The maximum diameter of a light point still perceived as sharp; determines a lens's depth of field.
Technical Details
For full-frame cameras (36×24 mm), the standard CoC is 0.030 mm, for Super-35 sensors it is 0.020 mm, and for Micro Four Thirds it is 0.015 mm. These values are derived from the geometric relationship between sensor size and a typical viewing distance of 25 cm for 20×25 cm prints. The CoC value directly influences depth of field calculations: smaller values result in shallower depth of field with identical camera settings. Modern hyperfocal distance tables and DoF apps use these base values for precise depth of field calculations.
History & Development
Lord Rayleigh first scientifically defined the circle of confusion for photographic applications in 1879. The currently used 0.030 mm standard for 35mm film was established in the 1920s with the Leica camera. Kodak used the 0.025 mm value for their exposure tables starting in 1936. With the transition to digital sensors from 2000 onwards, new CoC standards emerged for various sensor sizes, with pixel pitch development leading to ever smaller critical circles of confusion.
Practical Application in Film
Cinematographers use CoC calculations for precise follow-focus operations and depth of field planning. In "Blade Runner 2049" (2017), DoP Roger Deakins worked with CoC apps for exact hyperfocal distance calculation for wide landscape shots. Steadicam operators use CoC-based DoF markings on the lens for quick focus pulls without monitor control. Focus pullers use these values to create precise focus marks on follow-focus wheels, especially with long focal lengths and wide apertures.
Comparison & Alternatives
The circle of confusion differs from bokeh quality, which describes the aesthetic appearance of out-of-focus areas. While modern focus peaking systems visualize focus ranges in real-time, CoC calculations remain indispensable for pre-production planning. Waveform monitors and false-color displays complement, but do not replace, the mathematical precision of the circle of confusion concept. For anamorphic lenses, modified CoC values apply for the horizontal and vertical image axes due to different magnification factors.