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What to Look for in Binoculars

A Guide to Help You Purchase and Enjoy Quality Sport Optics.

What do the numbers mean?

Binoculars have one job – to magnify the view. They make things look closer or larger. There are three numbers that are important in understanding a particular binocular’s performance; magnification, diameter of the objective lens, and the field of view.

Magnification

When you see “8x40” on a model, that’s two of the three numbers. The 8 is the magnification. It means objects look eight times larger (or closer) than they do with the naked eye. A “10x50” would designate a ten-time magnification (sometimes shown as 10X).

Objective

The second number, or the 40 in the example given above, is the diameter of the objective lens (the big one facing the object being viewed) in millimeters. The diameter of this lens helps determine the brightness of the image - important to those who use their binoculars in low light (hunters, birders, boaters). Larger objective lenses let in more light. The downside is that larger objective lenses make the unit larger, heavier, and more expensive.

Field of view

Field of view is the width of the area viewed through the binocular. It is generally expressed on the binocular in degrees (angular field of view) and is often referred to in feet at 1000 yards (linear field of view). One degree of angular field of view equals 52.5 feet at 1000 yards. A binocular with a 5 degree angular field of view would have a linear field of view of 262.5 feet at 1000 yards (5x52.5 = 262.5). A wide field of view makes it easier to find the action. However, a wide-angle binocular often has short eye relief making it more difficult to use when wearing eye glasses.

Eye Relief

Eye relief is the distance from your eye to the rear most lens of the binocular’s eyepiece. This distance should be at least 17mm if you wish to use the binocu- lars with glasses and see most or all of the field of view. The long eye relief of PENTAX binoculars allows viewing of the entire field of view clearly and effortlessly. This is essential for eyeglass wearers and allows viewing for extended periods of time without added eye strain.

Specifications
Eye Relief

What determines image brightness?

Exit pupil

Exit pupil is the diameter in millimeters of the image forming circle of light that exits the binocular. It is the bright circle visible in the eyepiece when the binocular is held at arms length. Exit pupil diameter is calculated by dividing the diameter of the objective lens by the power. Example: For 8x40 binoculars, divide 40 by 8. The result is an exit pupil of 5mm. BaK4 Prisms Light rays at edge of the prism are refracted or bent, for even illumination across exit pupil. Even with BaK4 prisms, with some porro-prism designs, the prism cage or mounting may be visible in the ocular. This does not measurably affect image brightness or quality.

Generally speaking, the diameter of the exit pupil, is directly proportional to the brightness of the image. The maximum pupil dialation for most individual’s eyes is 7mm. However, as we age, a maximum of 5mm is more likely. Exit pupils of 5mm or greater are considered good for low light use, but only if the lens and prism quality and optical coatings are at the highest levels. Relative Brightness correlates to our perception of brightness. For example, a relative brightness factor of 25 will appear twice as bright as one of 12.5 if all other factors are the same. There are many ways to determine relatively how bright a binocular is (relative brightness, twilight factor, etc.), but these numbers don’t tell the whole story. A lower quality binocular will give the exact same number as the most expensive binoculars on the market. There are many other factors that must be considered.

Objective

The second number, or the 40 in the example given above, is the diameter of the objective lens (the big one facing the object being viewed) in millimeters. The diameter of this lens helps determine the brightness of the image - important to those who use their binoculars in low light (hunters, birders, boaters). Larger objective lenses let in more light. The downside is that larger objective lenses make the unit larger, heavier, and more expensive.

Field of view

Field of view is the width of the area viewed through the binocular. It is generally expressed on the binocular in degrees (angular field of view) and is often referred to in feet at 1000 yards (linear field of view).

Prisms

Image brightness is also directly related to the type of prisms and optical coatings used. All PENTAX binoculars use high index BaK4 prisms and fully multi-coated lenses.
Coating on front and rear elements only

  • Single or multi-coating is equally ineffective if only the front and rear elements are coated.
  • Up to 40% light loss!
  • Very low contrast.
  • Muted colors are difficult to discern detail.
  • Single coating on all elements (most common)
  • Up to 10% light loss!
  • Colors and details are greatly improved but still not best quality.
  • PENTAX super-multi-coating on all elements
  • Only 2.4% light loss!
  • Binoculars exhibit brilliant contrast, color, and it is easy to discern fine detail.

Aspherical elements

Aspherical optical elements, included in most PENTAX binoculars, greatly improve image brightness and sharpness, while minimizing disturbing distortions. The aspherical elements are specially shaped lenses that focuses light passing through the edge of the lens element at the same point as light passing through the center.
This is extremely critical in compact binoculars since the lens elements are much smaller than in full sized binoculars.

One degree of angular field of view equals 52.5 feet at 1000 yards. A binocular with a 5 degree angular field of view would have a linear field of view of 262.5 feet at 1000 yards (5x52.5 = 262.5).
A wide field of view makes it easier to find the action. However, a wide-angle binocular often has short eye-relief making it more difficult to use when wearing eye glasses.

Relative Brightness correlates to our perception of brightness. For example, a relative brightness factor of 25 will appear twice as bright as one of 12.5 if all other factors are the same.
There are many ways to determine relatively how bright a binocular is (relative brightness, twilight factor, etc.), but these numbers don’t tell the whole story. A lower quality binocular will give the exact same number as the most expensive binoculars on the market. There are many other factors that must be considered.

