Binoculars - An Introduction

Binoculars have 3 basic components:

Objective Lenses

The main lenses of a binocular (the end opposite the eye-piece) serve to collect light, thereby enabling the observation of distant objects. Each objective lens typically is manufactured with two separate glass elements. The refractive specifications of these elements permit the objective lens to image objects free of any false colours.

Prisms

The objective lenses form images that are both upside-down and reversed left-for-right. Prisms invert the primary image. Binoculars utilise either porro prisms or roof prisms. Porroprism Binoculars Porro prisms give binoculars their familiar zigzag profile, while roof prisms permit a straight-line design. Either type of prism could be designed BAK-7 or BAK-4. Both are economical and highly effective designs.The finer glass in the BAK-4 design is of high density and virtually eliminates internal light scattering, producing sharp and defined images.

Eyepieces

The ocular, or eyepiece, design included with a binocular has important performance implications. While the most basic function of an eyepiece is to magnify the image formed by the objective lens, in fact the eyepiece also largely determines the binocular’s field of view, edge-of-field image resolution, and other characteristics listed below. From two to five glass elements are usually used in the manufacture of the eyepieces.

Magnification and Lens Size

You'll normally see binoculars referred to as 8x40 or 20x50.

The first number is the magnification. Looking through an 8x40 pair the object will appear to be eight times closer than with the naked eye, 20x50s more than double that magnification. Different applications require varying degrees of magnification. . Higher power results in images that are less bright and in a binoculars that are more difficult to hold steady in your hands. Powers of 7x to 16x are the most popular magnifications. Tripods are recommended for binoculars with magnifications above 20x.

Binoculars described as 8-16x50 are known as zoom binoculars can magnify 8 to 16 times. These usually have a small lever near your right thumb to control magnification.

The second number is the size of the objective lens in mm. The objectives are the large lenses at the far end of the binoculars. Large objectives gather more light, doubling the lens diameter quadruples the light gathered. This is particularly important in poor light conditions, such as bird watch at dusk or astronomy. But while 70mm objectives offer excellent performance they are very large binoculars that are difficult to to hold and are bulky.

Field of View

The bigger the value, the wider the field of view, and the easier it is to locate the object or to watch large-scale objects . Field of view is measured in degrees of arc or as field-width (in metres) at 1000 metre distance. For example the Leica 10x25 Ultravid roof prism binoculars have 90 metres wide for an object 1000 metres distance from the user, yielding a field of view of 90 metres at 1000 metres. In comparison the Leica 7x42 Ultravid have 140 metres field of view for an object 1000 metres distance from the user.

Some binoculars are designed specifically with a wider field of view (known as wide-angle).

Exit Pupil Size - Brightness

This is an important factor in determining how well binoculars work in low light conditions. Exit pupil size is the diameter of the actual beams of light leaving the eyepieces.

It's calculated by dividing the diameter of the objective lens by the magnification. So 8x40 binoculars would have an exit pupil size of 40/8 = 5mm, 7x50 binoculars have an exit pupil size of 50/7 = 7.1mm. To calculate the Relative Brighness Index, simply square the result. The RBI for 7x50 is 7.1^{2 =} 50.4

In sunlight your eye's pupil will contract to a size of 2-4mm. But at night they'll open to 7mm. This varies with your age to an extent. Binoculars with 4mm exit pupil size seem as bright as those with 6mm in daylight. But in poor light your pupils may open to 6mm, so the binoculars with a 4mm exit pupil seem dimmer than those with 6mm.

Basically for daytime observation exit pupil size isn't significant, however, for night time and evening use a 6-7mm exit pupil is beneficial.

Eye Relief

Eye relief is basically the amount in mm your eye can be from the eyepiece and see the whole field of view afforded by your binoculars. In other words, it is the distance between the eyepiece and the eye point. Clearly this is an issue for eye-glass wearers as their eyes will be further back. When using binoculars with an eye relief of less than 10mm you're only seeing the centre of the image as you cannot place your eye at the eye point and the field of view is partially shaded off. High eye point binoculars with longer eye relief (15 mm or longer) are suitable for eyeglass wearers.

Lens Coatings

Binocular lenses have optical coatings to improve the amount of light they gather. The coating reduces the amount of light which is reflected away and lost. Early binoculars only transmitted 50% of the light they gathered. Coatings should be incredibly thin and must be applied with equal thickness to a very high degree of accuracy. This is not a simple or inexpensive process, that's why some brands may "cut corners".

Most binoculars have at least 8 lenses. Some of these are cemented together, but for the rest there is an air to glass boundry which light must cross. It is on these boundaries that the coatings are important. In a conventional set of binoculars there are an incredible 20 glass to air transitions. This illustrates why quality coatings are vital. Without quality coating some light is lost on each air to glass transition.

In addition, as light passes through the lens, some light is reflected by the front and rear surfaces of the lens. This light causes flare or ghosting, reducing image sharpness and contrast. Multi-layer coating on one or more surfaces can significantly increase image brightness and boost sharpness and contrast.

Binocular Styles

Compact binoculars are extremely portable and lightweight.
Full size binoculars offer superior light gathering capabilities and often wider fields of view.
Zoom binoculars allow you to flexibility in magnification.
Focus Free or Fixed focus binoculars offer a "no focus" mechanism which means the binoculars are always in focus but are dependant on your eyes to focus.
Wide angle binoculars offer greater field of view without sacrificing power.
Rubber armored binoculars absorb shocks, provide protection from the elements, and offer an ergonomic, non-slip gripping surface.
Waterproof binoculars are usually hermetically sealed and nitrogen charged to remain dry when submersed in water.