Camera Aperture Explained: F-Stops, Light and Depth of Field

Aperture is the adjustable opening in a lens through which light reaches the camera sensor. It affects exposure and depth of field: the range of distances that appear acceptably sharp. Learning it is mainly about connecting the f-number to those two results.

You can control aperture in Aperture Priority or Manual mode. Neither requires abandoning useful automatic features. Start with a stationary subject so changes in depth of field are easy to compare.

Why a smaller number means a wider opening

Aperture is written as an f-number, such as f/2.8, f/4 or f/8. It expresses the ratio of focal length to the effective diameter of the lens’s entrance pupil. For everyday use, remember that a lower number describes a larger relative opening.

At the same shutter speed and ISO, a wider opening produces a brighter exposure. A smaller opening admits less light, so maintaining brightness requires a longer exposure, a higher ISO or a change in the available lighting.

The familiar full-stop sequence includes f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11 and f/16. Moving one full stop towards the larger number approximately halves the light. Camera dials often also offer one-third-stop increments.

Nikon’s guide to maximum aperture explains how f-numbers relate to lens openings and the markings on a lens.

An exposure example

Suppose f/4, 1/250 second and ISO 200 give the brightness you want. Changing to f/5.6 reduces light by one stop. You could compensate with 1/125 second or ISO 400, but those alternatives affect motion rendering or image quality differently.

Moving from f/4 to f/8 reduces light by two stops, not one. It requires four times the exposure time, or an equivalent adjustment elsewhere, to maintain the same brightness under unchanged light.

This is why a lens described as fast has a relatively wide maximum aperture. The extra light can support a shorter exposure or lower ISO, but it does not mean the lens focuses faster in every situation.

What depth of field actually means

Only a particular distance is in precise focus. Depth of field describes the surrounding range that looks sufficiently sharp for the intended use. A wider aperture narrows that range when other conditions remain the same; a smaller aperture expands it.

Distance and magnification matter too. Close subjects can have very little depth of field even at a fairly small aperture. Background blur also depends on how far the background is from the subject.

Do not equate a low f-number with a guaranteed attractive background. The background’s texture, lighting and distance still matter. A change of viewpoint may remove a distraction more effectively than opening the lens.

Choosing aperture for people

For a single portrait, a wide aperture can separate the person from the surroundings. Start around f/2.8 to f/4 if available, focus on the important eye and inspect whether the rest of the face has enough sharpness.

For several people, their positions are crucial. Faces at different distances may need more depth of field, or a different arrangement. Shooting every group at the lens’s widest setting can leave some people visibly soft.

The nearest eye being sharp does not guarantee both eyes or the whole face are sharp. Review according to the pose, camera distance and intended output, then stop down if needed.

Choosing aperture for scenes and close subjects

For a landscape with foreground and distance, an intermediate aperture such as f/8 is a useful test on many cameras. Choose focus carefully and check the nearest important detail as well as the horizon.

Macro work has especially limited depth of field. A smaller aperture can help, but subject movement, lighting and diffraction may limit the result. A stable scene may suit focus stacking; a moving insect requires a different approach.

Our shallow depth of field guide explores selective focus and the relationship between subject and background.

Sensor format changes the comparison when you match framing and output. Settings that create one appearance on full frame may not produce the same depth of field on a smaller sensor. Compare actual results rather than treating f-numbers as universal recipes.

Why stopping down has limits

Small apertures increase depth of field, but diffraction can reduce fine detail. The best compromise depends on the scene, lens, sensor and display size. There is no single aperture that is sharpest for every photograph.

If f/16 gives the necessary depth but loses some fine detail, that may still be a worthwhile choice. The aim is a suitable photograph, not automatically avoiding every setting where diffraction exists.

Canon explains diffraction in its discussion of lens aberration correction. Correction features do not remove the need to choose an appropriate aperture.

Watch what the camera changes

In Aperture Priority, the camera adjusts shutter speed for its exposure target, with ISO also changing if automatic. Check that the resulting shutter speed still suits the subject. A well-exposed frame can contain unwanted movement blur.

A zoom lens marked f/3.5–5.6 has a maximum aperture that varies with focal length. If you zoom towards the longer end, the widest available opening can become smaller. That is normal operation, not a fault.

For a simple exercise, photograph a subject with a background several metres behind it at f/2.8, f/5.6 and f/11, using available settings. Keep viewpoint and focus consistent and compare both the background and the shutter speeds selected. The comparison shows the creative result and its exposure cost.