Choose binoculars for a specific purpose
The most important part in choosing binoculars is selecting the optimal magnification and features for your purposes.
Particularly focus on the magnification. If you mainly want to zoom in close on far away subjects, choosing a high magnification is best. But remember that although a higher magnification makes objects look closer, it narrows the actual field of view, making it more difficult to find the object you are looking for. Also, the higher the magnification, the more noticeable image shaking becomes. Instead of thinking ‘the higher the magnification, the better’, it is important to choose a magnification that fits your purposes.
Generally, binoculars with a magnification of 6 to 10x are easier to use, but for birdwatching, tracking moving objects, and keeping shaking to a minimum, 8 to 10x magnification is best. For theatergoing, a somewhat lower magnification is easier to use, and portability is an important factor.
Binocular features are also important in how you use binoculars. A waterproof construction is good to have for whale watching. Binoculars that are easy to focus are great for sporting events, etc. where you need to focus on fast-moving subjects.
As shown here, the magnification and features of binoculars differ depending on how you want to use them. Keep these ideas in mind in order to choose the best binoculars for you.
Although the part names differ depending on the type of binoculars, many prism variety binoculars such as the 8×42 PRO and 10×42 PRO have objective lenses, prisms, eyepiece lenses, a focus adjustment dial, and a diopter adjustment ring.
Many binoculars contain useful information in the model name, such as the “magnification”, which indicates how much larger an object appears and the “effective diameter of objective lens”, which is a guideline for relative brightness.
Magnification is a value that indicates how large objects appear when looking through the binoculars. For example, when using a pair of 10x binoculars, an object 100 meters away will appear to be the same size as when viewed by the naked eye from 10 meters away. Generally, the higher the magnification, the larger objects appear, but the narrower the field of view becomes.
Effective diameter of objective lens
This is the diameter of the objective lens and indicates the brightness of the binoculars. The larger the effective diameter of objective lens, the better its light collecting capabilities are, making for a brighter view. However, the larger the effective diameter, the heavier the binoculars become. The maximum diameter for handheld binoculars is about 50 mm.
Exit pupil diameter and relative brightness
The pupil of binoculars appears as a bright circle when viewed about 30 cm away from the eyepiece lenses. This size is known as the “exit pupil diameter”. The exit pupil diameter can be calculated by dividing the effective diameter of objective lens (mm) by the magnification (x), and the relative brightness of a pair of binoculars is expressed using “exit pupil diameter (mm)”2. The larger the exit pupil diameter, the brighter the image in the binoculars will be.
As a rule, if the exit pupil diameter of binoculars is larger than the pupil diameter of the human eye, the image in the binoculars will appear bright. Conversely, a smaller exit pupil diameter will make the image in binoculars appear darker than when a scene is viewed with the naked eye. The pupil diameter of the human eye changes significantly due to brightness and age, opening to about two to three mm in bright locations, and from five to seven mm in dark locations. For this reason, binoculars with the exit pupil diameter of about two to three mm or more are good for use in bright locations, whereas five to seven mm or more are best for use in dark locations.
In bright locations, the human pupil diameter opens from about two to three mm. The 8×25 WP II waterproof binoculars with the exit pupil diameter of 3.1 mm offer sufficient brightness.
Because the human pupil diameter in dark location is about five to seven mm, binoculars with the exit pupil diameter of five mm or higher, such as the 8×42 PRO (pupil diameter of 5.3 mm), are sufficiently bright for viewing in dim locations.
Three fields of view for binoculars
Actual field of view
This term indicates the visible range of binoculars from a fixed position, and is expressed by the angle measured from the centre of the objective lenses. The wider the actual field of view, the easier it is to find objects. As the magnification of the binoculars increases, the actual field of view narrows.
Field of view at 1,000 m
This indicates the range in meters visible 1,000 meters ahead when binoculars are in a fixed position.
Apparent field of view
This is the visual angle when looking through binoculars. A wide apparent field of view indicates a wide actual field of view even at a high magnification.
Even at the same magnification, a wide view type with a wide apparent field of view delivers images with greater impact. The basis for wide view type binoculars differs depending on the standard. In the old JIS standard (JIS B7121:1993), the apparent field of view was calculated by multiplying the actual field of view by the magnification, and binoculars with an apparent field of view of 65° or higher were designated as wide view binoculars.
In ISO standards (14132-1:2002) and the new JIS standard (B7157:2003, B7121:2007) the apparent field of view is calculated by following formula.
2ω‘ = 2 x tan-1（magnification x tan ω）
Apparent field of view: 2ω‘
Actual field of view: 2ω
In new standard, binoculars with an apparent field of view of 60° or higher designated as wide view binoculars.
This is the distance from the final surface of the eyepiece lenses to the eye at which the entire field of view is visible through binoculars without vignetting. If this distance is long, it is easier to use binoculars even while wearing glasses.
Closest focusing distance
The shortest distance where focusing is possible on binoculars is called the closest focusing distance. When observing close-up subjects such as flowers and insects, a pair of binoculars with a short closest focusing distance is best, such as the 8×42 PRO and 10×42 PRO with a closest focusing distance of 1.5 m.
When light passes through the lens or prism, it reflects off the surface, reducing the amount of light, making the image appear darker. Anti-reflective lens coating is applied to the surface of the lens to prevent loss of light. There is single-layer mono-coating, and multi-coating, which is formed of multiple layers. Lenses with multi-coating have higher light transmittance, resulting in a bright, clear field of view.
Types of binoculars
Many current binoculars use convex lenses for the objective lenses and eyepiece lenses and a prism to erect the image. There are two types of prisms, Porro prism and Roof prism. There are also Galilean and mini Porro type binoculars.
Porro prism binoculars
These binoculars use a Porro prism developed by an Italian inventor. It has excellent optical properties and enables a bright, sharp field of view from low to high magnifications.
Roof (Dach) prism binoculars
Dach means “roof” in German. Roof prisms are designed to be used in a straight line with the eyepiece lens and objective lens optical axis, making it possible to build lightweight, compact binoculars.
This is a simple structure that uses both convex and concave lenses and is featured in opera glasses. These binoculars can erect the image without the use of a prism. This type of binoculars has a simple lens structure that is relatively inexpensive while practical has a limited magnification of 4x. The name comes from Galileo Galilei, who first looked at celestial objects with a telescope.
Mini Porro binoculars
This is a modification of the Porro prism type, which reverses the order of the eyepiece and objective lenses. It features superb optical performance, making a lightweight, compact design possible, however, because the objective lenses are placed on the inside of the eye width, this design is limited to small objective lenses with a diameter of about 15 to 25 mm.
CF (Centre Focusing)
Centre focusing ring is used to adjust the focus simultaneously for both the left and right sides for quick focusing. Some models also have a diopter adjustment ring for dealing with a visual acuity difference between the left and right eyes.
IF (Individual Focusing)
Diopter adjustment ring is turned to focus both eyepieces individually.