Aperture is the adjustable opening in a lens, formed by the diaphragm blades, that controls how much light passes through to the sensor or film. In photography, it is one of the three exposure controls together with shutter speed and ISO, and it is expressed as an f-number such as f/2.8 or f/8.
Aperture affects far more than brightness. The same setting determines the depth of field, the character of background blur, the amount of diffraction softening and how well the lens performs optically. Choosing an aperture is therefore both an exposure decision and a creative decision.
How the f-number describes aperture
The f-number is the ratio of the lens focal length to the diameter of the entrance pupil, the apparent size of the opening seen from the front of the lens. A 50 mm lens at f/2 has an entrance pupil of 25 mm; at f/8 it is 6.25 mm. Because the number is a ratio, f/4 admits the same light per area on any lens, which makes exposure settings transferable between focal lengths.
Light transmission depends on the area of the opening, which scales with the square of the diameter. Each full stop multiplies the f-number by about 1.4, the square root of two, and halves the light. That is why the standard sequence looks irregular: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22.
| Aperture setting | Light relative to f/1.4 | Typical use |
|---|---|---|
| f/1.4 | 1 (reference) | Low light, strong subject isolation |
| f/2.8 | 1/4 | Events, indoor sports, portraits |
| f/5.6 | 1/16 | General use, small groups |
| f/8 | 1/32 | Landscapes, architecture, sharpest zone of many lenses |
| f/16 | 1/128 | Maximum depth of field, sunstars, bright light |
A smaller f-number means a wider opening, which confuses many beginners. “Opening up” means moving toward f/1.4; “stopping down” means moving toward f/16. Most cameras also offer one-third-stop increments, such as f/3.2 and f/3.5 between f/2.8 and f/4.
Aperture and depth of field
Depth of field is the distance range in front of and behind the focus point that appears acceptably sharp. Wider apertures produce shallower depth of field because light arrives at the sensor in a wider cone, so points away from the focus plane spread into larger blur circles.
Aperture is only one of the factors. Depth of field also shrinks as subject distance decreases and as magnification increases, which is why macro photographers often work at f/11 or f/16 yet still see only millimeters in focus.
| Factor | Change | Effect on depth of field |
|---|---|---|
| Aperture | Wider, such as f/2 instead of f/8 | Shallower |
| Subject distance | Closer to the subject | Much shallower |
| Focal length at equal framing | Longer lens from farther away | Roughly similar; background blur appears larger |
| Sensor size at equal framing and f-number | Larger sensor | Shallower |
Landscape photographers often focus near the hyperfocal distance, the closest focus point at which everything from half that distance to infinity is acceptably sharp, to maximize depth of field at a moderate aperture.
Aperture in the exposure triangle
Within the exposure triangle, aperture trades light against depth of field, shutter speed trades light against motion rendering, and ISO trades brightness against noise. Changing one control by a stop must be balanced by the opposite stop on another to keep the same exposure.
| Aperture | Shutter speed | ISO | Visual result |
|---|---|---|---|
| f/2 | 1/1000 s | 100 | Shallow focus, frozen motion |
| f/4 | 1/250 s | 100 | Moderate focus depth, most motion frozen |
| f/8 | 1/60 s | 100 | Deep focus, risk of motion blur |
| f/16 | 1/15 s | 100 | Very deep focus, tripod usually needed |
In aperture priority mode, labeled A or Av, the photographer sets the aperture and the camera chooses the shutter speed through its metering, optionally adjusted with exposure compensation. It is the most common mode for photographers who prioritize control of depth of field.
Optical effects of aperture choice
Most lenses are sharpest two to three stops down from maximum aperture. Wide open, residual aberrations and vignetting reduce corner sharpness and brightness; stopped down too far, diffraction spreads light and softens fine detail across the whole frame.
The onset of visible diffraction depends on pixel pitch and output size. High-resolution full-frame sensors typically begin to lose fine detail beyond about f/8 to f/11, and smaller sensors reach that point earlier. At f/22 the softening is visible on most modern cameras when viewed at full resolution.
| Symptom | Likely cause | Fix |
|---|---|---|
| Soft corners and dark edges | Shooting wide open | Stop down one or two stops |
| Overall softness at small apertures | Diffraction | Use f/8 to f/11 or focus stacking |
| Only one eye sharp in a portrait | Very shallow depth of field | Stop down to f/2.8 to f/4 or align the face |
| Visible sensor dust spots | Small aperture reveals dust | Clean the sensor or retouch |
| Busy, nervous background blur | Lens design and partially stopped-down blades | Increase distance to background or open up |
The number and shape of the diaphragm blades shape out-of-focus highlights and sunstars. Rounded blades keep highlights circular when stopped down slightly, while straight blades produce sharp-pointed sunstars at small apertures.
Common misconceptions about aperture
- A bigger f-number does not mean more light; it means a smaller opening and less light.
- f/1.8 is not always better: the image may be soft and the depth of field too thin for the subject.
- Maximum depth of field is not always sharpest: f/22 often looks softer than f/8 because of diffraction.
- f-numbers are not t-stops: cinema lenses use t-stops, which measure actual transmitted light including glass losses.
Stripped to the essentials
Aperture is the size of the hole that lets light into the camera. It works like the pupil of your eye: wide open in a dark room, small in bright sunshine.
A wide opening lets in plenty of light and makes the background blurry, which is great for portraits. A small opening lets in less light but keeps everything from front to back sharp, which suits landscapes. The confusing part is the numbering: a small number such as f/2 means a big hole, and a big number such as f/16 means a small one.
Where it is applied
A portrait photographer works outdoors in a park on a bright afternoon, photographing a client three meters away with an 85 mm lens. She wants the face sharp and the trees behind turned into a soft wash of color.
She sets aperture priority at f/2, which keeps a depth of field of only a few centimeters and isolates the subject. The camera chooses 1/2000 s at ISO 100. For a group shot of four people standing in two rows, she stops down to f/5.6, so both rows fall within the zone of sharp focus, and the camera lengthens the shutter speed to 1/250 s to compensate.
The final gallery shows creamy backgrounds in the single portraits and every face sharp in the group image, achieved by changing a single setting.