The newly formed point is only a point in theory. From traveling
through the air and the lens, enough of the original light beam has
been diffracted so that what actually strikes the focal plane is an
Airy disc, such as you encountered in the previous illustration. But,
assuming we’re using an optically perfect lens adjusted with absolute
precision, the Airy disc’s diameter would be the least circle of
confusion. That is the smallest point of light that quantum physics
will permit. Because of the limits to how much the eye can distinguish
details, objects at the plane of critical focus might actually have some
distortion. But because the distortion is smaller than the eye can
discern, the objects appear to be in focus.3
As objects get farther from the critical plane, their smaller details are subsumed into
overlapping Airy discs, losing individual identity. The process continues until even
large objects that are far enough away from the critical plane fall below a threshold
of focus that’s acceptable to the human eye and perception. That’s when we officially
declare them to be “out of focus.”6
Calculating DOF
Depth of field (DOF) might seem like a topic
for which a degree in higher math would be
handy. Well, in this case, it would. But lack-
ing the degree, you can take advantage of a
couple of dozen calculators that will figure
depth of field on your computer, handheld, or
even without the advantages of electricity at
all. The DOF Calculator, shown at the right, is a
circular slide rule you can print and assemble that
fits nicely on a lens cap. You’ll find it and more calcu-
lators for DOF and other optic esoterica at http:
//www.dofmaster.com/, http://www.outsight.com/hyperfocal.html,
http://www.betterphoto.com/forms/linkAll.asp?catID=71, and
http://www.tawbaware.com/maxlyons/calc.htm.CHAPTER 4 HOW LIGHT PLAYS TRICKS ON PERCEPTION^61