Aperture is one of the most important settings in photography. It controls how much light comes through the lens, and it also controls how much of your photo looks sharp from front to back. It is the setting behind portraits with soft, blurry backgrounds and landscapes that are sharp from the flowers at your feet to the mountains far away. The numbers can look confusing at first, but the idea behind them is simple.
What is aperture?
Aperture is the adjustable opening inside a lens that light passes through on its way to the sensor. It is made by a ring of thin metal blades, called the diaphragm, that can open wide or close down to a small hole.
It works just like the pupil of your eye. In a dark room your pupil opens wide to let in more light. In bright sunlight it becomes small. A camera's aperture does the same job, except you can choose its size.
- A wide (large) aperture is a big opening. It lets in more light.
- A narrow (small) aperture is a small opening. It lets in less light.
F-numbers: why smaller means bigger
Aperture size is written as an f-number, such as f/2.8 or f/16. This is the part that confuses almost every beginner: a small f-number means a big opening, and a big f-number means a small opening.
The reason is that the f-number is really a fraction. The "f" stands for the lens's focal length, and the f-number tells you what fraction of the focal length the opening is. f/2 means the opening is one-half of the focal length. f/16 means it is one-sixteenth. Just as 1/2 of a pizza is much more than 1/16 of a pizza, f/2 is a much bigger opening than f/16.
Example: on a 100 mm lens, f/4 means an opening about 25 mm wide (100 ÷ 4), and f/16 means an opening only about 6 mm wide (100 ÷ 16).
Tip:Read the f-number as a fraction: f/2 is "one-half", f/16 is "one-sixteenth". Then it is easy to remember that f/2 is the bigger opening.
The f-stop scale
The main f-numbers form a standard scale. Each step to the right halves the light. Each step to the left doubles it. In other words, each step is one stop, as you learned in understanding exposure.
| f-number | Opening | Light compared with the step before |
|---|---|---|
| f/1.4 | Very wide | Starting point |
| f/2 | Wide | Half as much light |
| f/2.8 | Wide | Half again |
| f/4 | Medium-wide | Half again |
| f/5.6 | Medium | Half again |
| f/8 | Medium-small | Half again |
| f/11 | Small | Half again |
| f/16 | Small | Half again |
| f/22 | Very small | Half again |
The numbers look odd because each step multiplies the previous number by about 1.4. That is what it takes to halve the area of a circle. You do not need to calculate it; just learn the sequence. Your camera will also show in-between numbers like f/3.5 or f/6.3, which are one-third steps.
Every lens has a limit. A lens marked f/1.8 can open up to f/1.8 at most. This widest setting is called its maximum aperture. Lenses with very wide maximum apertures, like f/1.4 or f/1.8, are often called "fast" lenses because they let in so much light.
The second job of aperture: how much is in focus
Aperture does more than control light. It also changes how much of the photo, from near to far, looks sharp. This range of sharpness is called depth of field.
| Wide aperture (e.g. f/1.8) | Narrow aperture (e.g. f/11) | |
|---|---|---|
| Light | More light | Less light |
| Depth of field | Shallow: only a thin slice is sharp | Deep: much more is sharp from front to back |
| Background | Soft and blurred | More detailed |
| Typical use | Portraits, low light, separating a subject | Landscapes, groups, architecture |
Imagine a person standing in a park. At f/1.8, their face is sharp while the trees behind them melt into a soft blur, so the person stands out. At f/11, the face and the trees are both much clearer, so the photo shows the whole place.
Aperture is only one of the things that control depth of field. Distance to the subject and the lens you use matter too. You will study depth of field fully in focus distance, depth of field and subject separation and shallow and deep depth of field.
Is there a "sharpest" aperture?
Lenses are not equally sharp at every aperture. Many lenses are a little softer at their widest setting. At very small apertures, such as f/16 or f/22, a different effect called diffraction makes the whole photo slightly softer. For many lenses, the clearest detail is somewhere in the middle, often around f/5.6 to f/8, though it depends on the lens. You will learn more about this in optical limitations.
This does not mean you must avoid f/16. If you need a lot of depth of field, a tiny bit of softness is often a fair trade.
How to choose an aperture
- Decide what should be sharp. Only your subject? Choose a wide aperture. The whole scene? Choose a narrower one.
- Check the light. In dim light, a wider aperture helps you avoid slow shutter speeds or very high ISO.
- Set the other controls. Adjust shutter speed and ISO to get the brightness right.
The easiest way to control aperture yourself is Aperture Priority mode (A or Av), where you choose the aperture and the camera sets the shutter speed. You will learn it in exposure modes.
Common mistakes
- Mixing up big and small: f/2 is a big opening, f/16 is a small one.
- Using f/1.8 for a group photo: with such shallow depth of field, people at the back can be out of focus. Use a narrower aperture for groups.
- Always shooting wide open: the blurry background looks nice, but sometimes the setting around your subject is part of the story.
- Using f/22 for everything in a landscape: you may lose sharpness to diffraction; f/8 to f/11 is often enough.
How professionals use aperture
Professionals choose aperture for what it does to the picture, not only for light. A portrait photographer may pick f/2 to separate a face from a busy background. A landscape photographer may pick f/8 or f/11 for front-to-back detail. An event photographer in a dark hall may open up to f/2.8 simply to get enough light. They know which job the aperture is doing in each photo, and they adjust the other controls around that choice.
Practical examples
A portrait in a busy park
- Situation
- You photograph a friend in a park full of people and benches, and the background is distracting.
- What to do
- You open the aperture to f/2, the widest your lens allows, and focus on your friend's eyes.
- Why it works
- A wide aperture gives shallow depth of field, which blurs the busy background.
- Result
- Your friend stands out sharply against a soft, calm background.
Flowers and mountains
- Situation
- You want both the flowers close to you and the mountains far away to look sharp.
- What to do
- You choose f/11 and use a tripod because the shutter speed becomes slower.
- Why it works
- A narrower aperture gives deeper depth of field, but lets in less light.
- Result
- The scene is sharp from front to back, and the tripod keeps it free of shake.
Key points
- Aperture is the adjustable opening in the lens that light passes through.
- A small f-number (f/1.8) is a big opening; a big f-number (f/16) is a small opening.
- Each full step on the f-stop scale halves or doubles the light: one stop.
- A wide aperture gives shallow depth of field and a blurred background; a narrow aperture gives deeper focus.
- Choose aperture for what should be sharp, then set the other controls for brightness.
Frequently asked questions
What is aperture in photography?
Why is f/2.8 bigger than f/16?
What aperture gives a blurry background?
What aperture is best for landscapes?
Conclusion
Aperture controls two things at once: how much light enters the lens and how much of the scene looks sharp. Remember that small numbers mean big openings, and choose your aperture for the look you want. Next, you will learn the second exposure control: shutter speed.
Finished reading? Track your progress through Exposure and metering.