Circles of Confusion and Sweet Spots — Part One

Author: Sara Goodnick

Circles of confusion? Sounds like the start of a joke, doesn’t it?

According to Wikipedia: “In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source.”

Whoa! OK, I’m going to try to really simplify this so you can understand and use it to improve your photography. This is closely related to depth of field. For those of you new to photography, depth of field refers to how much of your image has acceptable sharpness at a given distance. This can be very short, or extremely long.

Camera lenses work by allowing a certain amount of light to pass through them and onto a sensor. The lens elements bend the light rays to place them onto the sensor. We focus the light rays by moving the focus ring on the camera to put our subject into focus.

The aperture, or hole the light rays pass through before reaching the sensor, is measured by what we call an f-stop. The numbers related to f-stops are actually measurements of the size of the hole-but they are in fractions. So, f/2.8 is a larger hole than f/22. Think of it like this: wouldn’t you want 1/2.8th of a piece of chocolate cake more than a minuscule 1/22nd size piece?

Stopping down, or making the aperture (f-stop) smaller, gives a greater depth of field. Like when you squint your eyes to see better. It means more will be in focus front to back than with a larger aperture.

So, for instance, if you want your subject, such as someone’s face to be in focus, but not the area behind that person, you would focus on the eyes, and use a large aperture, such as f/2.8. If you were standing close enough to the person, the background would be out of focus if it were far enough away.

Conversely, for a landscape photograph, you probably would want everything in focus, from the boulders and bushes 30 feet in front of you to the horizon. In this case you would want a smaller aperture, f/11 or 16. There’s more to it than just this, but this is one element of increasing depth of field.

So, why not go for the f/22? Wouldn’t that include even more? Well, sometimes you might notice that stopping way down does not improve the sharpness, it actually makes the image softer in some areas.

This has to do with your particular camera sensor and lens combination. As the aperture gets smaller, the light rays are forced closer together and begin to interfere with each other, cause diffraction. This will result in a loss of sharpness in your image.

You will need to test your own camera and lens combinations to learn their specific limits.

Look at the examples below. It is hard to see at this resolution, but if you look at the tip of the cactus in the center you will see that the needles are clearer in the middle photograph, shot at f/8.0. At the opposite ends of the spectrum, f/2.8 and f/22, the needles appear soft.All images were with a Nikon D700 on a tripod, set for aperture priority, lens 70-200 set at 70mm.


Sara Goodnick is a trip leader with Arizona Highways Photo Workshops.