
Aperture, is quite simply, the hole where light travels through. If you look through your lens, you’ll see a hole formed by a bunch of little blades. The size of the hole is expressed in f-stops, which is a ratio between focal length and the diameter of the hole. That’s why the numbers seem to be pretty weird. I won’t bore you with the math because, quite frankly, it bores me too! If you are so inclined, however, you can can look at this Wikipedia link that explains it much better than I can.
The aperture is expressed as a number prefixed with ‘f’ or ‘f/’ for “focal” (again, referring to that ratio). In the image of the camera above, it’s the number on the top right of the LCD that’s prefixed with the ‘F’. So, what is important for our discussion is this: the smaller the number, the bigger the hole; conversely, the larger the number, the smaller the hole.
You may have come across the term ‘fast lens’ at some point. That term refers to the widest maximum aperture (smallest number) that the lens can use. With a fast lens, the aperture is so wide that it allows the shutter speed to be really fast because so much light can go through the lens. Lenses that have a maximum aperture of f/2.8 or less (remember: the smaller the number, the bigger the hole) are generally considered fast lenses. These types of lenses aren’t cheap though! Many are priced in the thousands, some specialized ones are in the tens of thousands.
So why spend all that money? Why would anyone want a fast lens? Well, for a couple of reasons:
It gives you the ability to shoot in darker conditions

I shot the above photo in a very dark church at ISO1250, 1/25, f/2.8. If everything but that f/2.8 didn’t make any sense to you, suffice to say, it was really dark in there!
It gives you the ability to isolate the subject by reducing the depth of field
The size of the aperture is one of the biggest contributing factors to depth of field (DOF). A wide aperture (small f/ number) reduces the depth of field, whereas a small aperture (large f/ number) increases the depth of field. With a small DOF, a thin portion the scene in front of you will be in focus. As your DOF increases, more and more of the background comes into focus. I know all of these inverse relationships can get confusing, so I’ll let some pictures do the explaining:
small DOF (wide aperture; small f/ number)

f/2.8
f/4.0
f/5.6

f/8.0
f/11
f/16
large DOF (small aperture; bigger f/ number)
Now that you (hopefully) have a better idea of how to use aperture, you can apply the settings to your camera by switching to ‘A’ mode (or ‘Tv’ mode for you Canon users) on your camera and dialing in the desired aperture based on the DOF you want. Notice that the other number (that’s the shutter speed) changes whenever you change the aperture. That’s because aperture is only one of three components to exposure (shutter speed and ISO are the other two). Each of these components must be balanced and there are tradeoffs to be made when changing settings. Stay tuned and I’ll demystify the other settings in a further blog post.
Until then, keep shooting and share your work!