Bokeh is the aesthetic quality of the out-of-focus areas in a photograph, meaning how a lens renders blurred backgrounds, foregrounds and point light sources. In the field of lenses and optics within photography, the term describes the character of the blur, not its amount: two images with identical depth of field can show very different bokeh because the lenses spread defocused light in different ways.
The word comes from the Japanese for blur or haze, and photographers use it to judge whether a background looks smooth and creamy or busy and nervous. Bokeh depends on the shape of the aperture, the correction of spherical aberration, vignetting and the distance relationships in the scene, so it is partly a property of the lens and partly a result of how the photographer composes.
How a lens creates bokeh
Bokeh forms because every point outside the plane of focus is projected onto the sensor as a blur disc, also called the circle of confusion. The size of that disc grows with the distance between the point and the focus plane, and its internal brightness profile is determined by the lens design. The overlapping of thousands of these discs is what the eye reads as background blur.
The brightness distribution inside each disc decides the character. A disc that is brighter at the edge produces double-line or nervous bokeh, where fine background lines appear doubled. A disc that is brighter in the center and fades at the edge blends smoothly with its neighbors and produces soft, smooth bokeh. This distribution is governed mainly by residual spherical aberration, which is usually over-corrected on one side of focus and under-corrected on the other.
The following table summarizes the main optical factors and their effect on the rendered blur.
| Factor | What it controls | Visible effect |
|---|---|---|
| Aperture shape | Outline of each blur disc | Round, polygonal or star-like highlights |
| Number of blades | Roundness when stopped down | 7-11 rounded blades keep discs circular |
| Spherical aberration | Brightness profile inside the disc | Smooth or edge-heavy, nervous blur |
| Optical vignetting | Disc shape near the frame edge | Cat’s-eye highlights in the corners |
| Aspherical elements | Surface texture of the disc | Onion-ring patterns in highlights |
| Longitudinal chromatic aberration | Color of disc edges | Magenta and green fringes in front and behind |
Bokeh versus depth of field
Bokeh and depth of field are related but distinct concepts. Depth of field describes how much of the scene appears acceptably sharp, while bokeh describes how the unsharp part looks. Opening the aperture or moving closer to the subject reduces depth of field and increases the amount of blur, but it does not by itself improve the quality of that blur.
The amount of background blur depends on the physical aperture diameter, which equals focal length divided by f-number, and on the ratio between subject distance and background distance. An 85 mm lens at f/1.8 has an entrance pupil of about 47 mm, while a 35 mm lens at f/1.8 has one of about 19 mm, so the longer lens blurs a distant background far more at the same framing distance. The table below separates the quantity of blur from its quality.
| Aspect | Amount of blur | Bokeh quality |
|---|---|---|
| Main driver | Entrance pupil diameter and distances | Lens correction and aperture design |
| Changed by the photographer | Yes, through aperture, focal length and position | Partly, through aperture and background choice |
| Measured by | Blur disc size relative to the frame | Disc profile, shape and edge character |
| Related neighboring concept | Depth of field, crop factor | Spherical and chromatic aberration |
Crop factor also matters. At the same angle of view and f-number, a smaller sensor uses a shorter focal length and therefore a smaller entrance pupil, so it produces roughly one stop less blur on APS-C and about two stops less on Micro Four Thirds compared with full frame.
Types of bokeh character
Photographers describe bokeh with a set of recognizable patterns, each tied to a specific optical cause. Knowing the cause helps predict how a lens will behave before it is used on an important assignment.
| Pattern | Appearance | Typical cause |
|---|---|---|
| Smooth bokeh | Soft discs that merge evenly | Center-weighted disc brightness |
| Nervous bokeh | Doubled lines, busy textures | Edge-weighted disc brightness |
| Cat’s-eye bokeh | Lemon-shaped highlights at the edges | Mechanical vignetting at wide apertures |
| Swirly bokeh | Circular flow around the subject | Strong field curvature and vignetting |
| Onion-ring bokeh | Concentric rings inside highlights | Imperfect aspherical surfaces |
| Soap-bubble bokeh | Discs with bright, sharp rims | Strongly over-corrected aberration |
| Donut bokeh | Ring-shaped highlights with a dark center | Mirror (catadioptric) lens designs |
Some of these patterns are deliberate. Apodization filters, built into a few specialized portrait lenses, darken the edge of the aperture so that every disc fades outward, producing exceptionally smooth blur at the cost of about one stop of light transmission.
Controlling bokeh in practice
Good bokeh is achieved through lens choice combined with scene geometry. The most effective technique is to increase the distance between subject and background while keeping the camera close to the subject, because the ratio of these distances determines how large the blur discs become.
| Choice | Recommended setting | Reason |
|---|---|---|
| Aperture | Widest to about one stop down | Large discs, stopping down rounds the cat’s eye |
| Focal length | 85-135 mm for portraits | Larger entrance pupil, compressed background |
| Background distance | Several meters behind the subject | Larger blur discs, simpler shapes |
| Background type | Small, scattered light sources | Clear, attractive highlight discs |
| Busy foliage | Avoid or move further away | Branches expose nervous bokeh |
Stopping down slightly from maximum aperture is a common professional habit. At f/1.4 to f/2.8, closing the aperture by about one third to one stop often reduces cat’s-eye shapes in the corners while keeping the blur discs large, but stopping down further brings the polygonal blade shape into view on lenses with fewer or straight blades.
Limitations and common misconceptions
Bokeh is a subjective judgment, and no single measurement fully captures it. Lens tests can show disc profiles and chromatic fringes, but preferences differ between photographers and genres.
- A wide maximum aperture does not guarantee good bokeh; a fast lens with strong over-correction can render harsh, nervous backgrounds.
- More blur is not better bokeh; a large amount of blur can still look busy if the disc profile is edge-heavy.
- Computational portrait modes simulate bokeh with depth maps, and they often fail around hair, glass and fine edges.
- Bokeh and sharpness are not opposites; modern designs can combine high MTF in the focus plane with smooth out-of-focus rendering.
- Diffraction is unrelated to bokeh; it limits sharpness at small apertures, where bokeh is minimal anyway.
Explained without jargon
When a photo has a sharp person in front of a soft, blurry background, the look of that blur is called bokeh. It is not about how blurry the background is, but about how pleasant the blur looks.
Think of looking at city lights through a rainy window. Some windows turn each light into a soft, gentle glow, while others turn them into hard-edged rings. Lenses behave the same way. Good bokeh feels calm and creamy, and poor bokeh feels busy and distracting, even when the amount of blur is the same.
A typical use case
A portrait photographer shoots an evening engagement session in a city park strung with small decorative lights. She chooses an 85 mm lens at f/1.8, places the couple about two meters from the camera and positions them so the lights hang roughly ten meters behind them.
At full aperture, the lights near the frame corners appear as lemon-shaped highlights, so she closes the aperture to f/2.2. The discs become rounder while remaining large, and the foliage between the lights melts into smooth color. The result is a clean, glowing background that separates the couple without visual clutter, achieved in camera rather than through later editing.