This method uses a centre-focus knob to simultaneously focus on the left and right, which enables a quick focusing.
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.
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.
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.
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.
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.
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.
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.
This is the angle of the field of view visible when the binoculars are viewed through and the wider the apparent field of view, the wider the actual field of view even at high magnification. To take the above diagram as an example, the apparent field of view can be obtained by multiplying the actual field of view (9.5-degree) by the magnification (7x).
Binoculars with an apparent field of view of 65-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.
This indicates the visible range (mm) at a distance of 1,000m that can be seen without moving the binoculars.