Definition
Circle of confusion is the largest a single point of light can blur into on the sensor before a viewer stops seeing it as sharp, at normal viewing size. Depth of field calculators use it to decide what counts as in focus. The usual value is about 0.03mm for full frame and smaller for smaller sensors.
A 50mm lens at f/4, focused at 3 metres on full frame, assumes a circle of confusion of about 0.029mm and gives a sharp zone of roughly 2.6 to 3.5 metres. Set the calculator to APS-C with the same lens and settings and the value tightens to about 0.019mm, so the sharp zone shrinks.
Calculate it for your own shot with the Depth of Field Calculator.
Also used in: Hyperfocal Distance Guide, Focus Stacking Calculator, Sharpness Analyzer
Every depth of field number a calculator hands you rests on a decision someone made about how much blur still counts as sharp. Change that assumption and the sharp zone moves. The standard figure for full frame is about 0.03mm, chosen for a print viewed at arm's length. View the same file much larger, or crop in hard, and that tolerance is effectively tighter, which is why a shot that looked fine on the back of the camera can disappoint at 100 percent on a monitor.
Sensor size enters through the same door. A Micro Four Thirds sensor is enlarged twice as much as full frame to reach an 8x10 print, so its circle of confusion is set near 0.015mm. That one difference is a large part of why a 25mm f/1.8 lens on that format holds more front-to-back sharpness than its field of view alone would lead you to expect.
Set a depth of field calculator to a 50mm lens at f/4, focused at 3 metres, on a full-frame body. It assumes a circle of confusion around 0.029mm and returns a sharp zone of roughly 2.6 to 3.5 metres. Switch the sensor setting to APS-C without touching the lens, aperture or distance, and the tool tightens the circle of confusion to about 0.019mm. The reported sharp zone shrinks to something closer to 2.75 to 3.3 metres, even though not one piece of glass has moved.
The number changed because the smaller sensor's image is magnified more to reach the same viewing size, so less blur is tolerated back at the sensor. Photographers who print big or inspect at full resolution often set a calculator to half its default circle of confusion on purpose, which pushes the recommended focus point farther and trims the claimed depth of field.
Guides for this tool, by niche
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