Based on the physics of Airy discs, lens makers have established a standard for measuring resolution based on pairs of black
and white lines. This, of course, assumes ideal lighting conditions, 20/20 vision, the distance from an object to the eye,
and various details concerning brightness and contrast. In photography, the assumptions also include how much a photo will
be enlarged from a negative or digital image. Studies on human visual acuity indicate that the smallest feature an eye with
20/20 vision can distinguish is about one minute of an arc: 0.003″ at a distance of 10″. Optics, dealing as it does with human
perception, is in many ways the most imprecise of the sciences that abound with measurements and complex math and formulas.
The standard that optical engineers use, regardless of human acuity tests, is that 13 pairs of black and white lines per millimeter
is the finest resolution that can be seen. More lines than that take the viewer into the gray Rayleigh territory. That’s equivalent
to about 30μm (.03mm).
A point of light can undergo the Airy disc distortion and be enlarged in a photographic print, and as long as it does not
exceed 30μm, it is still perceived as sharp. But if the point of light grows larger than that, it’s considered blurry; the
larger it gets, the blurrier it is. It is this visual definition of sharpness that we’re looking for when we say a picture is in focus.