Aperture is one of three settings any photographer should always be thinking about when creating a photograph. (The other two are shutter speed and ISO. All three settings comprise the "exposure triangle.") To understand aperture, you have to know that every lens has little blades inside that form a circle. If you look at the image above, you'll see a lens with six aperture blades forming a small hole or pupil. Depending on how you set your aperture, those blades adjust to make a smaller or bigger hole for light to enter the camera.
The basic principle of aperture is that a wider aperture (a bigger hole) will let in more light while a narrow aperture (a smaller hole) will let in less light.
Your aperture setting is represented with an f-number like f/1.8, f/2, f/8, f/11. Remember, those aperture, or f-stop, numbers denote the size of the pupil in the lens.
What may confuse beginners when it comes to aperture is that the numbers seem to be the opposite of what you'd expect. For example, f/2 is a small number, but it's actually a big (or wide) aperture. And f/16 is a bigger number, but it's actually a small (or narrow) aperture. Take a look at the diagram below, and you'll begin to understand what I mean.
You might be wondering, "Why do smaller numbers mean wider apertures and bigger numbers mean narrower apertures?" The f-number represents the ratio of focal length divided by diameter of the pupil. The equation looks like this: a = f/D, or aperture = focal length divided by diameter of pupil.
To put this in more concrete terms, I'll use the 50mm f/1.8 lens (or Nifty Fifty) that many budding photographers own. The Nifty Fifty has a maximum aperture of f/1.8, which means that the aperture blades create a hole 27.8mm in diameter when set to the widest aperture. (1.8 = 50/D, therefore D, (the diameter of the pupil) is 27.8mm.) At a setting of f/8, the Nifty Fifty now has a pupil only 6.25mm in diameter. Starting to make a little more sense? See how a small number, like f/2 or f/4, means a wider aperture opening than a bigger number, like f/8 or f/16?
If numbers aren't your thing, that's okay. You don't really need to understand the calculations above to use aperture to your advantage. Just remember, a smaller f-number means a wide-open aperture, and a bigger f-number means a narrower aperture.
Aperture, Light, and Depth-of-Field
You need to understand aperture for two reasons: LIGHT and DEPTH-OF-FIELD.
As I have said above, aperture is one way to control the amount of light that hits your sensor. The wider the aperture, the more light will enter the camera. The narrower the aperture, the less light will hit the sensor. Your lighting conditions will often dictate your aperture setting. To create an image with your desired exposure (or overall brightness), a dark scene might require a wide-open aperture, while a bright scene can accommodate a narrower aperture.
But that's a really boring use of aperture. Aperture is much more fun to play with when experimenting with your depth-of-field.
Your depth-of-field is essentially the amount of your scene that's in sharp focus. A photo with a deep depth-of-field will have a sharp foreground, middle ground, and background. A photo with a shallow depth-of-field may only have one of those three elements in sharp focus. For example, portrait photographers will often use a shallow depth-of-field to separate their subject from the background. Those great portraits you see with a clean, sharp model and a beautifully blurred-out background? That's a portrait using aperture and depth-of-field to its advantage.
How does aperture relate to depth-of-field? Wide apertures provide a shallow depth-of-field. Narrow apertures provide a deep depth-of-field. That means f-numbers like f/1.4, f/2, and f/4 will yield a shallow depth-of-field. F-numbers like f/8, f/11, and f/16 will yield a deep depth-of-field. The photo above was created with an aperture of f/2.8 using a 100mm lens.
Why do I mention the lens? Not only does aperture help determine your depth-of-field, but two other variables also contribute to your depth-of-field: your lens's focal length and the distance between your camera and the subject. I'm not going to describe how focal length and distance affect depth-of-field in this article--things start to get complicated. For more details about focal length and distance, look out for an article soon-to-be-published.
In the meantime, take a look at how changing your aperture can change the depth-of-field in a photo. The following photos are focused on the red car in the middle. Notice the difference in the blue and gray cars as well as the line of the end of the table.
Start Playing With Aperture
To begin experimenting with aperture, move your camera dial to Av or A for aperture priority mode. Now you can use the little click wheel to change only aperture and experiment with aperture's effects on your image.
As you use aperture priority mode, you might struggle with blurry photos. Blurry photos in aperture priority mode usually mean your camera used a shutter speed that was too slow. For most handheld photos, your shutter speed should probably be at least as fast as 1/50 or 1/100. To help your camera set a faster shutter speed while using aperture priority, you can either raise your ISO or use exposure compensation in the positive direction. One (or both) of those two solutions will allow your camera to set a faster shutter speed, which will make your photo less blurry.
(Exposure compensation is a feature represented by a symbol that looks like a tiny box with a plus- and minus-sign. Exposure compensation will allow you to move an arrow along a continuum. Going in the positive direction will make the image brighter, negative will make the image darker. You can tell the camera to make your photo darker or lighter by using exposure compensation to set the scene “plus” or “minus” on a scale that’s usually -3 to +3. Compensating exposure in the positive direction makes your image brighter, negative makes things darker. +1 makes your image twice as bright as your camera’s initial guess. +2 is twice as bright as +1. +3 is twice as bright as +2. The negative scale works in reverse. -1 is twice as dark as the initial guess, and so on.)
There's actually an easy rule you can follow to select your shutter speed when hand-holding your camera. To reduce blur from handheld camera shake, your shutter speed should be set at least as fast as 1/focal length. Are you using a 50mm lens? Then your shutter speed should be at least 1/50. Using a 200mm lens? Then you need to set shutter speed to 1/200 to prevent handheld blur.
Okay, so we moved away from aperture and began talking about shutter speed. That's because the three settings of the "exposure triangle" are so inter-connected. Once you start talking about aperture, you inevitably talk about shutter speed and ISO!
To Summarize Aperture and Depth-of-Field
In the end, here are the main ideas to take with you:
Your aperture is an f-number that tells you the size of the pupil of the aperture blades in relation to the focal length.
A small f-number (like f/1.4 or f/2) means a big pupil and more light, while a big f-number (like f/11 or f/16) means a small pupil and less light.
Aperture can be used to solve exposure issues by letting in more or less light.
Aperture can be used creatively to experiment with depth-of-field (the amount of your image in focus from foreground to background. A small f-number (like f/1.4 or f/2) creates a shallow depth-of-field, while a big f-number (like f/11 or f/16) creates a deep depth-of-field.
As you begin to experiment and get comfortable with aperture, you'll quickly want more and more control of your camera. You'll be confidently creating photos in Manual mode in no time.
Aaron Taylor is a stay-at-home-dad and professional portrait and product photographer. Aaron is forever blessed to be in love and married to his best friend and partner in parenting. Most of his time is spent chasing his curious, energetic kids, a three-year-old son and one-year-old daughter. Aaron lives in Columbus, Ohio. Before moving to Columbus in the summer of 2016, Aaron was a high school English and Drama teacher in Montgomery County, Maryland. He spent ten years in the classroom and earned National Board Certification in English Language Arts. Give him his family, a good cup of coffee, and a homemade cookie or three, and all is right in Aaron’s world.