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Circle of Confusion Calculator

Pick a sensor format to find its circle of confusion.

How it is calculated

The tool uses the common diagonal rule: CoC = sensor diagonal ÷ divisor, with diagonal = √(width² + height²), all in millimetres. Preset sensor sizes are built in: full frame 36 × 24, APS-C 23.6 × 15.7, APS-H 28.7 × 19.0, Micro Four Thirds 17.3 × 13.0, 1-inch 13.2 × 8.8 and medium format 645 56 × 41.5. For a custom sensor choose the custom format and enter width and height in mm; with a preset those two fields are ignored. The divisor sets how strict the sharpness standard is. If it is empty, zero or negative, the tool uses 1500. A larger divisor gives a smaller, stricter CoC. The CoC is shown to four decimals and the diagonal to two, both in mm.

Worked example

For an APS-C sensor (23.6 × 15.7 mm) with a divisor of 1500: diagonal = √(23.6² + 15.7²) = √(556.96 + 246.49) = √803.45 = 28.35 mm. Circle of confusion = 28.35 ÷ 1500 = 0.0189 mm. For comparison, a full-frame sensor with the same divisor has a 43.27 mm diagonal and a CoC of 0.0288 mm, so the full-frame value is about 1.5 times larger.

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Common mistakes

  • Typing the sensor's crop factor or a focal length into the width and height fields; they must be physical sensor dimensions in millimetres, and they only apply when the custom format is chosen.
  • Entering width and height for a preset format and expecting them to be used; with a preset the tool takes its own built-in dimensions.
  • Using the pixel dimensions of an image instead of the sensor's size in millimetres.
  • Assuming a bigger divisor means a looser standard: it is the opposite, 1730 gives a smaller (stricter) CoC than 1500 for the same sensor.
  • Expecting the result to depend on print size or viewing distance directly; the divisor is a stand-in for those conditions, so change it deliberately if you view very large prints or crop heavily.

About this calculator

The circle of confusion (CoC) is the largest blur spot the human eye accepts as a sharp point in a final print or screen image — it's the reference value every depth-of-field and hyperfocal-distance formula is built on. It's approximated from a sensor's diagonal divided by a viewing-condition constant, commonly around 1500 for critical sharpness or 1442–1730 depending on the standard used, so a full-frame sensor (43.3mm diagonal) lands near 0.029mm while a Micro Four Thirds sensor (21.6mm) lands near 0.015mm.

Because CoC scales with sensor size, the same aperture and focal length give visibly different depth of field on different formats — which is exactly why a full-frame f/2.8 shot looks shallower than an equivalent APS-C or Micro Four Thirds shot. Cinematographers, photographers, and lens and camera manufacturers use the CoC value for a given sensor to compute accurate depth-of-field tables, hyperfocal distances, and focus-pulling marks.

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  • Editorial team

    The divisor is where your own standards come in. A larger divisor gives a smaller, stricter circle, which suits big prints or close pixel-level inspection, while a smaller one suits casual viewing. The result goes into depth-of-field or hyperfocal tables as a value in millimetres, so use the same figure everywhere to stay consistent. Which sensor format would you like added to the presets?

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