This tab just exposes one of the intermediate calculations that goes into the depth of field calculation. ƒ/Calc could display it in the Depth of Field tab instead, and may one day do that to save space in the user interface, but for now, we have the space to do it this way.
The “hyperfocal distance” is the nearest focus distance for a given aperture and focal length combination where “infinity” is still in focus. Practically speaking, this is the deepest possible depth of field for that focal length and aperture combination.
Some people use the hyperfocal distance rule to “prefocus” their lens for a given aperture for snap shooting. For example, if you have a 50 mm lens on a 35 mm camera and want to use an ƒ/22 aperture, you could focus your lens at about 4.4 m and have everything from about 2.2 m to infinity in focus.
A more common use of the hyperfocal distance is maximizing the depth of field for a given aperture. This is a common requirement in landscape photography. Since smaller apertures give greater depth of field, but also cause greater image degradation due to diffraction, it is best to choose the largest aperture that will still give an acceptable in-focus field.
f is the focal length of the lens, A is the aperture value and c is the circle of confusion value.