What is aperture? Depth of field, bokeh and light control
Aperture is the opening that sets how much light passes through the lens, written as an f-number (such as f/1.8 or f/8); a smaller f-number means a wider opening and shallower depth of field. Besides exposure, aperture also shapes depth of field, bokeh character, lens sharpness, diffraction and the look of point light sources.
Changing the aperture from f/2.8 to f/4 cuts the amount of light in half; changing it in the opposite direction doubles it.
The result also depends on subject distance, framing, focal length, and sensor format.
Diffraction can soften details at small apertures; the “sweet spot” varies depending on the lens.
The number of blades, the blade shape, and the optical design determine the characteristics of the out-of-focus regions.
In this guide we treat aperture as a shooting decision that changes both the technical structure of an image and the effect it has on the viewer.
The f-number is approximately the lens’s focal length divided by the diameter of its entrance pupil. For example, a 50 mm lens at f/2 has an effective opening approximately 25 mm in diameter.
Aperture, f-number, and depth of field
Let’s first clarify the physical principles behind aperture, and then how this aperture interacts with exposure and depth of field.
It is the aperture that controls the size of the light beam reaching the sensor through the lens.
The aperture blades inside the lens open and close to change the entrance opening. The pupil comparison helps at the start: it widens in the dark and narrows in bright light. In photography this change affects not only light but also how deep the sharp zone is and the shape of out-of-focus areas.
The next value dims the light by half
Cameras also often include 1/3-stop increments. The following list shows the full-stop values.
Wide aperture
More light, a shallower depth of field, and, in most cases, more pronounced background separation. It is used for portraits, low-light photography, and selective focus.
Small aperture
Less light, greater depth of field, and the potential for a starburst effect with point light sources. This can be used in landscape, architectural, and multi-layered scenes.
The area in front of and behind the focus point that appears acceptably sharp.
Depth of field is not the same as sharpness. The focus plane is theoretically a single plane; depth of field describes the region perceived as sufficiently sharp for a given display size, print size, and viewing distance.
- A wide aperture generally reduces the depth of field.
- A small aperture generally increases the depth of field.
- Depth of field shrinks very quickly as you get closer to macro.
- The distance of the background significantly alters the appearance of the blur.
Aperture
A small f-number tends to produce a shallower depth of field, while a large f-number tends to produce a greater depth of field.
Subject distance
As you get closer to the subject, the depth of field narrows. This is why macro shots have areas of sharp focus measured in millimeters.
Focal length and framing
A long focal length and a close-up composition magnify the background, making the blur more noticeable.
Sensor format
For the same frame and perspective, a large format can produce a shallower depth of field because it requires a longer focal length.
Even phones with small sensors can produce real optical blur at close distances. The strong background separation seen in portrait mode is usually supported by depth maps and computational photography.
Aperture and depth of field in Burak Bulut Yıldırım’s work
I show the theory with examples from my own portrait, product, industrial and editorial work.
Bokeh, sharpness, aberration, and diffraction
When the aperture changes, the depth of field changes and so do the lens’s rendering and technical performance.
Bokeh is not the “degree of blur,” but rather the visual quality of out-of-focus areas.
The number and curvature of the diaphragm blades are important, but they are not the only determining factors. Spherical aberration correction, the surface structure of aspherical elements, optical vignetting, and the texture of the foreground and background also affect the bokeh character.
The "sweet spot" is not a fixed rule
Many lenses balance centre and corner performance when stopped down 1-3 stops from wide open. Modern lenses can be very strong wide open, though; the best value depends on the model, focal length and focus distance.
Stopping down can reduce some optical defects
Stopping down can reduce vignetting, spherical aberration, coma, and some color fringing. It also reduces background separation and light-gathering capacity.
A very small aperture softens the details
Diffraction occurs at all apertures; its effect on the image becomes more noticeable at small apertures. High-resolution sensors may reveal this softening sooner.
As depth of field increases, microcontrast and fine detail may be lost.
When photographing products, jewelry, and macro subjects, rather than closing the aperture all the way to show the entire subject in focus, shooting at a medium aperture with multiple planes of focus and applying focus stacking often yields more controlled results.
- First, test your lens's performance at f/5.6, f/8, and f/11.
- Adjust the depth of field as needed by adjusting the shooting distance and camera angle.
- Use focus stacking for still products, and choose a sensible middle value for moving scenes.
How does aperture affect the appearance of light sources?
Lens flare, volumetric light rays, and the starburst effect are distinct optical phenomena. The aperture affects each of them in a different way.
It is an internal reflection within the lens
When a strong light source enters the lens directly or at an oblique angle, reflections and a loss of contrast may occur between the lens elements. The shape of the aperture can affect the flare pattern; the main factors are the angle of the light, the coatings, and the optical design.
It becomes visible due to particles in the air
Volumetric light beams become visible when fog, smoke, dust, or moisture scatters light. Aperture can affect how sharp or spread out a beam appears; the presence of a scattering medium is what makes it visible.
It is diffraction at a small aperture
Bright point sources such as the sun, a street lamp or a small reflection can turn into stars at around f/11-f/16. The ideal value depends on the lens and the scene.
The result depends largely on the number of diaphragm blades.
An even number of blades typically produces as many star-shaped spokes as there are blades. With an odd number of blades, since the spokes do not overlap directly opposite each other, the number is usually doubled: 7 blades can form approximately 14, and 9 blades can form approximately 18 spokes.
Do not look at the sun through the optical viewfinder. Frame your shot quickly using the electronic viewfinder or the screen; pointing the camera directly at the sun for an extended period can damage the equipment.
