Binocular Terminology

The Two Identification Numbers – Magnification and Objective Lens Diameter

Binoculars are normally identified by two numbers.

1. The first number is the magnification.
2. The second number is the diameter of the objective lens in millimeters (mm).

For example:
A 7x35 binocular magnifies objects 7 times and the objective lens of 35mm in diameter.
A 10x50 binocular magnifies objects 10 times and has an objective lens of 50mm in diameter.

Objective Lens Diameter

The Objective Lens Diameter is important because many of the performance characteristics of a binocular are determined by the diameter of this lens. The function of the objective lens is to collect light and direct it to the prisms or eye pieces. The larger the diameter, the more light will reach your eye thus providing a brighter and sharper image.

The larger the diameter, the heavier and bulkier the binocular will be. You will find there are many different combinations

Prisms

A prism is a solid piece of glass that functions as a mirror, but without a mirror's reflective backing. Light rays that have entered a prism cannot get out if they strike a surface at too great an angle. Instead, they reflect back, as if from a perfect mirror. The better prisms are made from a high density glass, BAK-4 and have circular exit pupils in the eyepieces. The less expensive BK-7 prisms will have squared-off exit pupils in the eyepieces.

There are generally two categories of prisms, porro prisms and roof prisms. Each type provides excellent viewing. Both types have two prisms set at right angles between the objective lens and the ocular (eyepiece) lens. This arrangement allows the object to be seen correctly, as apposed to upside-down.

Porro and roof prism binoculars are distinguished by their shape.

• Porro prism binoculars tend to be larger and have a definite offset to their body design. They have the original, simpler prism design. They are more light efficient and have the better contrast feature (the variance between shades of darkness).

• Roof prism binoculars are more straight and streamlined. They have a more complex light path and require greater optical precision in the manufacturing. They are lighter and compact, which makes them easier to hold.

Field of View

The Field of View (F. O. V.) describes the size of the area that can be seen through the binocular or spotting scope. When expressed as the number of feet per 1000 yards of distance, the higher the magnification the narrower the field of view.

On wide angle binoculars, a special lens design allows a wider field of view than normal. An example field of view for a standard 7X binocular would be 372 feet, whereas a wide angle would be 487 feet. Wider field of views are often preferred by hunters who want to scan large areas for game or for viewing sporting events.

Exit Pupil

A measurement of how much light is usable by a binocular or spotting scope. Generally, the larger the exit pupil, the brighter the image will appear, particularly in dim light.

Exit pupil is determined by dividing the diameter of the objective lens by the magnification. For example, a 7x35 binocular has an exit pupil of 5 mm. A 10x50 also has an exit pupil of 5 mm, so these are about equal in dim light. A 7x50 binocular however has an exit pupil of 7 mm. Lens coatings and the human eye also affect how well a binocular performs in dim light.

For general use in bright light, an exit pupil of 2.5 mm is sufficient and 4 mm and higher generally allow better viewing in dim light.

Eye Relief

Eye Relief is the distance, expressed in millimeters, from the ocular (eyepiece) lens to the point where the eye is positioned to view the entire image. Eye relief is affected by magnification, the number of lens elements, and field of view.

Most binoculars provide 8-13mm of eye relief. Binoculars with "long eye relief" provide from 14-20mm (or more) eye relief. Binoculars designed with long eye relief are usually preferred by those who wear eyeglasses.

Lens Coating

Lens coating allows more light to pass through the binocular by reducing the reflection. This allows for a sharper image. "Coated" means some surfaces have been coated. "Fully coated" means all "air to glass" surfaces have been coated with at least one layer of coating. The very best optics are the fully multi-layered coated lens, which can allow as much as 95% of the light through.

Types of Coating
• Coated – One or more surfaces are coated.
• Fully Coated – All air-to glass surfaces are coated.
• Multi-Coated – One or more surfaces are multi-layer coated.
• Fully Multi-Coated – All air-to-glass surfaces are multi-layer coated.

Resolution

Describes how sharp or clear an object appears when viewed. Resolution is primarily determined by the objective lens diameter. The larger the diameter of the objective lens, the better the resolution. However, lens coatings, lens alignment, quality of glass, and quality of prisms also affect how sharp a viewed image will be.