What does the circle of confusion size tell me?

It is the diameter of the blurred image of a single background point. A larger circle means a smoother, more dissolved background and bigger bokeh balls from point highlights, so it is a direct measure of how much background separation you will get.

What formula does this use?

It uses the geometric-optics defocus formula: c = (f squared / N) times the background-to-focus distance difference, divided by the background distance times the focus distance minus the focal length. All distances are converted to millimetres.

Why does percentage of frame width matter?

A 30 micron disc looks huge on a 13 mm one-inch sensor but tiny on a 36 mm full-frame sensor. Expressing the disc as a fraction of the frame width gives a sensor-independent sense of how prominent the bokeh will appear in the final image.

Why does a wider aperture create bigger bokeh?

The blur diameter is inversely proportional to the f-number, so opening from f/4 to f/2 doubles the disc size. Longer focal lengths and closer subjects with more distant backgrounds also enlarge the discs.

Is this the same as depth of field?

They are linked. Depth of field is the zone where the circle of confusion stays below a sharpness threshold (often about 30 microns on full frame). This tool reports the actual disc size at a chosen background distance rather than the in-focus zone boundary.

Bokeh Circle of Confusion Size Calculator

This calculator predicts how big your bokeh will be before you press the shutter. Enter the lens, aperture, subject distance, and background distance, and it returns the diameter of the blur disc that a single background point produces on the sensor — the direct physical measure of background separation.

How it works

When you focus at distance s_focus, a point at background distance s_bg does not converge to a sharp point on the sensor; it spreads into a small disc. Geometric optics gives its diameter on the sensor as:

c = (f² / N) × |s_bg − s_focus| / (s_bg × (s_focus − f))

with the focal length f and both distances in millimetres, and N the f-number. The tool converts your metre inputs to millimetres before applying it.

Three things make this number grow, and all three are intuitive:

Wider aperture (smaller N): the disc scales as 1/N, so f/2 doubles the disc versus f/4.
Longer focal length: the disc scales with f² at a fixed framing.
Bigger subject-to-background gap: the further the background sits behind the subject, the larger the disc, approaching a maximum as the background goes to infinity.

Sensor size and apparent size

A given disc diameter looks far more dramatic on a small sensor than on a large one, because it occupies a larger share of the frame. The tool therefore also reports the disc as a percentage of the frame width. Pick your sensor from the list and the preview circle scales to show roughly how prominent the bokeh will be in the final image.

Example and tips

An 85 mm f/1.8 lens focused at 2 m with the background 6 m away produces a disc of roughly 30 microns on full frame — strong, creamy separation. Stop down to f/8 and the disc shrinks to about 7 microns, and the background snaps back into recognisable detail. To maximise bokeh, open the aperture, move closer to the subject, and put as much distance as possible between the subject and the background. Every value is computed locally in your browser.