What’s the circle of confusion in photography?

If you want to master depth of field and learn how to shoot sharp, clear photos, then you must first learn the circle of confusion. Here’s how.

Have you looked at a photo and noticed the subject was clear and sharp while the rest of the picture was blurred? This is because of the depth of field, or the area in a photo that’s in focus when contrasted with the area behind or around it.

There can be a large depth of field, where there’s a large area of sharpness, or a shallow depth of field, where only a small portion is in focus. But why are we mentioning depth of field when this post is about the circle of confusion? Because understanding the latter is key to making the most of the former.

The circle of confusion is defined as the indistinct circular patch formed by a lens representing the out-of-focus image of a single object point. In other words, it’s the area that appears sharp even though it may not be technically sharp.

Depth of field doesn’t immediately change from clear to blurry. There’s an incremental transition. The circle of confusion defines how much an area can be blurred before it appears unsharp or out of focus to our eyes. Anything outside this area of acceptable sharpness (the circle of confusion) is outside the depth of field.

To better understand it, let’s look at the diagram below, which shows how camera lenses work to create a sharp image. When light passes through the lens, refraction causes rays of light to bend. The point where they converge is called the focal point.

When the focal point is aligned to the focal plane, the subject will be sharp. When you adjust the focus on a camera, the focus element inside moves either backward or forward, changing where the converging light rays are on the focal plane.

When the light rays aren’t in alignment, the light creates a circle of light on the focal plane that’s slightly larger and not perfectly in focus, although it still appears clear. This point of light is the circle of confusion. Anything that falls within this point of light will appear to be in focus. Anything outside it will be blurry.

Here’s another way to explain it.

Think of a magnifying glass as a camera lens and the ground as the focal plane. When you shine a light through the magnifying glass, the light forms a circle on the ground. If you move the magnifying glass closer to the ground, the circle becomes smaller and sharper. As you move it away, the circle becomes larger and less in focus. This circle of confusion would be the largest point of light on the ground that still appears in focus.

Let’s look at some examples with this in mind:

This photo showcases a narrow depth of field and focal plane that achieves crystal-clear sparks but leaves the blurred subject in the center, drawing the eye into the foreground. Photo by Todd Quackenbus.

This photo uses a narrow depth of field to achieve the opposite effect, focusing on the center of the image and leaving the blurred foreground to lead the eye into the center. Photo by Philippe Leone.

In the photo on the left, you can see how the subject isn’t clear throughout. In the photo on the right, the flowers are in focus and the background is softly blurred. Photo by David A. Kurtz.

Hopefully, this brief explanation gives you a better understanding of the circle of confusion and helps you better understand how we determine what’s in or out of the depth of field. Mastering depth of field will help you become a much better photographer.

Did we confuse you more or is everything crystal clear? Let us know in the comments below or start one on Twitter, Instagram, and Facebook.