Aperture is the opening in your lens that lets light reach the camera sensor
Think of aperture like the pupil of your eye. When you walk into a dark room, your pupil opens wider to let more light in. When you step into bright sunlight, it shrinks to protect your eye. Your camera lens works the same way. The aperture is a circular opening inside the lens that expands and contracts to control how much light enters the camera. A larger opening (wider aperture) lets more light through; a smaller opening (narrower aperture) lets less light through.
Aperture is measured in f-stops, written as f/1.8, f/4, f/16, and so on. This number represents the ratio of the lens's focal length to the diameter of the aperture opening. The confusing part: a smaller f-number means a wider aperture. So f/1.8 is a much larger opening than f/16. Understanding this backwards relationship is the key to controlling both exposure and focus in your photos.
Key Takeaways
- Aperture controls how much light reaches your sensor, so a wider aperture (lower f-number like f/1.8) brightens your image, while a narrower aperture (higher f-number like f/16) darkens it.
- Aperture directly affects depth of field—the amount of your image that appears sharp—so f/1.8 creates a blurry background while f/16 keeps everything in focus.
- Not all lenses can open as wide as others; a kit lens might max out at f/3.5 or f/5.6, while a prime lens can open to f/1.4 or wider.
- You control aperture by adjusting the f-stop number in your camera's settings, usually in Aperture Priority mode (A or Av on your camera dial) or Manual mode (M).
How aperture affects the brightness of your photo
Aperture is one of three tools that control exposure—how bright or dark your final image looks. The other two are shutter speed and ISO. When you open your aperture wider (move to a lower f-number), more light enters the camera, and your photo becomes brighter. When you close it down (move to a higher f-number), less light enters, and your photo becomes darker.
This matters most when you are shooting in low light. A portrait taken indoors under household lamps will be too dark if you use f/16, because very little light passes through such a narrow opening. The same shot at f/2.8 will be properly exposed because the wider aperture gathers enough light. This is why photographers often talk about "fast" lenses—a fast lens is one that opens very wide (like f/1.4), allowing you to shoot in dim conditions without needing a long shutter speed or high ISO.
How aperture creates blur and sharpness (depth of field)
Aperture's second job is controlling depth of field—the zone of your image that appears sharp and in focus. A wide aperture (f/1.8, f/2.8) creates a shallow depth of field, meaning only a thin slice of your scene is sharp and everything else blurs. A narrow aperture (f/11, f/16) creates a deep depth of field, keeping most or all of your scene in focus.
This is why portrait photographers love wide apertures. At f/1.8, the subject's face is sharp but the background melts into a soft, creamy blur. This blur is called bokeh, and it draws the viewer's eye to the subject. Landscape photographers do the opposite—they use narrow apertures like f/11 or f/16 to keep mountains, trees, and foreground all sharp from edge to edge.
The relationship between aperture and depth of field is not linear. The difference between f/2.8 and f/4 is noticeable but not dramatic. The difference between f/1.8 and f/2.8 is much more pronounced. And the difference between f/16 and f/22 is barely visible because both are so narrow that nearly everything is sharp anyway.
What aperture range your lens can actually open to
Not every lens can open as wide as every other lens. A kit lens that came with your camera body—often an 18–55mm zoom—typically opens to f/3.5 at the wide end and f/5.6 at the telephoto end. This is a limitation of the lens design and cost. A professional-grade zoom might open to f/2.8 across its entire range. A prime lens (fixed focal length) can open much wider: f/1.4, f/1.2, or even f/0.95 in specialty lenses.
The widest aperture a lens can achieve is called its maximum aperture, and it is printed on the lens barrel. You will see it written as something like "1:1.8" or "f/1.8". This number tells you the widest opening available. You can always close the aperture down narrower than the maximum—a lens with a maximum aperture of f/1.8 can also shoot at f/2.8, f/4, f/8, and so on. But you cannot open it wider than its design allows.
How to set aperture on your camera
Most cameras have two ways to control aperture: Aperture Priority mode and Manual mode. In Aperture Priority (labeled A or Av on your camera dial), you choose the f-stop you want, and the camera automatically adjusts shutter speed to get the correct exposure. This is the easiest way to learn because you focus on one variable at a time.
