A Beginner’s Guide to the Aperture

by Kerry Garrison ; Updated October 17, 2017

The aperture is likely the least understood setting on your camera. Shutter settings, for example, are relatively easy to understand — too long a shutter speed results in blurring. Same with ISO: Too high an ISO setting and you introduce digital noise. But learning how to use aperture properly is often confusing and vague. This week, I’ll try to explains it in way that doesn’t make your brain hurt.

What is this aperture thingy anyway?

Inside each lens is a diaphragm that can open and close. The size of the opening at any given setting is what is referred to as the aperture setting. On cameras, you refer to specific size settings as f-stops such as f/2.8, f/5.6,f/11 and such. While the obvious value of adjusting the aperture is to control how much light enters the camera, a secret hidden world lies beneath the hood that you need to understand. Let’s start with the basics.

Aperture settings

For a given ISO setting and shutter speed, you can adjust the amount of light entering the camera by adjusting the aperture. A smaller diameter (higher f-stop value) will allow less light in, while a larger aperture (smaller f-stop number) will allow more light in. If you refer to the Sunny 16 rule, you know that on a bright sunny day, if you are using ISO 200 and a shutter speed of 1/200th, then you will get a good exposure at f/16. As the sun goes down and there is less light, you need to allow more light into the camera. You can do this by opening the aperture up (again, using a smaller number) to allow more light in. This sounds easy enough right?

Image showing relative sizes of aperture

Image showing relative sizes of aperture

Have you collimated your rays lately?

The opening of the diaphragm also controls the cone angle of light coming into the camera. This is why adjusting the aperture can affect the depth of field. To see how this works, use your thumb and pointer finger to make a big circle. With one eye closed, look through the circle at an object about 12 inches away. This simulates a large aperture: You will be able to focus on the finger, but you will have far less focus on something across the room. Next, make the circle very small and look through it again (simulating a small aperture), you can focus on both your finger and something across the room equally well. The reason for this has to do with the angle of light coming into your eye. With the large circle, light is able to come in very straight which creates a shorter focal plane, with a smaller aperture opening the light comes in as a cone shape giving a longer focal plane. While this simple experiment has a very subtle effect (not everyone may even notice it) in your camera it can have a dramatic effect on how your images look.

Using aperture to control light

Factors that determine depth of field

Three factors determine the amount of depth of field you will have in your image. Taking from a comment post from Photo Larry, this points out the different factors:

  • Distance from the subject (close=shallow, far=deep)
  • Focal Length (short=deep, long=shallow)
  • Aperture (small=deep, open=shallow)

Aperture Setting
This one’s already covered: The larger the opening (smaller f-stop number), the shorter the depth of field.

Focal Length
The longer the focal length, the shorter the depth of field. This is why you will get more blurring effect on a telephoto lens than you will with wide angle lens. The most popular portrait lens is the 70-200 f /2.8 because as you back up from the subject, and use the longer focal length with a large aperture, the more the background will get blurred out.

Distance to Subject
The further you are away from the subject, the more depth of field you will have as well. This is much easier to see with a long telephoto lens. If you focus on something very close to you, much of the background will be blurry. As you focus on things further away, less of the foreground and background will be blurry.

aperture focus
using aperture to focus

Two images with different aperture settings

The basics of your aperture setting can help you begin to use it to your advantage, both in getting enough light into your camera and by being able to add some extra flair to your images using the depth of field.

Photo credit: Kerry Garrison