Binocular knowledge base

Binocular component names

Centre Focusing (CF)

This method uses a centre-focus knob to simultaneously focus on the left and right, which enables a quick focusing.

Individual Focusing (IF)

This allows the rotation of the diopter adjustment ring to adjust the focus for each individual eye. As it provides a strong airtightness this method is used for waterproof binoculars.

Main specifications of binoculars

Binocular types


Prism binoculars use convex lenses for both objective and eyepiece lenses and, incorporates an erecting prism that enables an inverted image to appear upright. There are two types of prisms with different shapes: roof prism and porro prism.

Roof prism

Binoculars that use a roof prism can be made more compact and lightweight as the optical axis of the eyepiece and objective lenses can be designed in a straight line.

Porro prism

This type binocular uses a porro prism invented by Ignazio Porro, an Italian inventor.
It has excellent optical properties and enables a bright, sharp field of view from low to high magnifications.


This is a simple design that uses both convex and concave lenses in the creation of opera glasses. This type of binocular has a simple lens structure that shows to the image the correct way up without the need of a prism and whilst practical has a limited magnification of 4x. The name originates from the first telescope used by Galileo Galilei to observe celestial objects.

Objective lens

Objective lens: Effective diameter of objective lens divided by magnification = Exit pupil diameter
The diameter of the exit pupil is calculated by dividing the effective diameter of the objective lens by the magnification. When the effective diameter of the objective lens is the same, the greater the magnification, the smaller the exit pupil diameter, making it more difficult to view through the binoculars.

Effective diameter of objective lens:
The diameter or size of the objective lens indicates the brightness of the binoculars. The larger the effective diameter the greater the light-gathering ability and the higher the resolution and brightness. However, because binoculars become heavier as the effective diameter increases, 50 mm is the approximate size limit for hand-held binoculars.

Eye relief

The distance between the position at which the eyes can see the entire field of view when looking through the binoculars and the eyepiece lens surface is called "eye relief". The longer this distance, the easier it is to use the binoculars for long periods of time and the more convenient it is to use the binoculars even while wearing eyeglasses.


Magnification indicates how large an object appears when viewed through the binoculars. For example, a magnification of 4x means that an object viewed at a distance of 100 m through the binoculars appears at the same size you would view the object at a distance of 25 m with the naked eye (This means viewing with the actual distance scaled down to 1/4). Essentially the higher the magnification, the larger the object appears, but as the magnification increases the field of view decreases.

Three fields of view for binoculars

Actual field of view

This is the range at which you can view without moving the binoculars and is the angle measured from the middle point of the objective lens. A wide field of view makes it easy to search an object. As the magnification of the binoculars increases, the actual field of view narrows.

Apparent field of view

This is the angle of the field of view visible when the binoculars are viewed through and even at high magnifications, a wider apparent field of view also means a wider actual field of view.

In the old JIS standard (JIS B 7121: 1993), the apparent field of view had been calculated according to the following formula: Apparent field of view = Actual field of view x Magnification
(Defined by this formula, binoculars with an apparent field of view of 65° or more are referred to as wide field of view binoculars.)

While in the ISO standard (14132-1: 2002) or new JIS standard (B7157:2003,B7121:2007), it has been changed as follows.
2ω‘ = 2 × tan-1( Γ × tanω)
2ω‘: Apparent field of view
2ω : Actual field of view
Γ : Magnification
(Defined by this formula, binoculars with an apparent field of view 60° or more are referred to as wide field of view binoculars.)

Wide field of view

Binoculars with an apparent field of view of 60-degree or more are called "wide field of view binoculars". Binoculars with a wide field of view but the same magnification provide a wider actual field of view, which is ideal for enjoying fast-moving subjects, viewing sports, and bird watching.

  • Reference: Calculation based upon ISO 14132-1:2002

Field of view at 1000 m

This indicates the visible range (mm) at a distance of 1,000m that can be seen without moving the binoculars.