Digital Prints

Visual Acuity & Maximum Resolving Power: The ability of the human eye to distinguish
fine detail is called visual acuity, and it is directly related to distance. As you move
farther away from the visual source, you reach a point where you no longer see the detail,
and everything merges together. This can be determined scientifically by using alternating
black-and-white lines of a specified width and then measuring the angle made from
the eye to these lines at the maximum resolvable distance. It has been shown that the visual
acuity of a normal eye with 20/20 vision is somewhere between 30 seconds of arc (when
lighting is “ideal”) and one minute of arc (when the lighting is “ordinary”). This is the
maximum visual resolution possible for most humans.
From this information, all kinds of interesting formulas [c = 2 × d × tan(RADIAN
ANGLE SYMBOL ÷ 2)] and conclusions can be drawn (see Table 2.2). One is that at any
given viewing distance, you gain nothing by having higher resolution than the maximum
resolving power of the eye because no finer details can be perceived. This is the upper limit,
so there’s no point going beyond that.
However, things are not so simple. These resolving power charts are based on highcontrasting,
black-and-white lines or letters (see illustration above and think of the chart
at your eye doctor’s office). The images that most of us print are anything but that. We
have complex patterns of dots or device pixels, overlapping dots, and all the rest. So how
does Table 2.2’s “details per inch” relate to the dots per inch of inkjet printing? It is generally
believed that printer resolution (dpi) must exceed maximum visual resolution
(“depi”) by a significant amount, on the order of double, triple, or more.
Plus, as digital imaging writer and publisher Wayne Cosshall explains it, there are other issues
like presentation. If you print on fine art or textured paper, you could get away with a lower
resolution because the paper’s texture will create its own detail and somewhat fool the eye. Also,
if you frame a print behind glass that lowers the contrast of the print a little, so again, you can
get away with less print resolution.
Chapter 2 ■ Understanding Digital Printing 51
A standard USAF 1951 resolution
target for measuring visual acuity.
Courtesy of Edmund Industrial Optics