth of field. So in this chart, the wide open f/2.8 aperture, will have very shallow depth of field, while the narrow f/16 aperture will have very deep depth of field. So, say I am shooting a portrait, and I want to blur out the background. I will choose a large aperture, which means smaller f-stop number. If I am shooting a landscape and want very deep depth of field, I will choose a small aperture, which means larger f number. In the old days, these whole stop apertures were all that your camera might have offered, but digital cameras offer apertures in 1/3rd-stop increments.

So what you will see on your camera is a progression like this. This aperture depth of field thing can be a difficult thing to learn, because you can't reason it out on your own without knowing a lot of physics and math. Again, over time, you will simply learn all of this by rote. If you wear glasses though, you have got kind of a built-in reference card for remembering whether smaller apertures yield more or less depth of field. Here is how it works. You've got to take your glasses off, so now I can't see anything. If I take my finger and curl it up into a tiny little aperture, a little hole, and look through it, the world will be much, much sharper.

What's going on here is I am giving myself a tiny little aperture, and that's dramatically increasing my depth of field. I am actually turning my eye into a pinhole camera. One of the kind of defining characteristics of pinhole photography is that pinhole pictures have infinite depth of field, because a tiny little aperture gives you infinite depth of field, and that depth of field correction is what's allowing me to see. But now, it's time to actually learn how to control the aperture in your camera.