Bokeh Calculator: Background Blur Circle of Confusion
Bokeh refers to the aesthetic quality of out-of-focus blur in a photograph, especially the soft circular discs that appear where background highlights and detail fall outside the depth of field. The size of these blur circles on the camera sensor depends on four variables: focal length, aperture (f-number), the distance from camera to subject, and the distance from camera to the background. This calculator uses the standard optical formula to compute the diameter of the blur circle on the sensor, expressed in millimetres. It also converts that diameter to a percentage of a full-frame sensor width (36 mm) so you can judge how significant the blur will look in the final image. A blur circle below 0.1 mm is barely noticeable; 0.3 mm or above is clearly visible as smooth, creamy background separation; and circles above 1 mm on the sensor produce the distinctive large bokeh discs seen in fast prime portrait photography. To maximise bokeh, use the widest aperture your lens offers, increase your focal length, move closer to your subject, and put as much distance as possible between the subject and the background.
Blur circle on sensor: -- mm
How the bokeh blur circle is calculated
The blur circle diameter on the sensor is derived from the thin-lens approximation and the geometry of how an out-of-focus background point projects through the lens aperture onto the sensor plane.
Lens diameter: d = f / N
Subject magnification: m = f / (s − f)
Background magnification: mb = f / (b − f)
Blur circle (mm) = d × |m − mb| / (1 + mb)
where: f = focal length (mm), N = f-number, s = subject distance (mm), b = background distance (mm)
Worked example
85 mm lens, f/1.8, subject at 3 m (3,000 mm), background at 10 m (10,000 mm):
- Lens diameter: d = 85 / 1.8 = 47.22 mm
- Subject magnification: m = 85 / (3,000 − 85) = 85 / 2,915 = 0.02917
- Background magnification: mb = 85 / (10,000 − 85) = 85 / 9,915 = 0.008573
- Blur circle = 47.22 × |0.02917 − 0.008573| / (1 + 0.008573) = 47.22 × 0.020597 / 1.008573 = 0.964 mm
- As fraction of 36 mm sensor: 0.964 / 36 = 2.68% (Very strong bokeh)
Bokeh blur strength reference
| Blur circle (on sensor) | Strength rating | Appearance |
|---|---|---|
| Below 0.10 mm | Minimal | Background appears almost sharp; little visible separation |
| 0.10 to 0.30 mm | Moderate | Noticeable softening of background detail |
| 0.30 to 1.00 mm | Strong | Background clearly blurred; subject stands out well |
| Above 1.00 mm | Very strong | Large, smooth blur discs; classic portrait bokeh look |
These thresholds are indicative and depend on print size, viewing distance, and personal taste. A 0.30 mm blur circle on a sensor printed at 20x30 cm and viewed at arm's length is clearly visible, but may look subtle on a small phone screen.
Tips for maximising background blur
- Open your aperture: Moving from f/2.8 to f/1.4 roughly doubles the blur circle diameter. The widest aperture your lens offers will always give the most blur.
- Use a longer focal length: An 85 mm lens produces more blur than a 35 mm lens at the same aperture and subject distance, because the longer lens has a physically larger diameter opening.
- Get closer to your subject: Halving the subject distance increases blur significantly. The blur circle scales roughly with the square of the reciprocal of subject distance.
- Move the background farther away: The greater the distance between subject and background, the larger the blur circles on the background. Even a few extra metres makes a noticeable difference at typical portrait distances.
Bokeh calculator: frequently asked questions
What is bokeh?
Bokeh is the aesthetic quality of the out-of-focus blur in a photograph, particularly in the background behind a sharp subject. The word comes from the Japanese 'boke', meaning blur or haze. Technically, bokeh is produced when points of light or detail in the background are rendered as soft, overlapping circles on the camera sensor. The size and shape of these circles depend on the lens aperture, focal length, subject distance, and the distance of the background from the subject. Smooth, round, evenly lit blur circles are generally considered pleasing bokeh.
How do I get more bokeh and background blur?
Four factors increase background blur: a wider aperture (lower f-number such as f/1.4 or f/1.8), a longer focal length (85 mm or longer for portraits), a shorter subject distance (get closer to your subject), and a greater distance between the subject and the background. Any combination of these increases the blur circle size on the sensor. The most effective single change is usually widening the aperture, as blur scales with the square of the f-number ratio.
Does crop factor affect bokeh?
Crop factor affects the effective field of view, which indirectly affects bokeh. To frame the same composition on an APS-C camera (1.5x crop) as on a full-frame camera, you use a shorter focal length, which reduces blur. For example, a 50 mm f/1.8 on full frame gives similar framing to a 35 mm f/1.8 on APS-C, but the 50 mm produces noticeably more background blur because of the longer focal length. Sensor size itself does not change the physics of the blur circle calculation, but the lens choice required for equivalent framing does.
What is the best lens for bokeh?
Fast prime lenses with wide maximum apertures are best for bokeh. The 85 mm f/1.4 and 85 mm f/1.8 are classic portrait bokeh lenses, giving pleasing subject isolation at typical portrait distances of 2 to 4 metres. The 50 mm f/1.4 and 50 mm f/1.8 are popular general-purpose bokeh lenses. For extreme blur, 135 mm f/2 and 200 mm f/2 lenses produce very large blur circles. Lens optical design also affects the character of bokeh: the number, shape, and coating of aperture blades influence whether out-of-focus highlights appear round, hexagonal, or oval.
What focal length is best for portraits with background blur?
The traditional portrait focal lengths for background blur are 85 mm to 135 mm on a full-frame camera. At these focal lengths and typical portrait distances of 2 to 5 metres, a wide aperture of f/1.4 to f/2.8 produces strong subject isolation with large, smooth blur circles. Shorter focal lengths such as 35 mm or 50 mm can also produce nice blur at f/1.4 or f/1.8, but require getting closer to the subject, which can introduce perspective distortion of facial features. Telephoto lenses above 135 mm compress perspective and produce very large blur circles, but require more space between photographer and subject.
Sources
- Wikipedia: Bokeh.
- Cambridge in Colour: Depth of field in photography.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. For professional optical engineering work, consult a qualified optical physicist.