Quotes from Charles A. Bergman, Audubon, November 1981
Porro prism (see Figure #1 above)
"But how do binoculars present an upright image to the viewer? ...An Italian named
M. Porro invented the first prismatic inverting system in the middle of the nineteenth
century...Porro's system consists of two identical prisms in each barrel of the
binocular, placed at right angles to each other. Each prism looks rather like one
of Napoleon's hats, the bicorne. Like mirrors, each prism reflects and reverses
the light off its steep sides. One prism reverts the image left to right and the
other prism inverts the image top to bottom. With an acrobat's grace, the image
somersaults through the binoculars, springing to our eyes, normal and erect, a natural
Roof prism (see Figure #2 above)
"Roof prisms were invented in the nineteenth century, in the optical workshop of
Carl Zeiss at Jena, Germany. A young professor of physics at the University of Jena,
Ernst Abbe, formulated the mathematical laws for the paths of light through microscopes.
Together with the chemist Otto Schott, he also invented the first high-quality,
reproducible optical glass. At age twenty-six, he became director of research for
Zeiss. Using his own mathematics and glass—both historical contributions to the
optical industry—Abbe created not only the microscopes of Louis Pasteur, but also
a pentaprism, the "Abbe roof prism." Abbe's roof prism was the progenitor of a vast
array of modern roof prisms. Inside, roof prisms juggle the light rays through an
invisible labyrinth of angles, a convoluted path, a mathematical marvel. On the
outside, a roof-prism binocular is elegant in its simplicity."
The differences in durability, image and optical quality in binoculars and scopes
are most pronounced between lower and middle priced optics with less dramatic changes
between middle- and high-priced optics. The two types of optic designs reach peaks
in each of these factors at different price levels.
The only difference between the two designs in full-sized binoculars that does not
change with price is their size and shape. Porro prisms are larger and more bulky
than roof prisms. Most bird watchers are inclined to stick with the slim roof prism
design although there are those with big hands that claim that the larger porro
prisms are more comfortable for them (I have large hands and do not find this so).
Porro prism binoculars reach very good optic quality at about $250. Roof prisms
usually do not reach an equivalent level of optical quality until about $400-$700.
Roof prisms, because of the engineering required, are more expensive than porro
prisms of equivalent optical quality. Modern roof prism binoculars tend to have
two separate prisms that are cemented together, rather than two offset prisms of
the porro prism design. Porro prism designs consequently tend to be much more susceptible
to alignment problems if dropped. This can be very costly to fix. Thus, both weather
resistance and durability are better in roof prism binoculars.
Roof prism designs are almost exclusively internal focusing and that makes them
much easier to seal and keep waterproof. Even expensive waterproof porro prism designs
typically are sealed with "o" rings and are not internal focusing. The mechanical
movements of the eyepiece (or objective) moving back and forth, and wear on an "o"
ring can cause problems over time. First is loss of the seal, allowing moisture
and dust into the interior. Moisture and dust settling on the mirrors degrade the
performance. Next, wearing of an "o" ring causes focusing friction. As the "o" ring
degrades, the focus becomes progressively stiffer. The need for nitrogen (or other
dry gas) fill and sealed optics to prevent internal fogging and condensation are
discussed in the weatherproof section.
Reverse Porro Prism