Fear not, it’s not as complicated as you may have been lead to believe. Let’s talk first about the word Aperture. The simplest definition of Aperture is “opening”. So when we set an f-stop on our camera we are simply controlling the size of the opening that the light will pass through in our lens on the way to the sensor (or film).
Now the easiest way to address this is to talk a little bit about fractions. Ok, I understand that some of you just went to hide under the bed because you’re a bit insecure if we talk about anything mathematical; so let me try to help. If I say that the larger the denominator the smaller the fraction, some of you are convinced that I’m speaking Greek. So let’s make it as simple as possible, the bigger the number on the bottom of a fraction, the smaller the fraction.
So, if you’re really hungry, you might eat ½ of a pizza, but otherwise you might only eat ¼ of that pizza. Well, if you set your lens to ½ open you’re letting more light in than if you set it to ¼ open. So, if you convert this discussion to f/stops it would be the equivalent of using f/2 to let more light in or f/4 to let less light in. By now I hope you see where I’m going with this discussion. If we substitute the “f” in f/stop with the number “1”, your f/stop becomes a simple fraction. Therefore the f/stop representing a fractional opening controlling how much light passes through your lens.
Let’s talk about common f/stops for just a moment so you’ll be able to relate this directly to your camera. Each of these f/stops is what we call a ‘full stop’ – f/2.8 • f/4 • f/5.6 • f/8 • f/11 • f/16 • f/22, and this is the most common range you’ll find on your camera. However, if you sit down with your camera and scroll through the f/stops, you’ll find a range that looks something like this; f/2.8 • f/3.2 • f/3.5 • f/4 • f/4.5 • f/5 • f/5.6 • f/6.3 • f/7.1 • f/8 • f/9 • f/10 • f/11 • f/13 • f/f14 • f/16 • f/18 • f/20 • f/22 • f/25 • f/29… DON’T PANIC, we’re still on the same page. The list of ‘full stops’ that I listed above still stands, and everything you see in between those stops are 1/3 stops. These 1/3 stops give us finer control over how much light we allow through the lens.
Now, here’s the important thing to remember about these stops; for each ‘full stop’ that we change our aperture we either double or cut in half the amount of light coming through the lens. So, if we stop our lens down by one stop, say from f/8 to f/11, we are now allowing ½ as much light to come through the lens. I refer to this as “stopping down” because we are closing the aperture down to a smaller size. Conversely, if we open our aperture by one stop, say from f/16 to f/11, we are allowing twice as much light to come through the lens.
Now you may be wondering why we would change the aperture in the first place and that’s going to take just a little discussion to make clear.
First of all, let’s translate this to something that you can relate to easily. If you want to see something far away, you squint to bring it into focus. Well, I’ve got news for you; you just changed the aperture on the lens in your eyes. In short that’s the same reason we change the aperture on our camera lens. With a smaller aperture we have the ability to focus over a greater distance. That gets into a discussion on Depth-of-field, which I write about elsewhere.
It is important to note that if we close our lens (stop down) 1 f/stop, we reduce the amount of light making it to the sensor by ½. Therefore, if we had a proper exposure to begin with, we now have to expose the image for twice as long in order to get a proper exposure. So you can see that changing our f/stop is directly related to our shutter speed. We generally will change both, aperture and shutter speed, to maintain proper exposure. In order to get more familiar with this, I recommend that you set your camera to “Aperture Priority” and practice shooting at various f/stops, to see the effects on your images. While doing this pay attention to the shutter speed the camera selects for each f/stop. This will give you a good deal idea of how the two are related.
On another note: if you want to slow down your shutter speed, or speed up your shutter speed, you can change your f/stop to allow less or more light in, as appropriate. For instance, if you want to make an attractive photo of a fountain, it’s best to use a slow shutter speed. By reducing the size of the aperture, we reduce the amount of light coming through the lens, creating the need for the light to come through for a longer period of time (a.k.a. a slower shutter speed). Conversely; if you want to stop action on a fast moving subject, you may have to open the aperture to allow more light to come through the lens. By doing so, you can use a faster shutter speed, producing the ability to stop the action of your subject.
The images below are of a diaphragm from a camera lens, set to two different f/stops, one stop apart. The image on the left, the smaller opening, is set to f/16, while the image on the right is set to f/11. Opening the aperture by this one-stop, doubles the amount of light making it through the lens.
©2010 Jeff Cowell, jeffcowell.com