Aperture is the adjustable opening inside the lens through which light passes on its way to the sensor. It works like the pupil of an eye: it widens to admit more light and narrows to admit less. Of the three exposure settings, aperture has the strongest effect on how a photograph looks, because it also controls how much of the scene is in focus.
How the aperture works
Inside the lens is a ring of thin overlapping blades called the diaphragm. The blades move together to form a larger or smaller hole. On most modern cameras the aperture stays fully open while you compose, so the viewfinder is bright, and closes to the chosen size only at the moment of exposure.
The size of the opening is written as an f-number, such as f/2.8 or f/11. The numbering runs opposite to what most beginners expect: a small number means a large opening, and a large number means a small opening. The reason for this is explained in the lesson on f-stops.
Two jobs at once
1. Brightness
A wider opening admits more light per unit of time. In dim conditions, opening the aperture is the first and best way to get a brighter exposure, because it adds real light instead of amplifying a weak signal.
2. Depth of field
Depth of field is the zone, from near to far, that appears acceptably sharp. A wide aperture such as f/2 gives a shallow zone: the subject is sharp and the background dissolves into blur. A narrow aperture such as f/11 gives a deep zone, with foreground and background both sharp.
Aperture is not the only factor. Depth of field also becomes shallower when you move closer to the subject and when you use a longer focal length. A portrait at f/4 from one metre away has a much more blurred background than a street scene at f/4 focused ten metres away.
Typical apertures by subject
| Subject | Aperture | Why |
|---|---|---|
| Single-person portrait | f/1.8–f/2.8 | Separates the person from the background |
| Couple or small group | f/4–f/5.6 | Keeps every face sharp |
| Street and travel | f/5.6–f/8 | Forgiving focus, good sharpness |
| Landscape and architecture | f/8–f/11 | Sharp from foreground to horizon |
| Close-up of small objects | f/8–f/16 | Depth of field is very thin at close range |
| Indoor events in low light | Widest available | Maximum light |
Maximum aperture and lens names
Every lens has a widest possible aperture, and it is printed in the lens name. A “50mm f/1.8” lens opens to f/1.8. A kit zoom labelled “18–55mm f/3.5–5.6” opens to f/3.5 at 18 mm but only to f/5.6 at 55 mm. Lenses with a wide maximum aperture are called fast lenses, because they allow faster shutter speeds in the same light. An inexpensive 50mm f/1.8 lets in roughly eight times as much light as a kit zoom at f/5.6.
Aperture and sharpness
Lenses are usually not at their sharpest when fully open. Most reach peak sharpness two or three stops down from maximum, commonly around f/5.6 to f/8. At very small apertures such as f/16 and f/22, an optical effect called diffraction softens the whole image slightly. Use these only when the extra depth of field is really needed.
How to set it
Choose Aperture Priority mode, marked A on most cameras and Av on Canon. Turn the main dial to change the f-number. The camera sets the shutter speed to match. This is the mode many experienced photographers use for the majority of their work.
Common mistakes
- Reading the numbers backwards and choosing f/16 when a blurred background was wanted.
- Shooting a group at f/1.8, so that only the front row is in focus.
- Using f/22 for every landscape and losing sharpness to diffraction.
- Expecting strong background blur from a kit lens at f/5.6 with the subject standing right in front of a wall.
Practice exercise
- Place an object about one metre from the camera, with a background at least three metres behind it.
- In Aperture Priority, focus on the object and take a photo at the widest aperture the lens allows.
- Without moving, repeat at f/5.6, f/11 and f/16.
- Compare the backgrounds, and note how the shutter speed changed in each frame.
Next in this series: What Is an f-Stop?, Wide Aperture vs Narrow Aperture Explained and What Is Shutter Speed?. You can also browse all photography lessons.
