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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 perspective."
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."
Practical differences
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
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