Think of aperture as the hole that controls the amount of light that enters your camera. It works very similar to your iris; it opens to let in more light, and closes to let in less.
The size of the aperture (the opening) is written in what we call “f-stops”. For instance, if the specs of a lens is “50mm ƒ/1.4″, ƒ/1.4 is the widest aperture of that lens. Unlike shutter speed and ISO, aperture is counterintuitive. The smaller the f-stop, the wider the aperture, letting in more light. The following is a list of f-stops in 1-stop increments, from bright to dark:
ƒ/1, ƒ/1.4, ƒ/2, ƒ/2.8, ƒ/4, ƒ/5.6, ƒ/8, ƒ/11, ƒ/16, ƒ/22
Here is a more visual diagram showing you the correlation between f-stop and aperture:
Just to make it clear, an ƒ/2.8 gathers twice as much light as an f/4 lens because it is a stop faster. You will probably end up memorizing these numbers because they are very useful. Again, like shutter speed, your camera will be able to adjust aperture in 1/3 of a stop increments
Remember, any lens that has the same f-stop gathers the same amount of light regardless of focal length. Therefore, if you are shooting an object at ƒ/4 with a lens and later switch to another lens and shoot at ƒ/4, the exposure should be the same as long as the lighting is the same.
Variable Aperture Lenses
Some lenses will give you a range for the f-stop and say “18-55mm ƒ/3.5-5.6″. That kind of lens is called a variable aperture lens, since the widest aperture changes throughout the focal length of the zoom, and is it common with cheaper lenses. The widest focal length is always the brightest, so this lens the widest aperture is ƒ/3.5 on the 18mm end and ƒ/5.6 on the 55mm end.
Depth of Field
Technically speaking, a lens can only focus at one distance, though since the falloff for sharpness isn’t extreme, objects a bit closer and further from that distance can still be reasonably sharp. That wiggle-room for objects in focus focus is called the “depth of field” (DOF), and it is significantly affected by the aperture.As you open your aperture wider, less things will be in focus, so you would say that the DOF gets shallower. Here’s a are several photos I took with the same exposure and composition, just different aperture. Put your mouse over the labels underneath the photo to see the change in DOF.
ƒ/1.4 | ƒ/2 | ƒ/2.8 | ƒ/4 | ƒ/5.6 | ƒ/8 | ƒ/11 | ƒ/16