To set aperture in Aperture Priority mode, rotate the command dial or press the appropriate button while looking at your camera's LCD screen or viewfinder. The f-number will change as you turn. Once you have chosen your aperture, half-press the shutter button to focus, and the camera will show you the shutter speed it has selected. If the shutter speed is too slow (below 1/60 of a second, for example), you may get motion blur, and you will need to either open the aperture wider or increase your ISO.
In Manual mode (M), you control both aperture and shutter speed yourself. This gives you complete control but requires you to understand how both settings work together. Manual mode is useful once you are comfortable with the relationship between aperture, shutter speed, and ISO.
Common aperture choices and when to use them
| Aperture (f-stop) | Depth of Field | Common Uses |
|---|---|---|
| f/1.4 to f/2.8 | Very shallow; blurry background | Portraits, low-light scenes, isolating a subject |
| f/4 to f/5.6 | Shallow to moderate; some background blur | Portraits with context, street photography, general shooting |
| f/8 to f/11 | Deep; most of scene is sharp | Landscapes, group photos, product photography |
| f/16 to f/22 | Very deep; nearly everything sharp | Landscapes, macro photography, situations requiring maximum sharpness |
The aperture you choose depends on what you are photographing and what you want the final image to look like. If you are shooting a portrait and want the background out of focus, start at f/2.8 or wider. If you are shooting a landscape and want the entire scene sharp, start at f/8 or narrower. If you are shooting indoors without a tripod and need a fast shutter speed to avoid blur, open the aperture as wide as your lens allows.
How aperture interacts with shutter speed and ISO
Aperture does not work alone. It is part of the exposure triangle—three settings that together determine how bright your image is. The other two are shutter speed (how long the sensor is exposed to light) and ISO (how sensitive the sensor is to light).
If you want a shallow depth of field (f/2.8) but your scene is very bright, the camera will want to use a very fast shutter speed to avoid overexposure. If that shutter speed is faster than your camera can handle, you have a problem. The solution is to use a neutral density filter (a piece of glass that darkens the scene without changing color) or to shoot at a different time of day. Conversely, if you want a deep depth of field (f/16) in dim light, the camera will want a very slow shutter speed, which requires a tripod to avoid motion blur.
Learning to balance these three settings is how you gain creative control over your images. Aperture is just one piece of that puzzle, but it is the piece that most directly affects how your subject looks.
Frequently Asked Questions
Why do photographers say f/1.8 is "wider" than f/4 when the number is smaller?
The f-stop is a ratio, not a direct measurement. F/1.8 means the aperture diameter is 1/1.8 of the focal length. F/4 means it is 1/4 of the focal length. Since 1/1.8 is a larger fraction than 1/4, the actual opening is larger even though 1.8 is a smaller number. It is backwards from what you might expect, which is why it confuses everyone at first.
Can I use a very wide aperture like f/1.4 all the time?
You can, but you may not want to. A very wide aperture creates such a shallow depth of field that only a tiny part of your subject is sharp. In a portrait, this might mean only the eyes are in focus and the nose is blurry. In bright sunlight, a wide aperture will overexpose your image unless you use a very fast shutter speed or a neutral density filter. Use wide apertures when you specifically want a blurry background or when you need to gather light in dim conditions.
What does it mean when a lens says f/3.5–5.6?
This means the maximum aperture changes depending on the zoom position. At the widest zoom setting (usually 18mm on an 18–55mm lens), the lens opens to f/3.5. As you zoom in, the maximum aperture narrows. At the telephoto end (55mm), it only opens to f/5.6. This is a cost-saving design; professional zooms maintain a constant maximum aperture across the entire zoom range.
Does aperture affect autofocus speed?
Yes. A wider aperture (lower f-number) lets more light into the camera, which helps the autofocus system lock on faster, especially in low light. A very narrow aperture (f/16 or narrower) can slow autofocus because less light is available for the focus sensors to work with. This is another reason why fast lenses (wide maximum apertures) are popular for video and fast-moving subjects.