Aperture: Technical Use

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The Details

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The Hole That Controls the Light

Let’s start with the simplest possible explanation. Aperture is the opening inside your lens that controls how much light can pass through it. Inside your lens are a series of blades that open and close. You can think of them a little like the pupil of your eye.

When it’s dark, your pupil gets larger to let in more light. When it’s bright, your pupil gets smaller to let in less light. Your camera does essentially the same thing with the aperture.
Bigger opening = more light.
Smaller opening = less light.
Pretty simple, right? Then photography goes and gives us numbers that seem to say exactly the opposite.

The Weird Part: f-Numbers

Aperture is measured using something called an f-number or f-stop. You’ll see numbers like f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Here’s the part that confuses almost everybody at first.

The smaller the f-number, the larger the opening. And the larger the f-number, the smaller the opening. So: f/2.8 = big opening, f/22 = small opening. It seems backward. There is a mathematical reason for it, but for now, just remember: Small number = big hole. Big number = small hole.

Why Is It Called f/2.8?

If you’re curious about the math, here’s the simple version. The “f” refers to the focal length of the lens. The f-number is related to the focal length divided by the diameter of the aperture opening.

So if we take my favorite Canon 100mm macro lens: 100mm / 2.8 = about 35.7mm. At f/2.8, the aperture opening is approximately 35.7mm across. Now let’s stop the lens down to f/22: 100mm / 22 = about 4.5mm. So at f/22, that same lens has an opening of only about 4.5mm.

Same lens. Very different opening. You don’t need to memorize the math. I just want you to understand why the numbers work the way they do.

So Why Not Always Shoot at f/2.8?

Great question. Because… Nothing is free. A larger aperture lets in more light. That’s great. But it also changes something else: Depth of Field.

Depth of Field is basically how much of the photograph is sharply focused. At f/2.8, you can have a relatively shallow Depth of Field. That means a small portion of the scene may be sharply focused while things closer to you or farther away become blurry. That’s often exactly what you want, for example, portraits — you might focus on someone’s eyes and let the background become beautifully soft.

But imagine you’re standing at the edge of the ocean photographing a landscape. You may want the rocks in the foreground, the water in the middle distance and the distant horizon to all be reasonably sharp. A very shallow Depth of Field isn’t necessarily going to help you. So you may want to use a smaller aperture. Maybe f/11. Maybe f/16. Maybe f/22.

The Paintbrush

Here’s another way I think about it. This is not a technical definition. It’s just a way to visualize what aperture is doing creatively. Think about painting.

A big paintbrush can cover a lot of area quickly, but it’s not very good at painting tiny details. A small, fine-tipped brush lets you work with much more detail. I think about aperture somewhat like that. A large aperture, such as f/2.8, gives you a relatively narrow area of focus. A smaller aperture, such as f/16 or f/22, gives you much more Depth of Field to work with. Again, the paintbrush isn’t what aperture technically is. It’s just a mental picture.

Why I Tend to Use Smaller Apertures

I’m a landscape guy. And I love photographing the ocean. So I often want a lot of the scene to be sharp. That means I tend to favor smaller apertures. f/16, f/18, f/22. You’ll see me use those a lot, because of the kind of photographs I make.

But notice something important. I’m not saying: “Landscape photography should always be shot at f/16.” That’s not how this works. I’m saying that I often want a lot of Depth of Field, so a smaller aperture can help me get what I want. Your photograph may need something completely different.

There Is Another Reason I Love f/22

Here’s a fun one. Look at a photograph with a small, bright point of light in it. The sun. A streetlight. A bright light in a night scene. At very small apertures, the physical shape of the aperture blades can cause those points of light to produce beautiful starbursts.

That is one of the reasons I sometimes love shooting at f/22. I’m not just controlling the amount of light. I’m using the aperture to create a visual effect. And that’s an important distinction.

Technical vs. Creative

Technically, aperture controls the size of the opening through which light enters the camera. That’s the technical answer. Creatively, aperture gives me control over Depth of Field. That’s where things get interesting.

I can use a large aperture to isolate a subject. I can use a smaller aperture to keep more of a landscape sharp. I can use a very small aperture to create starbursts around bright points of light. Same control. Different reasons for using it.

The Trade-Off

Remember our Exposure Triangle? Changing the aperture changes the amount of light entering the camera. But it also changes Depth of Field. So every time you change it, you’re making a trade.

A larger opening: More light. Less Depth of Field.
A smaller opening: Less light. More Depth of Field.
And there can be other trade-offs too. Very small apertures can eventually introduce diffraction, which can reduce overall sharpness. So even f/22 isn’t automatically “better” than f/8. There is no magic setting. There is only the setting that makes sense for the photograph you’re trying to create.

How I Think About Aperture

When I look at a scene, I don’t ask: “What aperture am I supposed to use?” I ask: “How much of this photograph do I want in focus?”

If I want a shallow area of focus, I’ll consider a larger aperture. If I want more of the scene sharp, I’ll consider a smaller aperture. Then I look at the light. Because remember: Nothing is free. If I stop down to f/22, I’m letting in much less light. That means something else may need to change. Maybe my shutter speed gets slower. Maybe my ISO goes higher. That’s the Exposure Triangle at work again.

Try This

Take your camera outside. Find something relatively close to you with something interesting in the background. Put your camera in manual mode. Start at the widest aperture your lens allows. Take a photograph.

Now stop down a little. Take another. Keep going. Try f/4. Then f/5.6. Then f/8. Then f/11. Then f/16. If your lens allows it, try f/22.

Don’t just look at whether the photograph is correctly exposed. Look at what is in focus. Look at the background. Look at the foreground. Look at how much of the scene is sharp. You’re not trying to find the “right” aperture. You’re learning what the aperture does. And once you understand what it does, you can start deciding when you want to use it.

The camera gives you the aperture.
You decide what to do with it.

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