About Binoculars

In roof prism binoculars, the prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, streamlined shape in which the lenses and prisms are in a straight line. Roof prism binoculars are less bulky and more rugged than an equivalent porror model.

In porro prism binoculars, the objective or front lens is offset from the eyepiece. Porro prism binoculars provide a greater depth perception and generally offer a wider field-of-view. Because of the simplicity of this system, some of the best values can be with a porro design. Porro prisms generally offer a rich depth of field, wide field of view, and three-dimensional image.

So which is best? A group of bird watches did an amateur test between the two and came to this conclusion.

The results? Although we expected the Porros to be brighter, no one could see any difference in brightness or contrast between the two. We repeated the test on a dark, overcast day, with the same results. However, all the judges reported they saw slightly more detail through the Venturer roofs than through the Superior E Porros. In this individual case, roof bested Porro.

Of course, this test used only one sample of each model binocular. A better test would have compared multiple samples.

Nevertheless, we believe that, whatever optical deficit roof prism designs once had, the improvements in roof prism optics such as phase-corrected coatings have erased them. And in the marketplace, where consumers vote with their wallets, birders who are buying top-end binoculars tend to elect roofs over Porros. This is why almost all the new advances in birding binoculars have been in roof prism designs.

This is part of an article that first appeared in Bird Watcher’s Digest, January, 2000.

Some binoculars are O-ring sealed and nitrogen-purged for total waterproof and fogproof protection. These modesl can withstand complete immersion in water and stay dry inside. The interior optical surfaces won’t fog due to rapid temperature change or humidity.

Binoculars are often referred to by two numbers separated by an “x”. For example, 8×32. The first number is the power of magnification of the binocular. With an 8×32 binocular, the object being viewed appears to be eight times closer than you would see it with the unaided eye.

The second number in the formula (8×32) is the diameter of the objective front lens. The larger the objective lens, the more light that enters the binocular and the brighter the image.

Most optical prisms are made from boroscillicate (BK-7) glass or barium crown (BaK-4) glass. BaK-4 is the higher quality glass yielding brighter images and high edge-to-edge sharpness.

BAK-4 is a high density glass of better optical quality than BK-7. With BAK-4, if you hold binoculars away from your eyes and up to the light you can see the circular exit pupils in the eyepieces. The less expensive BK-7 prisms have squared-off, non-circular exit pupils.
The prism is alike: Bak4 prism costs more than Bk7 prism – Bk7 prism is fine to most of the users, but, to some “picky” users, Bak4 prism is a must. The difference is the quality of glass and anti-reflective coatings. The design of the prisms can affect the quality of the exit pupil. The BK7 prism typically found in lower priced binoculars produces an exit pupil with shaded (blurred) edges, whereas the BAK4 prism has a perfectly round exit pupil. Whilst the BK7 prism design would look no different to the BAK4 in daylight, as the light levels drop, and the eye pupil expands you start to observe the effects of the shaded regions as the image quality drops and becomes prone to chromatic aberration around the periphery of the image.
See the illustrations bellow

Lens surface coatings reduce light loss and glare due to reflection for a brighter, higher-contrast image with less eyestrain.

Coated – A single layer on at least one lens surface.
Fully Coated – A single layer on all air-to-glass surfaces.
Multi-Coated – Multiple layers on at least one lens surface.
Fully Multi-Coated – Multiple layers on all air-to-glass surfaces.

But note how the manufacturer describes his coatings. “Coated” means a single layer antireflection coating on some lens elements, usually the first and last elements–the only ones you can see. “Fully Coated” means that all air to glass surfaces are coated. This is good. “Multi-Coated” means that at least some surfaces (again, usually the first and the last) have multiple layers of antireflection coatings. (The others presumably have single layer coatings.) Multiple layers are about an order of magnitude more effective than a single layer. “Fully Multi-Coated” means that all air to glass surfaces have received multiple layers of antireflection coatings, and this is what you want in your binoculars.

The latest fad in coatings is ruby or red multi-coatings. These are intended to reduce glare in bright light.

The side-to-side measurement of the circular viewing field or subject area. It is defined by the width in feet or meters of the area visible at 1000 yards or meters. A wide-angle binocular features a wide field-of-view and is better for following action. Generally, the higher the magnification, the narrower the field-of-view.

Resolution, or definition, is the ability of a binocular to distinguish fine detail and retain clarity.

Refers to the size of the circle of light visible at the eyepiece of a binocular. The larger the exit pupil, the brighter the image. To determine the size, divide the objective lens diameter by the power (an 8×32 model has an exit pupil of 4mm).

The distance a binocular can be held away from the eye and still present the full field-of-view. Extended or long eye relief reduces eyestrain and is ideal for eyeglass wearers.

The distance a binocular can be held away from the eye and still present the full field-of-view. Extended or long eye relief reduces eyestrain and is ideal for eyeglass wearers.

A “fine focus” adjustment ring usually provided around one eyepiece to accommodate for vision differences between the right and left eyes.

Found on the best roof prism binoculars, this chemical coating is applied to the prisms to enhance resolution and contrast. Would not provide an advantage on porro prism models.

Rubber armor provides multiple benefits. It helps protect the binocular from the bumps and scratches that come with day-to-day use. It provides a comfortable gripping surface for making them easier to hold on to. It’s easy to wipe clean after a tough day in the field. And it suppresses noise if the binocular bumps aluminum or other non-rubber surfaces, which might otherwise spook wildlife.

Most of the information provided here courtesy of Bushnell.com