Bokeh (from the Japanese word for blur) refers to the aesthetic quality of out-of-focus areas in a photograph — particularly how background highlights render as smooth, round discs. The size and shape of those blur circles determines the character of the bokeh.

How is the blur diameter calculated?

The blur disc diameter for a background point at distance d_bg is: blur = (f² / (N × (d_subject - f))) × |1 - d_subject / d_bg|, where f is focal length, N is the f-number, and distances are in the same unit. A larger blur diameter means more out-of-focus rendering.

Does sensor size affect bokeh?

Sensor size affects the depth of field and the required field-of-view crop, but not the physical blur disc size in absolute terms. However, to frame the same subject at the same size, a larger sensor requires a longer focal length or closer distance — both of which increase blur. That is why full-frame systems produce more bokeh for equivalent compositions.

What is the blur ratio?

The blur ratio compares the blur disc diameter to the circle of confusion threshold. A ratio above 1 means the background point is visibly blurred beyond the acceptable sharpness threshold. Higher values mean more pronounced bokeh.

Why does increasing aperture (lower f-number) produce more blur?

A wider aperture (lower f-number) increases the diameter of the lens entrance pupil. Out-of-focus points project a larger blur disc proportional to entrance pupil diameter, so f/1.4 renders backgrounds significantly more blurred than f/5.6.

What background distance gives the most blur?

Blur increases as the background moves farther behind the subject (or the subject moves closer to the camera). A subject very close to the background will appear only slightly separated, while an infinite background distance produces the maximum blur disc diameter.

Bokeh Calculator

Enter focal length, aperture, subject distance, and sensor size to calculate background blur diameter and depth of field for beautiful bokeh.

Lens & Scene Settings

Sensor Format

Focal Length (mm)

Aperture (f-number)

Enter the number only: 1.4, 2.8, 5.6, 8…

Subject Distance

Background Distance

Distance to the background element you want blurred (same unit).

Fill in your lens settings and click Calculate Bokeh.

Summary

Bokeh quality depends on how large background highlights render as blur discs relative to the frame. This calculator computes the blur circle diameter for a background object at any distance, alongside near/far focus limits and total depth of field, for any combination of focal length, f-stop, subject distance, and sensor format. Use it to compare lens and aperture choices before a shoot, or to understand why a 50 mm f/1.8 on full-frame produces smoother subject separation than a kit zoom at f/5.6.

How it works

Select your sensor format — or enter a custom circle of confusion (CoC).
Enter the focal length of your lens in millimeters.
Enter the aperture as an f-number (e.g. 1.4, 2.8, 5.6).
Enter the distance from your camera to the subject.
Enter the distance to the background object you want blurred.
Read the blur disc diameter, depth of field, and blur ratio in the results panel.

Use cases

Choose the right lens and aperture to achieve creamy background separation for portraits.
Compare how full-frame vs. APS-C vs. Micro Four Thirds affects bokeh at the same framing.
Determine the minimum aperture to blur a distracting background element.
Plan macro shots by understanding how thin the in-focus zone is at close distances.
Evaluate whether a background at a given distance will be pleasingly blurred or still recognizable.
Teach photography students the relationship between aperture, focal length, and background blur.
Compare two lens options (e.g. 85 mm f/1.8 vs. 135 mm f/2) for subject isolation.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu