Types of binoculars

Binoculars are classified as follows:

Differences between astronomical and terrestrial telescopes

An astronomical telescope (Keplerian telescope) uses convex lenses for both objective and eyepiece lenses, and the image is inverted. It is designed to give objective lens optical performance top priority and minimize light loss, so a prism to rectify the image is not incorporated.

A terrestrial telescope incorporates a prism between the objective and eyepiece lenses to rectify the image. It is convenient to observe erect images of landscapes and objects.

Galilean binoculars

Galilean binoculars are so called because they feature the same structure as that used in the instrument first used by the Italian astronomer Galileo Galilei for astronomical observation in 1609. These binoculars consist of convex lenses for objectives and concave lenses for eyepieces and form erect images.

Because no prism is employed, the binoculars can be made compact and lightweight. However, maximum magnification is up to about 4x. Generally, the field of view is not wide, and the peripheral areas of the field are likely to be out of focus. Opera glasses are usually of this type.

Prism binoculars

Convex lenses are used for both objectives and eyepieces. A wider field of view and high magnification can be attained than is the case with Galilean binoculars. An erecting prism system is incorporated in the optical path to rectify the image. Also, employing prisms has the effect of making the length of such binoculars shorter. There are two types of prism binoculars: Roof (Dach) prism type and Porro prism type.

Porro prism binoculars

Porro prism binoculars create an erect image with two prisms (four prisms total with both sides). This was invented by Ignazio Porro in Italy in the middle of the 19th century and has the longest history among prism binoculars.

Roof (Dach) prism binoculars

The roof prism system is used to rectify the image for this type of binoculars. "Dach" means roof in German, and this type of prism features a roof-shaped surface. The optical path at the objective side and eyepiece side is virtually straight, making it possible for the binoculars to be compact and lightweight. However, manufacturing and adjusting the prism require very high technology. Demand for this type of binoculars is expected to grow as more customers prefer its slim, stylish design.

Magnification

This is the ratio of the apparent size of an object in comparison with what a viewer sees with the naked eye. For instance, an object 800m away viewed through 8x binoculars looks about equal in size to an object 100m away viewed with the naked eye. The higher the magnification, the more unstable the image will be due to hand movements. Higher magnification also results in a narrower field of view and lower brightness. Binoculars with magnifications up to 12x are recommended for general use. Also, using a tripod to prevent hand movements is highly recommended. Binoculars incorporating a VR (vibration reduction) function reduce image shake even at a high magnification.

Effective diameter of the objective lens

The effective diameter is the inside diameter of the objective lens frame. With the binoculars designated with a numerical formula 8x42 7.0°, 42mm is the effective diameter of the objective lens.

Given the same magnification, the larger the objective diameter, the greater the light-collecting power. This results in higher resolution and a brighter image. However, large-diameter objective lenses make binoculars bigger and heavier.

Binoculars are classified according to the effective objective lens diameter as follows.

Effective diameter

• Below 25mm:Compact-type binoculars
• 30 - 49mm:Standard binoculars
• Over 50mm:For astronomical observation and business use

Exit pupil

The exit pupil is the bright circle that can be seen in the center of each eyepiece when you hold the binoculars about 30cm away from your eyes with the objective lenses pointed toward a bright light. The larger the diameter is, the brighter the viewfield is, which is an important consideration when using binoculars in dark situations and for astronomical observation.

Exit pupil = The effective diameter of the objective lens ÷ Magnification

With 8x42 binoculars, the formula is 42 ÷ 8 = 5.3.
Therefore, the diameter of the exit pupil is 5.3mm.
This figure indicates the brightness of the image in view.

What is the relationship between bright/low-light conditions and the exit pupil of binoculars?

The pupil diameter of human eye changes depending on the ambient light conditions.

In low-light conditions (comparing 8x20 and 7x50 binoculars)

8x20 binoculars

7x50 binoculars

In bright conditions (comparing 8x20 and 7x50 binoculars)

Relative brightness

Relative brightness value is obtained by squaring the diameter of the exit pupil. The greater the relative brightness is, the brighter the image will be. With 8x42 binoculars, the brightness is (42÷8)2= 28.1. This means that if the magnification is the same, the larger the effective diameter of the objective lens, the brighter the image will be.

Do binoculars with the same exit pupil offer the same brightness?

No. Brightness may vary even if the exit pupil is the same. This is because the amount of light reaching the viewer's eyes varies according to the number of lens elements and quality of lens/prism coatings. Superior optical design and highquality coating greatly contribute to the brightness of binoculars. Brightness values specified in product brochures, etc. are theoretical ones calculated in the design process. Please note these factors when comparing actual brightness values.

Eye relief

Eye relief is the distance from the outer surface of the eyepiece lens to the position where the exit pupil is formed (eyepoint).
Looking through binoculars from the eyepoint, you can obtain the whole field of view without vignetting.
It is recommended for eyeglass wearers to use binoculars with a longer eye relief (high eyepoint).

For eyeglass wearers

Field of view

Real field of view

Real field of view is the angle of the visible field, seen without moving the binoculars, measured from the central point of the objective lens. The larger the value is, the wider the viewfield available. For example, binoculars with a wider field of view are advantageous for locating fast-moving wild birds within the viewfield. This also applies for finding small nebulas or a cluster of stars in astronomical observations.

Apparent field of view

Apparent field of view is the angle of the magnified field when you look through binoculars.
The larger the apparent field of view is, the wider the field of view you can see even at high magnifications.
With the conventional method used previously, the apparent field of view was calculated by multiplying the real field of view by the binocular magnification. (With this formula, apparent field of view wider than 65˚ is called wide field of view.)
After revision, Nikon's figures are now based on the ISO 14132-1:2002 standard, and obtained by the following formula:

tan ω' = Γ x tan ω
Apparent field of view:2ω'
Real field of view:2ω
Magnification:Γ

(With this formula, apparent field of view wider than 60° is called wide field of view.)

Field of view at 1,000 meters

Field of view at 1,000 meters is the width of the visible area at a distance of 1,000 meters, which can be seen without moving the binoculars.
For example, with 8x42 7.0° binoculars:
Field of view at 1,000m = 2 x 1000m tan (7.0÷2) = 122m