Circle of Confusion
The largest blur circle that still reads as a point to the viewer, and the value on which every depth-of-field calculation depends.
Formula / rule
c ≈ diagonal / 1500
Measured in
mm
In depth
The standard figure of about 0.03 mm for full frame assumes an 8×10 inch print viewed at 25 cm by someone with normal vision. Change any of those assumptions — a bigger print, closer inspection, a 60-megapixel file examined at 100 per cent — and the acceptable circle shrinks, which is why published depth-of-field tables are always more generous than modern pixel-peeping allows.
Examples in practice
Using 0.015 mm rather than the standard 0.03 mm halves every published depth-of-field figure for critical work.