Depth of Field and the Circle of Confusion
In the previous post, I discussed the relationship between aperture and depth of field. A larger aperture means a smaller depth of field and a smaller aperture means a larger depth of field. The depth of field of an image is the distance between near and far objects in a photograph which appear acceptably sharp. But how do we define “acceptably sharp?”
Although many factors impact sharpness such as eyesight and medium, there is a standardized measurement for determining what is acceptably sharp. This is based off of the optical idea of “circle of confusion,” or the cross-sectional circle of a cone of light projected on the focal plane from a point-source as a result of a lens not being in perfect focus (see image below). Ideally, a point source would project perfectly to a point on the focal plane, but this is never the case due to technological limitations. Therefore, when we want to look at acceptable sharpness, we’re really looking at the largest size this circle can become before it starts looking blurry, or the maximum permissible circle of confusion. It is important to note that although this is only one case of circle of confusion, the maximum permissible circle of confusion is often referred to by photographers as only circle of confusion. From here on out, I’ll be referring to the photographic definition of circle of confusion (CoC), which is implied to be this maximum size. Somewhat confusing, isn’t it?
As the aperture size decreases, the depth of field falling within the circle of confusion increases, so more of the image is in focus. (source)
Alternatively, CoC may be explained by the maximum amount a single point must be blurred to be considered unsharp. The acceptable sharpness is influenced by two main factors: size of the print and viewing distance. As an image is enlarged or viewed more closely, out of focus points will be more noticeable. Based on this, a loose definition of circle of confusion is one that would still be seen as a point by a human eye when an image is printed as an 8x10 and viewed from one foot away (Cambridge in Colour). Camera manufacturers define this circle of confusion to be 0.01in. However, this changes based on the scaling factor, medium, and distance. For example, different cameras may have different circles of confusion if they have a crop factor.
The size of the circle of confusion on the image sensor itself (not how our eyes perceive it) is only based on aperture and focusing distance. This acceptable range in size is the depth of field.
Although circle of confusion may seem like a complicated topic at first, the important thing to remember is that it is a threshold maximum value in its photographic definition: the largest size of a point projected as a circle on the image plane which is nearly indistinguishable from a point. Depth of field is simply the subject depth where this condition is fulfilled.