Image-erecting prisms (porro or roof prisms) are used in binoculars
to provide correctly-oriented (erect and right-reading) images. With porro prisms
(named after Ignazio Porro, the Italian scientist who invented them), the quality
of the glass in the prisms affects the performance of the binoculars.
When you hold a binocular up to a light source, the exit
pupil is the small circle of light you see in the eyepiece. The best porro prism
binoculars, those that use costly high density/high transmission BaK4 (barium crown
glass) prisms, have circular exit pupils.
Less-expensive porro prism binoculars use inexpensive BK7
(borosilicate) glass in their prisms. This glass produces gray areas cutting across
the edges of the exit pupils, making them look like square pegs in round holes.
(This is due to some of the light passing through the faces of the low refractive
index BK7 glass prisms instead of being 100% internally reflected as it should be.)
Image brightness in these vignetted areas is reduced by
a clearly visible 25%, which is often objectionable in dim light as it affects overall
brightness. In addition, exit pupils may not only be vignetted by cheap prisms,
but may also have totally cutoff edges due to poor design.
Because roof prisms (named after their resemblance to a
house roof) use a different light path than porro prisms, they cannot vignette the
image the way porro prisms can, no matter what type of glass is used. Accordingly,
they do not need high density BaK4 prisms to perform properly. Examining the exit
pupil of roof prism binoculars for prism cutoff will therefore not
the presence (or absence) of BaK4 glass.
There are as many as ten
lenses and prisms between a binocular’s objective and your eye. Each has two surfaces
that reflect back and lose about 4% of the light striking each surface. Coating
these surfaces with a vacuum-deposited antireflection layer of magnesium fluoride
one-fourth of a wavelength of light thick (a few-millionths of an inch) increases
the light transmission through the surface. This reduces light loss to 1.5% per
surface (0.5% or less with multiple layer "multicoatings" of zirconium oxide, cryolite,
zinc sulfide, and other arcane materials.)
These coatings give you brighter images because of their
increased light transmission. They also yield sharper, less hazy, and higher contrast
images, because they reduce the amount of light scattered randomly within the binocular.
Inexpensive binoculars usually have only their outer lens
surfaces coated, to give a false impression of quality. Every binocular we carry
is fully coated, with at least one antireflection layer on all
Some are multicoated on lenses and eyepieces, with single-layer coatings on their
prisms. These are described as "multicoated." Those listed as "fully multicoated"
have multiple coatings on all
You can check the coating quality by examining the color
of the image of a fluorescent light reflected in them. Magnesium fluoride coatings
of the correct thickness give a purple/violet tint to the largest reflected image
(the one on the outer lens surface), although the tint can range from pale blue
to magenta, depending on the type of glass in the lens. Magnesium fluoride coatings
that are too thin yield a pink reflection, while coatings that are too thick look
green. The smaller reflections (from the internal surfaces of the lenses) are generally
off-white and tinted a faint violet, amber, or green.
Multicoatings are usually green in color, although the reflected
image can be any of a number of tints, even red and amber, depending on the type
of glass and the coating materials used.
Binoculars come in several body styles
– the two-piece German Z (Zeiss) body, the one-piece American B (Bausch & Lomb)
body, and H-shaped one- or two-piece roof prism bodies.
Z-body and B-body binoculars use porro prisms to provide
correctly oriented images. Porro prisms are two triangular prisms offset from each
other so that the light travels in an S-shaped zigzag path, as shown above.
This porro prism zigzag results in a longer light path so
magnifications can be greater; the ability to use larger objectives for better performance
in low light; and objective lenses that are spaced further apart than your eyes
to provide a more three-dimensional image when used close in (nearer than 25 to
H-body (roof prism) binoculars typically use two roof-shaped
prisms that are aligned side by side. They provide the same image-erecting and magnification-increasing
functions as porro prisms.
Roof prism designs result in physically compact H-shaped
binoculars that have objective lenses more or less in line with your eyes, making
the close-in three-dimensional effect not as noticeable as it is with the larger
and usually heavier Z-body or B-body porro prism models.
binoculars (first developed by Carl Zeiss Optical in Germany) mount the objectives
in separate front barrels that are screwed into the prism-holding body. This provides
possible entry points for dust and moisture where the barrels join the body, although
rubber armor will minimize this possibility. Inexpensive
such two-piece construction potentially can have their optics knocked out of alignment
more easily than one-piece B- or H-body models.
Also, Z-body prism mounting plates are part of the Z body
itself, so the prisms are clamped into the body in a predetermined position, which
may or may not be optimal, depending on how carefully the body is machined. As Z-body
binoculars can be less expensive to manufacture and align, most no-name "department
store special" binoculars are of this type.
However, excellent Z-body binoculars are
from some of the name-brand manufacturers we carry – such as Nikon, Swift, and Celestron.
We have no reservations about recommending Z-body binoculars from manufacturers
of this quality, as their products are capable of a lifetime of fine performance
when treated with reasonable care.
B-body binoculars use sturdy one-piece body castings to
hold both objectives and porro prisms, providing a better dust and moisture seal
than an inexpensive Z body and offering less opportunity for the optics to be knocked
out of alignment. In addition, B-body prisms are usually collimated (aligned) and
mounted onto separate alignment plates before being installed in the body, allowing
for a more precise optical path.
roof prism binoculars use one- or two-piece body castings to hold objectives and
prisms in alignment. Those using one-piece castings provide the same sealing and
durability benefits as B-body binoculars and are often waterproof. Those using two-piece
castings are more rugged than two-piece Z-body binoculars, due to their more compact
design, but are usually only water-resistant or showerproof, rather than fully waterproof.