Any time light moves from one medium to another, for example air to glass, light tends to disperse at the interface. Some will reflect off the surfaces, some is absorbed by the material, and some light bounces off internal imperfections in the material. The goal of lens coatings is to minimize the surface reflections so that the maximum amount of light passes through the lens and reaches your eyes. Some manufacturers only coat the front and rear lens elements, or if they do coat all surfaces, they only use a three or four layer coating. While this does reduce some of the light loss due to reflections, it doesn’t do the best job in giving a bright, clear image and can lead to 10 to 40 percent light loss. All optical elements within every PENTAX product are treated with our time-proven multi-layer coating. This coating is impregnated in the glass elements which make it very durable, and not only improves light transmission to deliver bright, high contrast images without flare, but also reduces eye fatigue even during extended observation. With the 7-layer PENTAX smc coating, light loss is limited to as little as 2.4%.

Phase coating

By design, roof prism binoculars split the light entering the barrels into two separate paths. Although they are again combined when they reach the viewer’s eye, the result is reduced contrast and image sharpness, which is sometimes referred to as “out of phase” or “phase shift.” PENTAX has incorporated a thin coating on the roof prism surface of all roof prism binoculars, which forces the light beams back into phase, thus creating enhanced contrast and a sharper view.
Phase coating is not necessary in porro-prism binoculars since the light path is different than in roof prism binoculars.

Exit Pupil
A 10x50 model w/ a 5? real view angle
BaK4 Prisms VS BK7 Prisms
Super-multi-layer Coating
Aspherical elements
Phase coating

Binoculars Designs

Binocular designs fall into two broad categories; roof-prism and porro-prism. The earliest designs, porro-prism existed first. Roof-prisms were originally developed because the new design made it possible to build smaller and lighter binoculars. As porro-prism technology has developed, however, size and weight differences between the two designs have become less significant. Both are available in full size or compact models from PENTAX. In terms of practical use, the differences between the two are now relatively small, but let’s look at the advantages and disadvantages of porro-prism vs. roof-prism design.

Porro-prism
Advantages
Less expensive and very good optical quality.
Disadvantages
Less compact, less comfortable to hold for long periods, generally less rugged than roof-prism.

Roof-prism
Advantages
More compact than comparable porro-prism model and usually easier to hold for long periods. Roof-prism binoculars are generally manufactured to closer optical and mechanical tolerances, making them more rugged than comparable quality porro-prism binoculars.
Disadvantages
Higher cost.

Porro-prism VS Roof-prism

Quality Features

Durable Construction

Every PENTAX Sport Optics instrument is built to the highest of standards using some of the toughest materials available. PENTAX binoculars and spotting scopes are encased in highly durable rubber-armored housings, which are designed to protect the optical mechanisms inside from shock and damage, while making them easier to hold. PENTAX waterproof products are fully sealed and feature airtight construction. They are purged with laboratory grade nitrogen to provide a fog proof view in the harshest weather conditions, and extreme temperatures.*
*Not designed for underwater use.

Inner-focus Optical Design

Incorporated in all DCF series models, plus the UCF and PCF WP II models, this sophisticated optical design is ideal for waterproof models that require a fully airtight body construction. It also contributes to a considerable reduction in size and weight and improves overall balance by minimizing a weight shift during focusing.

Functional, Maneuverable Body Design

The body and optical mechanism of PENTAX binoculars and spotting scopes are computer-designed to ensure the easiest handling possible and minimize overall size and weight, without sacrificing optical performance.

What determines quality in binoculars?

There are three measurements of quality in binoculars: lens and prism optical quality, mechanical quality and alignment, and quality of design. Let’s look at these factors one at a time.

  • Lens and prism quality
    The best quality binoculars, such as PENTAX, use photographic quality lenses, multi-layer optical coatings, and BaK4 prisms, the best available. This insures the brightest, highest contrast, and sharpest image possible.
  • Mechanical quality/alignment
    While optical quality is very important, optical quality can be achieved inexpensively. Without proper construction and alignment, however, the resulting binoculars will be of poor quality. Alignment is often poor in inexpensive binoculars initially, and deteriorates rap- idly with use, and especially abuse.
Inner-focus Optical Design

Categories of use

Birding

8x to 20x with exit pupil of at least 5mm for low light use (8x40, 8x42, 10x42, 10x50). The greater powers are usually for backyard bird watching or a second pair for field use, and require a support of some sort for extended viewing (tripod, monopod or other). Close focus, waterproof, rug- ged, high resolution and dependability are what’s important with the birding enthu- siast. Birders encounter some of the most extreme conditions nature has to offer from the tropical rain forest to Antarctica.

Hunting

Large exit pupil, ruggedness, waterproof (8x42, 10x42, 8x40,10x50, 20x60, 8x25, 10x25, 12x25). Higher powers are usually for long range spotting and trophy identifi- cation, and require a tripod or other support.

Hiking, Backpacking

Compact, lightweight binoculars are key for this customer. However, durable binoculars are a must since they are usually jostled in a backpack. Low light is usually less important than size and weight. The Papilio binoculars are especially appropriate here as they double as a high quality field microscope.

Spectator Sports

Moderate power and wider angle of view is valuable here (8x-10x compacts). Compacts are very popular for spectator sports even though field of view is generally not as great as with full size binoculars.

Theater

6.5x-10x compact binoculars are particularly suitable for this use, due to their light weight and compactness.

Astronomical

8x-20x with larger objectives provide an advantage. Useful for lunar observation, comets, and spotting for telescopes. Tripod capability important.

Surveillance

For daylight surveillance, up to 20 power is appropriate. A exit pupil is desirable for night surveillance. 7x50 and 8x42 are two configurations popular with law enforcement agencies.