What is circle of confusion, and why does it matter for depth of field?

Circle of confusion is the largest blur disc a viewer still perceives as an in-focus point rather than a visibly soft spot — a chosen threshold, not a fixed physical law, and every depth-of-field and hyperfocal-distance figure this site computes depends directly on it.

Why It's a Disc, Not a Point

A lens focuses light from one exact distance to a genuinely sharp point on the sensor; light from every other distance converges slightly before or after the sensor plane, landing as a small disc instead of a point. Closer to the true focus distance, that disc is small enough to be indistinguishable from a real point at normal viewing; further away, it grows large enough to read as visible blur. Circle of confusion is the size at which that transition is considered to happen.

Where the Number Actually Comes From

This site computes circle of confusion as the sensor's diagonal measurement divided by 1500 — full frame's 43.27mm diagonal gives 0.0288mm, for example. Other widely used conventions divide by 1442 or 1730 instead, producing a slightly different acceptable-blur threshold and, downstream, slightly different hyperfocal-distance and depth-of-field numbers from the identical physical setup. None of these divisors is objectively correct; each encodes a different assumption about typical print size and viewing distance.

Why Sensor Size Enters Here Specifically

A smaller sensor needs more enlargement to reach a given print size than a larger sensor capturing the same scene, which means a smaller physical blur disc becomes visible at that same print size — this is why smaller sensors get a smaller, stricter circle of confusion under every common convention, not an arbitrary choice.

See circle of confusion drive a real result for your own sensor and settings in the DoF & Hyperfocal Calculator.