Following my post on shutter speed, the next item in the trifecta of exposure control is aperture. The aperture determines how much light passes all the way through the lens. It’s a diaphragm located near the back of the lens that opens and closes (although it never closes all the way) and it works just like your eye’s pupil. In the dark, your pupils dilate to let as much light in as possible. If you go from a dark room to bright sunlight, your pupils will constrict to little pinpoints, limiting the amount of light that enters your eye. You can adjust your camera’s aperture to be wide open (like your dilated pupils) which is better for taking photos in low light, or you can make it restrict almost all of the light, only letting in a little bit – best when you’re in bright sunlight.
The size of the aperture is measured in increments called f-stops. The language of aperture is confusing because the larger the actual opening, the smaller the f-stop, or f-number. It’s counter-intuitive, but it’s because the f-number is a ratio between the actual size (in mm) or the aperture opening and the focal length (in mm) of the lens. At any rate, you may have seen numbers like f/2.8, f/3.5, f/5.6, f/8, etc. All you really have to know is the lower the f-number, the larger the aperture and the more light is passing through the lens. Lenses that can allow more light through, therefore having a smaller maximum aperture number, are considered faster, because they can capture images in lower light settings.
But remember, the larger the actual aperture size (more light getting through), the lower the f-number (like f/1.4, f/2.8).
A lens will list its largest available aperture right on the lens, but in order to make it even more confusing, it will most likely use a different format to list it. If you look at your lens and see 1:2.8, that means that the largest aperture you can shoot with that lens is f/2.8. If your lens has two f-numbers (like 1:3.5-5.6) then you have a zoom lens with a variable minimum aperture. The two f-numbers refer to the largest available aperture at each end of the zoom. For example, if you have a 17mm-55mm zoom lens that lists 1:3.5-5.6 on the barrel, it means that at 17mm the largest aperture available to you is f/3.5, and at 55mm, the largest aperture available to you is f/5.6. As you zoom from 17mm to 55mm, your minimum f-stop will vary between f/3.5 and f/5.6.
So what’s fast? You’ve may have heard of people talking about fast lenses. Generally speaking an f-number like f/1.4 or f/1.8 is considered really fast. That means that the aperture can let in a lot of light – making it a great lens for low-light situations. But low-light is only half of the story with a fast lens.
But wait, there’s more!
There’s another really important thing about the aperture that makes it special – it affects depth of field. Basically, depth of field is how quickly the background of an image gets out of focus. See the example photos below. The one on the left was shot with an aperture of f/3.5, and the one on the right was shot with an aperture of f/22. Notice the two glasses, the pitcher and the background and the level of focus between the two shots.
So a low f-stop number gives you a sharp focus on the subject and a really out-of-focus background. The slower the lens, the less dramatic the out-of-focus background will be. There are other factors that affect depth of field, but i’ll talk about those some other time.
You can experiment in making adjustments to your aperture by putting your camera in aperture-priority mode. Most cameras have this option, and it lets you choose the aperture while the camera adjusts all the other settings to compensate the exposure.