Lens and ray diagram for calculating the circle of confusion diameter c for an out-of-focus subject at distance S2 when the camera is focused at S1. The auxiliary blur circle C in the object plane (dashed line) makes the calculation easier.

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Lens and ray diagram for calculating the circle of confusion diameter c for an out-of-focus subject at distance S2 when the camera is focused at S1. The auxiliary blur circle C in the object plane (dashed line) makes the calculation easier.