What is the circle of confusion?

It is the largest blur spot a point of light can form on the sensor while still being perceived as sharp in the final image. Points blurred more than this appear out of focus. Every depth-of-field and hyperfocal calculation depends on the chosen value.

How is the circle of confusion calculated?

Start from the smallest detail the eye can resolve at the viewing distance, scale it down by the print enlargement factor (print diagonal divided by sensor diagonal), and the result is the CoC on the sensor. The common Zeiss shortcut is sensor diagonal divided by 1500.

Why does sensor size change the value?

A small sensor must be enlarged more to reach the same print size, so any blur on it is magnified more. The acceptable on-sensor blur therefore has to be smaller. Full frame typically uses about 0.029 mm, APS-C about 0.019 mm and Micro Four Thirds about 0.015 mm.

What viewing conditions are standard?

The traditional reference is an 8 by 10 inch print viewed at 25 cm by a person with normal vision resolving about 5 line pairs per millimetre. Larger prints, closer viewing or pixel-peeping all demand a smaller circle of confusion.

Should I use a tighter value for modern high-resolution cameras?

If you inspect images at 100 percent on screen, the effective enlargement is enormous and the classic CoC is too generous. Many photographers use half the standard value, or base it on a one or two pixel blur, for critical work on high-megapixel bodies.

Circle of Confusion Calculator

The circle of confusion is the single number that every depth-of-field, hyperfocal-distance and diffraction calculation quietly depends on. It defines how much a point of light can blur on the sensor while still looking sharp in the final image. This tool computes the right value from your sensor size and how the photo will actually be viewed.

How it works

Sharpness is judged in the final image, not on the sensor, so the calculation works backwards from the viewer’s eye:

Eye resolution. A person with normal vision resolves detail subtending about 1 arc-minute. At a viewing distance d, the smallest discernible detail is roughly d × tan(1 arc-minute) ≈ d / 3438.
Enlargement factor. The print is bigger than the sensor by print diagonal ÷ sensor diagonal. Any blur on the sensor is magnified by this factor.
Circle of confusion on the sensor. Divide the eye’s detail limit by the enlargement factor:

CoC = (viewing detail limit) ÷ (print diagonal ÷ sensor diagonal)

The Zeiss shortcut

For the classic conditions (print viewed at its own diagonal distance, normal vision) all the geometry collapses to a famous rule of thumb:

CoC ≈ sensor diagonal ÷ 1500

A full-frame sensor (43.3 mm diagonal) gives 43.3 ÷ 1500 ≈ 0.029 mm, which is exactly the value found in standard DoF tables. The tool offers both the full viewing-condition method and this shortcut.

Worked example

A Micro Four Thirds sensor has a 21.6 mm diagonal. Using the Zeiss rule: 21.6 ÷ 1500 ≈ 0.0144 mm. If instead you plan a large 50 cm print viewed close at 35 cm, the full method returns a noticeably smaller value because the enlargement is greater — meaning you must stop down further for the same apparent sharpness.

Notes and tips

Match the value to the use. Web thumbnails tolerate a large CoC; gallery prints and 100% screen inspection demand a small one.
Half the standard value is a common rule for high-resolution sensors where pixel-level scrutiny is likely.
The CoC you get here is the input for the hyperfocal distance formula H = f² ÷ (N × c) + f, where c is this circle of confusion.

All calculations run locally in your browser.