Some Binocular Features

Prisms: 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.)

BK7 prism cutoff



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 reveal the presence (or absence) of BaK4 glass.

Anti-Reflection Coatings: 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 optical surfaces. 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 optical surfaces.

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.

Body Styles: 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 30 feet).

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 Light Path



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.

Parts of porro prism binocularsZ-body 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 binoculars using 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 available 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.

Parts of roof prism binocularsH-body 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 permanent collimation.

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.

Armor: 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.

Focusers: 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 focusing knob.

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 for birders. 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. Avoid them!

What those words and letters mean: Most manufacturers 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:

A or GA: rubber armored.

B: 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 "eyeglass").

C: compact roof prism binocular.

CF: center focus (not "close focus" as many people think).

D: roof prism binocular (abbreviation of "dach," the German word for "roof").

H: H-body roof prism binocular.

IF: individual focusing eyepieces.

P: phase-corrected prism coatings.

W, WA, or WW: wide angle.

Z: porro prism binocular with Zeiss-style two-piece Z-body.

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: Grey market 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 be perfect.

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 manufacturer’s 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 available only 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.


. . . our 34th year