The camera lens changes the direction of travel of incoming light rays. After their transient through the lens, the rays trace out a cone shaped path. Hard sharp focus is achieved when the distance, lens to film or digital sensor is such that the apex of this cone just kisses the prepared surface. In perfect world, a cross-section of the point of contact would be a point with no discernable dimension. In actuality, due to residual uncorrected aberrations, we get a diffused circle with scalped margins. We are taking about the circle of confusion. This is the smallest fraction of an image that that conveys intelligence. The entire image area consist of countless such circles, some tiny some huge. Size matters. For us to perceive a image as tack sharp, the circles must be so small that they are observed to be points not circles. The permissible size to have this happen is based on viewing distance, image contrast, lighting level, and the acuity of the observer’s eye.

The basis used to determine the permissible size of the circle of confusion is the resolving power of the human eye. Under bright light, an observer with 20/20 vision will perceive a coin at a distance of 3000 diameters, to be a point and not a disk. Stated another way, a 1 meter wagon wheel viewed from 3 kilometers (1 yard viewed from 9,000 feet or 1.7 miles. Such is too stringent for photographic work. This is because of the contrast of our media and the ambient light that we typically work in. More realistic for photo purposes are a disk viewed from 1000 times its diameter. This works out to 3.4 minutes of arc. Some examples: 1/100 of an inch viewed from 10 inches corresponds to 0.254mm viewed from 245mm. Another way to state this is 1/50 of an inch (0.5mm) viewed from 20 inches (500mm).

When we talk about the size of the circle of confusion at the focal plane of a camera, the size of the camera’s format must be taken into account. A 35mm film camera yields a tiny image that must be enlarged about 8X to yield an 8x10 inch image. Therefor the size of the circle of confusion must be made tiny to allow this 8X enlargement. So we end up with a hodgepodge of circle sizes that may or may not fit the viewing requirement. The industry has generally settled on using a circle size of 1/1000 of the focal length. Such a scheme roughly takes into account that the image will be enlarged to make a satisfactory display. For critical work Kodak uses 1/1750 of the focal length and Leica 1/1500.

Using the 1/1000 rule of thumb with a 50mm lens, the permissible circle size is 0.050mm. Such a circle size will permit 10X enlargement 0.05 X 10 = 0.5mm viewed from 20 inches.

That’s how this stuff goes, lots of gobbledygook ?