What Is Aperture? Depth of Field, Bokeh, and Light Control
The aperture is not merely an opening that illuminates the photograph. It is one of the fundamental creative tools that collectively determine exposure, depth of field, bokeh character, lens sharpness, diffraction, and the appearance 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.
Conclusion: It is determined by the 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.
We view the aperture not as a sequence of numbers to be memorized, but as a decision that governs the technical and psychological structure of the image.
The f-number is approximately equal to the lens’s focal length divided by the diameter of the exit pupil. For example, a 50-mm lens at f/2 has an effective aperture of 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 aperture. The analogy with the pupil is helpful as a starting point: it dilates in the dark and constricts in the light. In photography, however, this change affects not only the amount of light but also the width of the depth of field and the shape of the 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.
It is the area in front of and behind the point you have focused on that appears “in focus.”.
Depth of field is not the same thing as sharpness. The plane of sharpness is, in theory, a single plane; depth of field, on the other hand, defines the area that is perceived as sufficiently sharp depending on the viewing angle, print size, and viewing distance.
- A wide aperture generally reduces the depth of field.
- A small aperture generally increases the depth of field.
- As you get closer to the subject, the depth of field decreases dramatically.
- The distance of the background significantly alters the appearance of the blur.
Diaphragm
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 true optical bokeh in close-up shots. The strong background separation seen in portrait mode, however, is mostly supported by depth mapping and computational photography.
Aperture and depth of field in Burak Bulut Yıldırım’s work
Here, I’m illustrating the theory not with examples taken from the internet, but with the actual aperture choices I’ve made in my portrait, product, industrial, and editorial work.
Bokeh, sharpness, aberration, and diffraction
When the aperture changes, it’s not just the depth of field that changes; the lens’s image character and technical performance also change.
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 center and corner performance when stopped down by 1–3 stops from wide open. However, modern lenses can be very powerful at wide open; the optimal setting varies depending on the model, focal length, and focusing distance.
Adjusting the aperture can reduce certain optical defects
Vignetting, spherical aberration, coma, and some chromatic aberrations may decrease when the aperture is stopped down. Conversely, background sharpness and light-gathering capacity also decrease.
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 stationary subjects and find an appropriate compromise for moving scenes.
How do light sources appear through a diaphragm?
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
When fog, smoke, dust, or moisture scatters light, volumetric light streaks become visible. The brightness can affect the perception of the beam’s sharpness and spread; the primary condition for the beam’s formation is the presence of a scattering medium in the environment.
It is diffraction at a small aperture
Point-like, bright light sources—such as the sun, a streetlight, or a small reflection—may take on a star-like shape at f/11–f/16. The ideal setting varies depending 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.
