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This table is only designed as a general guide, if you need any help deciding which magnification would best suit your needs, just e-mail us.

The Numbers??

For these examples we will use 8x30

The first number is the magnification, ie the subject will appear 8 times closer than with the naked eye.

The second number represents the diameter of the objective lens (wide, front end) in mm, this effects the amount of light entering the binocular and the image brightness (ie the bigger the hole in the front, the more light can fit through it and the brighter the image).

Exit pupil the size of the hole you look through. If held at arms length, whilst looking towards the eyepiece, a circle of light can be seen in the middle of the eyepiece, this is the exit pupil.

Exit pupil diameter - determines the amount of light reaching the eye and can be found by dividing the objective diameter by the magnification. In the case of 8x30, this would be 30 / 8 = 3.75mm
The human eye in low light opens to a maximum of about 7mm, which explains why low-light models are usually 8x56 (which equates to a 7mm exit pupil.)

Eye Relief is the distance between the eye and the binocular. The longer eye reliefs can be useful for users who prefer to wear their spectacles whilst viewing. However many models with short eye reliefs can also be used with spectacles if they have fold down rubber eyecups.

Field of View Can be expressed in many ways: 'xxx feet in 1000 yards', 'xxx yards in 1000 yards', 'xxx meters at 1000 meters' or in degrees (°). We have converted the various manufacturers preferred methods into degrees for this website, as this makes comparisons between various models/makes easier.

Twilight Factor Can be used to compare different binoculars with the same magnification. A 10x25 has a factor of 15.8, whilst a 10x50 has a factor of 22.3 thus a 10x50 should be better in low light. It should be noted that the Twilight Factor is purely mathematical using the magnification and the objective lens and does not take account of lens quality. In practice a good pair of 10x40 can give a brighter image than a low cost pair of 10x50.

Roof prism binoculars [41Kb] Porro prism binoculars [19Kb]

Roof prism binoculars have the prisms back-to-back this makes for a much smaller design. Due to the more complicated shape of the prisms in this design binoculars made using this technique were more expensive, however the popularity of this design has reduced production costs and these days the prices are only slightly above those of porro prismed binoculars.


Porro prism binoculars have the prisms offset, the more simple prism design is easier to manufacture, but the binoculars are physically larger than those with roof prisms. With the objective (front) lenses being further apart than binoculars of roof prism design the 3D, or depth perception, is enhanced.

This page updated 30 March 2020
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