Design Nomenclature / Light & Optics
Circle of confusion
Also called CoC, blur circle, blur spot
- Light & Optics
- Systems & Spec
- 2D & 3D
This entry has not been translated yet and is shown in English.
The disc a point source projects onto the sensor when it sits away from the focus plane. Depth of field is entirely a decision about how large this may grow before it reads as unsharp — a threshold set by print size, viewing distance, and eyesight, not by physics.
In practice
The 0.03 mm figure baked into every DOF calculator is a convention for a 35 mm frame at a 25 cm viewing distance with normal vision. Change any of those and it is wrong.
Not to be confused with
- Depth of fieldThe band of subject distances rendering acceptably sharp — where the blur circle stays under a chosen threshold. Exactly one plane is truly in focus; everything else is a compromise about how much unsharpness passes. It narrows with wider apertures, longer lenses, and closer subjects.
- BokehThe character of the out-of-focus rendering — how a blur circle is filled, whether its edge goes bright or soft, whether it turns to onion rings or cats' eyes toward the corners. A judgement about quality, set by the lens's aberration correction and the shape of its iris.