Circle Of Confusion
Circle of confusion is the largest out-of-focus blur disc a viewer still perceives as an in-focus point rather than visibly soft — a chosen threshold, not a fixed physical constant, and the single value every depth-of-field and hyperfocal-distance calculation is built on.
This site computes it as the real sensor diagonal divided by 1500, giving full frame 0.0288mm and APS-C roughly 0.0179-0.0188mm. A larger circle of confusion (a bigger sensor) is a more tolerant threshold, giving more depth of field at a fixed real focal length and aperture; a smaller one (a smaller sensor) is stricter, giving less depth of field under that same fixed comparison.
Frequently Asked Questions
Why do different calculators disagree on depth of field for identical settings?
Almost always a different circle-of-confusion convention — dividing the sensor diagonal by 1442 or 1730 instead of 1500 produces a genuinely different acceptable-blur threshold and, downstream, a different depth-of-field figure from the identical camera, lens and shooting settings.
Is a stricter circle of confusion always more accurate?
Not necessarily 'more accurate' — it reflects a different assumption about typical viewing distance and print size, calibrated to closer scrutiny than a more generous convention, with neither one objectively correct for every possible use case.