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.

Technique: Using Hyperfocal Distance

Your lens aperture setting will affect depth-of-field - the amount of an image that will appear to be in focus. The smaller the f-stop number, the smaller the depth-of-field. Practically speaking, if you open your lens all the way up and focus on something, chances are that less of what you see will ultimately seem in focus than if you stop the lens down.

But you may find that you need to use a wider aperture because of the specific lighting conditions. If you still want s big a DOF as possible, then you want to know about the hyperfocal distance. If you focus the lens on something that distance away, you'll get everything from that point to infinity and some about in front of it. Better lenses come with a scale that can help, as you can see in the image below:
The small numbers at the bottom are different f-stops. The numbers in the window are the distance at which you are focusing. Say you wanted the hyperfocal distance at f/16 (a smaller aperture, but the technique works there, as well). Then you'd set the focus on manual and turn it so that the infinity sign on the right was lined up over the 16. The center line on the scale below will point to the hyperfocal distance. The distance over the 16 on the right shows the closest distance that will appear in focus.

Ah, but what do you do when the lens doesn't have this feature? Use either a hyperfocal distance chart or calculator. Here's a spot that has some free downloads that should be useful. You might notice the term circle of confusion - I'll get to that tomorrow.