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The Terms used to describe Binoculars
Binoculars are usually described with a set of numbers such as 7x26, 10x50, etc. This is often printed somwhere on the body.

The numbers printed on a binocular and what they mean

Magnification

The first figure describes the magnification: 7x, or 10x etc. This means that the image seen using the binoculars will be 7x and 10x bigger than when viewed with the naked eye. For example a bridge 1000 metres away would appear to be 100 metres away when viewed with 10x binoculars.
This is illustrated in the two diagrams below:

As a general rule, (when all other parameters are kept constant) the higher the magnification, the smaller the field of view and the duller the image in low light.

Another important point to bear in mind is handshake. With increasing magnification handshake is more evident. When exceeding 10x magnification it is usually the case that what is gained in further magnification is lost in handshake.

We therefore do not recommend buying binoculars with a magnification higher than 10x for hand-held use. When going over 10x, the use of a tripod is strongly recommended.
binocular view at 7 times magnification view at ten times magnification
7x 10x
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Objective Lens

The figure ‘50’ in '7x50' indicates the diameter of the objective lens in millimetres. For a set magnification, increasing the diameter of the objective lens has no effect on the field of view, but does increase the light gathering capabilities of the binocular.

Another important quality of binoculars is the Lens Coating used. Coatings help to reduce reflected light and increase the percentage of light reaching the eye. The effects of coatings can be seen in the diagrams (right).

It can be seen that the reflected light is dramatically reduced on the coated lens. A single uncoated objective lens will transmit about 95% of all incident light, while a coated lens will transmit about 99% (depending on coating type and quality). Therefore in a binocular with 10 surfaces (lenses and prisms) no coatings will result in an overall light transmission of about 65% whilst good coatings can improve this up to 95% resulting in a much brighter image.

Lens with no coating Lens has no coating
Coated Lens Coated Lens
     
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Exit Pupil
Exit pupils example
If you hold your binoculars at arms length and look at the centre of the eyepiece lens, you can see a small circle of light. This is called the 'exit pupil' and is illustrated in the left figure.

All the light leaving the binoculars is transmitted through the exit pupil, the diameter of which is determined by the ratio of objective lens diameter and magnification.

For example:

Exit pupil = Objective lens diameter/Magnification

For a 7x26, the Exit pupil diameter = 26/7 = 3.71mm,
But for a 7x50, Exit pupil diameter = 50/7 = 7.14mm.

Hence increasing the diameter of the objective lens increases the size of the exit pupil and the amount of light entering the eye. During daylight hours when the eye pupil is only about 2-3mm dilated, you would see no difference in brightness between a 7x26 and 7x50. However, as the light levels drop, the eye pupil can dilate up to a maximum of 7mm. In these conditions the 7x50 would provide a brighter image than the 7x26. Hence binoculars with a large exit pupil are more suitable for astronomy and use in low light levels.



Prism comparisonsThe design of the prisms can affect the quality of the exit pupil. The BK7 prism typically found in lower priced binoculars produces an exit pupil with shaded edges, whereas the BAK4 prism has a perfectly round exit pupil. Whilst the BK7 prism design would look no different to the BAK4 in daylight, as the light levels drop, and the eye pupil expands you start to observe the effects of the shaded regions as the image quality drops and becomes prone to chromatic aberration around the periphery of the image.

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Field of view

field of viewField of view can be expressed as an angle, or a distance at 1000 metres (e.g. 7.1¡ or 124m at 1000m). Consider figure 9: A scene 1000m away is viewed through the binocular, which appears as a circle when the instrument is set correctly.
At 1000m, the width of the image seen across the diameter of the circle will be 124m, or 7.1º as illustrated.

The Apparent field of view is the visible width of the field as seen through the binocular, and can be obtained by multiplying the real field of view by the magnification. Hence for a 10x50 with a 5º field of view, the apparent field would be 50º.


Wide angle field of view

example of Wide Angle field of view

By varying the configuration of the ocular lens system, the field of view can be made wider. A binocular is said to have a wide field of view if the apparent field is greater than 65º. However, excessively widened fields of view can cause distortion; a topic discussed later.

The Figure on the left shows a comparison between what is seen using a binocular with a normal, and wide angle field of view.
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