BetterPhoto Q&A
Category: New Questions

cercles of confusion

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November 02, 2001

"Since all lenses contain a certain amount of spherical aberration and astigmatism, they cannot perfectly converge rays from a subject point to form a true image point (i.e., an infinitely small dot with zero area). In other words, images are formed from a composite of dots, (not points) having a certain area, or size. Since the image becomes less sharp as the size of these dots increases, the dots are called "circles of confusion." Thus, one way of indicated the quality of a lens is by the smallest dot it can form, or is "minimum circle of confusion." the maximum allowable dot size in an image is called the "permissible circle of confusion."

Confused yet? No? Let me continue, but I'll stop quoting Canon.

What does this mean to us? Firstly, if the minimum circle of confusion occurs exactly on the focus plane. Not a centimetre in front or behind it. As you get further away from the in focus plane, the circles of confusion start becoming larger, and the photo begins to look "Out of focus" to the human. Depth of field is the area where the human eye believes that the circle of confusion is still a point (hence the photo is still sharp)

So much more physics I could throw in here, but you probably didn't want that level of answer. Let me know if you did. (or if anyone else is interested for that matter)

In a short answer, a circle of confusion is the "basic unit" of a photograph, similar to a pixel in digital photography. As the circle of confusion gets smaller, the resolution of the photo gets better. The size of the circle of confusion is determined by:

1) The quality of the lens,
2) the distance from subject the point of critical focus,
3) the distance between the point of critical focus and the lens,
4) and the aperture of the lens.

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November 03, 2001