Lens__ Circle of confusion

[Doremus Scudder]

Correct me as a non native English speaker, but I thought that the correct English expression was: circle of diffusion and not circle of confusion?
I find it a bit confusing...

I guess we should be serious for a while and clear this up. The responses you've got so far are indeed humorous, but not really to the point

The Circle of Confusion (CoC for short) is a function of focus. At sharpest focus, a lens makes a spot, not a point, since there are always some aberrations and imperfections in the lens. As that spot is defocused (e.g., changing lens-to-film distance), it becomes a circle, which grows in size the more out-of-focus the spot gets (aperture affects the growth of the CoC, hence small apertures for more depth of field). This is the CoC. I'm not sure how the word "confusion" got attached to this, but that's the term.

Diffraction causes a similar phenomenon, but the defocused areas in this case are called Airy Discs. Again, I'm not sure why the nomenclature ended up this way. Diffraction is always present, caused by the scattering/bending of photons as they pass an edge. Smaller apertures cause more apparent diffraction because the diffraction quotient of the total light passed is larger at smaller apertures.

"Diffusion" usually refers to the spreading of a concentrated substance through a less concentrated carrying medium, such as water or air, till equilibrium is reached. Think of a sugar cube slowly dissolving in your tea until it is all equally sweet. See https://en.wikipedia.org/wiki/Diffusion

Best,

Doremus