True fully-submersible waterproof and fogproof binoculars have to be nitrogen-purged
of all oxygen and moisture, then hermetically sealed – an expensive process. Waterproof
binoculars that are not nitrogen purged won’t leak, but they may fog internally
under extremes of cold and humidity.
Roof prism binoculars require much more precise (and therefore
more expensive) optical fabrication and collimation than porro prism binoculars.
The angle between the two prism faces in a roof prism binocular, for example, must
be accurate within 2 seconds of arc, a mere 1/1800th of a degree and 300 times more
precise than the 10 arc minute accuracy allowed with porro prisms. Because of the
accuracy required, roof prisms are usually laser collimated and are rigidly mounted
on alignment plates prior to being installed in the binoculars, allowing virtually
The design of most roof prisms requires that one surface
of the prism be an aluminized mirror. This means that roof prism binoculars typically
will not be as bright as similar-aperture porro prism models, due to the 12% light
loss typical of a mirror. During daylight observing, however, this small light loss
is rarely visible. Most observers feel that the convenience of the roof prism’s
compact size and light weight more than make up for the light loss. Premium roof
prism binoculars that use fully multicoated optics and high-reflectivity silver
mirrors, on the other hand, can be as bright or brighter than similar-aperture porro
prism models, although much higher in price.
Some binoculars are available clad in rubber
or polyurethane armor. This soundproofs the binoculars, so the metallic clink of
binocular against tripod or spotting scope won’t alarm birds or wildlife. But any
binocular will benefit from rubber armor, as it also protects precisely-aligned
optics from unavoidable knocks and shocks, extending their useful life. This is
particularly important with two-piece Z-body binoculars.
In addition, armor makes binoculars easier to hold when
wet – and more comfortable to hold in very hot or very cold climates. Armor also
improves the water-resistance of binoculars. While it won’t provide protection against
a dunk in a mountain stream, armor makes an occasional shower less of a threat to
a binocular’s well-being.
Most binoculars have a central focusing
knob that changes the focus by moving both eyepieces simultaneously. Porro prism
binoculars usually move the eyepieces in and out of the binocular body itself, leading
to potential entry points for moisture, although the chance is lowered if an eyepiece
rainguard is used during showers. Roof prism binoculars usually focus by moving
optical elements internally, leaving few places for dust or moisture to enter.
Center focus binoculars have a diopter adjustment for one
eyepiece to let you compensate for any difference in the strength of your eyes.
Correct for that difference once, and you can ignore the adjustment thereafter,
as the difference will be taken care of at all distances and positions of the central
Some binoculars have rocker-type fast focusing (Insta-Focus).
While this has some advantages over a central knob – the ability to change focus
quickly while tracking birds in flight, for example – it is not as durable or precise
as central focusing. Instant focusing is only found on inexpensive binoculars, where
good optics are rare and precise focusing is therefore not required. We neither
carry nor recommend Insta-Focus binoculars.
Some waterproof binoculars focus by rotating the eyepieces
individually. This is much slower than central focusing and so is not recommended
for birding. It is usually reserved for nautical binoculars that are primarily used
for scanning the horizon and consequently have no need for fast or frequent focusing.
"Permanent focus" binoculars are not
They are usually prefocused at a 50’ distance, so that close-in subjects are always
out of focus. They often offer no way to compensate for eye strength differences.
What those words and letters mean:
describe their binoculars in their model designations. For example, "10 x 40 B/GA"
binoculars are 10 power, 40mm objective lens, long eye relief, armored binoculars.
Here’s what the code letters mean:
either a porro prism binocular with a Bausch &
Lomb-style one-piece body (American or Japanese usage), or a binocular with long
eye relief for eyeglass use (German usage, from the German word "briller," meaning
compact roof prism binocular.
center focus (not "close focus" as many people
roof prism binocular (abbreviation of "dach,"
the German word for "roof").
H-body roof prism binocular.
individual focusing eyepieces.
phase-corrected prism coatings.
porro prism binocular with Zeiss-style two-piece
Some manufacturers use other letters (such as T*, FMT, RC,
SL, SLC, etc.) to denote specific features or models in their binoculars, but the
definitions above are the only ones in industry-wide use.
Why the grey market is something to avoid:
imports are products sold in this country that are meant to be sold overseas only.
They usually don’t have the same accessories as those meant to be sold in the U. S.
(cases and rainguards may be deleted, for example).
Grey market products are purchased from a retailer overseas
by a U. S. dealer (usually a camera store) and brought into this country without
going through the manufacturer’s quality control process.
Manufacturers of quality optical equipment have a vested
interest in seeing that you receive perfect optics. Unfortunately, their products
are sometimes jarred out of alignment during shipment from their plants. The manufacturers’
U. S. quality control centers inspect and fine-tune the optics before shipping
them to dealers, so your optics will
Grey market imports bypass this final quality control check.
Since the manufacturers don’t get to inspect the optics, they can’t guarantee their
performance, so grey market imports are not covered by any
warranty valid in this country.
If a grey market product is defective when you get it, the
manufacturer’s U. S. repair department will not
repair it under warranty.
You will have to pay for the repairs – or send it back at your expense to the manufacturer’s
repair department in the country where it was made for warranty service. Grey market
imports also do not
qualify for the special perks often standard with U. S.
warranty products – such as the three-year "Passport" no-fault replacement warranty
on U. S.-legal Leica USA imports.
Since quality optics last a lifetime, the difference in
price between a legitimate import and a grey market no-warranty import amounts to
only a few pennies a week over the life of the instrument. We don’t think it’s worth
giving up a lifetime of protection to save the price of a cup of coffee every other
month. That’s why we won’t sell grey market products.
We are a manufacturer-authorized dealer for every brand we sell (many grey market
outfits are not even dealers for the products they advertise). Everything we sell
has the full manufacturer’s warranty valid in the United States – and worldwide.
Don’t settle for less.