Depth of Field and the Circle of Confusion
In one of my previous articles on this website I have delved about the concept of depth of field (DoF) as well as the parameters that govern DoF. As you can recall, DoF depends on several aspects, chief among those being the aperture value in use. Bigger the aperture, shallower the depth of field and vice versa. There is, however, yet another concept that is closely connected with DoF and that is the circle of confusion.
Before we understand what the circle of confusion is we need to understand how an average lens projects an image on to the sensor. Inside the lens are a number of elements (or groups of elements) which work in tandem to make the image project perfectly on to the image medium at the back of the camera. These are all important but for this discussion we will consider only one group and that is the focusing group.
The Circle of Confusion: Exploring Photography with Mark Wallace
Light travels in a straight line. That means when light originates from a point source it should appear as a point source when it’s perfectly sharp on the image plane. This is the task of the focusing group. The focusing group refracts the light coming through the lens and then projects it on to the sensor / film (image medium) at the back of the camera. The image medium is also known as the focusing plane and the point where the light merges (or at least theoretically should merge) is known as the focusing point.
When the focusing point is perfectly aligned with the focusing plane the result is a tack sharp image. When it is not aligned perfectly, we get what is known as a blurred image. It is an image where the elements are not perfectly sharp. There are a number of reasons for an image to be blurry. However for this discussion we shall consider it is because the lens / we were unable to focus properly.
If the elements are not wildly out of focus, they may still appear to be reasonably sharp. This happens because our eyes accept them to be sharp. This is where the concept of circle of confusion comes in.
The circle of confusion is basically about the degree of tolerance that the human eye has before it starts to make a clear distinction between an object that is out of focus and one that is in focus. Let’s say that there you are shooting an arrangement of dinner spoons laid out on a straight line. You are using an aperture of f/4. You set your AF point on one of the spoons and click. Does all the spoons appear in focus? No. While the spoon you focused on appear tack sharp, the one immediately in front and immediately behind appear to be out of focus. Now change the aperture to f/8 and make the exact same image again. Did you notice the difference? The spoon in focus is tack sharp. But the spoon immediately in front and behind also appear to be reasonably sharp. If you set your aperture to f/11 and remake the shot all three spoons will appear perfectly sharp.
Why this happened? This is because with the aperture stopping down by one stop each time, you are basically increasing the depth of field. With it, however, something else is also happening and that is you are involving the concept of circle of confusion. Even at f/11 only the spoon that you had focused on is truly sharp. The other two spoons are merely appearing sharp because even though they are slightly out of focus, your eyes are unable to tell the difference. That in essence is the concept of circle of confusion.