In the simplest of terms, aperture is a hole that allows light through. This also happens to be the third part of the exposure triangle, and can be one of the hardest concepts to grasp. Aperture is measured in “f-stops” and the higher the number, the smaller the opening. For example f/4 is a larger opening than f/22. Moving from one f-stop to the next doubles or halves the amount of light that gets in.
Aperture also controls the “depth of field” (DOF) or the parts of the image that are in focus. Having a large depth of field means most of the image, foreground to background, will be in focus. This means a relatively small aperture was used, like f/16 for example. A small, or shallow depth of field means only a small part of the image is in focus and the rest will be fuzzy. To get this kind of look, you want a big or wide aperture; something around f/2.8.
This can be a difficult concept to grasp. It’s somewhat opposite of what you think. But something that helped me when I was first learning it is small numbers means small DOF and large numbers means large DOF.
Like with shutter speed, different styles of photography favor deep or shallow DOF. More often than not you’ll find landscape photographers using small apertures in ensure the whole scene is in focus, from the foreground all the way back to the horizon. Portrait and macro photographers will use wide apertures to isolate the subject by allowing the background to fall out of focus.
I hope you have found this helpful. Next week we’ll combine the three parts of the exposure triangle and talk about how they’re related to each other.