Ocular lens - the lens in the eyepiece

Prism - a block of glass with triangular section having accurately polished edges which are used to reflect light. Two forms of prism binoculars are available:

Porro Prism binoculars consist of two right-angled prisms joined so as to reflect the light path 3 times, resulting in a corrected image.

Roof Prism binoculars require very precise angles and polishing. The light path is reflected 4 times. This design is more compact, but as the light path is longer and the prisms more complicated, the glass and manufacturing need to be of higher quality than the Porro prism design for equal performance.

Magnification

The first figure describes the magnification, eg: 8x or 10x etc. This means that the image seen using the binoculars will be 8x and 10x bigger than when viewed with the naked eye.

As a general rule the higher the magnification, the smaller the field of view and the duller the image when viewing in low light conditions.

It should also be noted that with as increase in magnification, there will be an increase in handshake. When exceeding 10x magnification it is usually the case that what is gained in further magnification is lost in handshake.

Objective Lens

The figure ‘40’ in '8x40' 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.

Lens Coating

This is another important quality of binoculars. Coatings help to reduce reflected light and increase the percentage of light reaching the eye thus resulting in a much brighter image.

Exit Pupil

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 and this is called the 'exit pupil'. 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.

An increase in the diameter of the objective lens increases the size of the exit pupil and thus the amount of light entering the eye. This has a particular advantage when viewing in low light conditions.

The 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.

Field of view

The 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.

Eye relief

Eye relief is the distance between the ocular lens and the exit pupil. Ideally, when the binocular is raised to the eye the exit pupil should be focussed onto the front of the eye. This allows the observer to see the whole field of view correctly. Wearing glasses whilst using binoculars increases the distance between the eye and the ocular lens making the observed field of view narrower. To correct this, many binoculars have rubber eyepieces that can be rolled down to enable the eye to be closer to the ocular lens.

Collimation

This refers to the alignment of the two halves of a binocular. For perfect stereoscopic vision, both sides should be parallel such that one circle is seen when looking through them. A binocular that is out of collimation (or out of alignment) usually the result of a knock, will have slightly different images in each side, and can cause considerable discomfort and eyestrain when used. The collimation of a binocular can be re-set and Monk Optics are able to do this on the majority of binoculars.

IPD

IPD stands for 'inter-pupillary distance' or the distance between the pupils of your eyes. As this distance is different for each person, the binocular can be adjusted to fit by opening or closing the hinge. Many binoculars include an IPD scale in millimetres, on the hinge mechanism (fig opposite). IPD is set correctly by first opening the binoculars right out, then observing a distant object whilst folding them shut until a perfect circle is formed.

Focussing and diopter adjustments

As there is often a difference in focus between the left and right eye, a diopter adjustment is necessary. On most centre-focus binoculars the left eye is first focussed using the central wheel, after which the right eye can be balanced using the diopter adjustment on the right eyepiece. Once the dioptre adjustment has been set, the main central wheel will focus both eyes equally. Binoculars having individual eye focussing allow the diopter of both eyepieces to be set individually. However a change of object distance requires the adjustment of both eyepieces unless the depth of field is particularly good.

Distortion and aberration

There are many different types of aberrations which can flaw the vision when using a pair of binoculars, some of these are:

 Spherical aberration

  - This results in a blurring or softness of the image

 Astigmatism

  - Horizontal or vertical lines may appear out of focus.
  - This is more noticeable around the periphery of the field of wide angle binoculars.

 Curvature of field

  - A flat object does not appear flat when viewed using the binoculars. -This means that the centre of the image can be in focus whilst the periphery is out of focus and vice versa.

 Chromatic aberration