Terminology: Circle of Confusion

Yesterday I wrote about hyperfocal distance. I mentioned the concept of a circle of confusion. This sounds more confusing than it is.

It all comes down to perception. The human eye can only see so much detail. In fact, someone with perfect sight under good lighting can only distinguish between things that are at least a minute of arc apart or one-sixtieth of one degree. At reading distance, that means you could only distinguish between two points if there were at least a fifth of a millimeter apart. The distance is significantly larger if you're looking at a billboard from 200 feet away. That's because you're farther away, so a minute of arc traces out a bigger linear distance. (Think of stretching a long string from a central point outward. As you move the string around the point, the far end has to travel a greater distance than, say, the middle of the string to keep up.)

The circle of confusion actually has two meanings. In optics, it refers to the fact that no lens is perfect, so it won't actually focus images to a precise point. Instead, you get a little circle that is as close to a point as you can get. That is the circle of confusion.

To make that confusion a bit more confusion, photographers have a separate use of the term - the biggest circle that will appear as a point under the viewing conditions. It's really the maximum permissible circle of confusion, but photographers have shortened the phrase for convenience.

However, it all comes down to whether the circles the lens makes will look as though they're in focus. How this fits in with hyperfocal distance and depth of field is that when you focus your lens, you're actually focusing on a plane. Look at the image below:


Everything on the plane will be equally in focus. But as objects are farther from the plane, the image you get from the lens is a larger circle. When that circle gets too big - grows beyond the maximum permissible circle of confusion - it looks blurry. That's why you can take a photograph that looks in focus, hold a magnifying glass over it, and suddenly notice things that now look a little blurry.

If you're curious about the circle of confusion for a digital camera, I'll refer you back to DOFMaster, which has an extensive reference table and calculator.