Binoculars from the specialist

Astronomy

Eye relief

The 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 (fig 11). If the eye relief is too short 'blacking out' is often found to occur around the periphery of the vision as shown in the figure below:

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. However some binoculars have an exceptionally long eye relief, making them particularly suitable for use with spectacles. The Monk Explorer is one such binocular.

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 'interpupillary 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 13). 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 (refer to figure 14). N.B. It is a very common misconception that the right eye dioptre adjustment should be made before the centre-focus whereas it should in fact be the other way round! 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.

Fixed focus binoculars

These are sometimes known as 'auto-focus' a name which is not strictly correct since there is no automatic mechanism. The focus is simply pre-set at infinity at the factory for clear vision with no adjustment necessary from about 15m to infinity. Fixed focus requires a deep field of view to allow pre-setting and for this reason is usually best used on lower magnification binoculars such as the 7x. As the focus is pre-set, normal vision is assumed, so spectacle wearers need to keep their spectacles on whilst using these binoculars.

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:

o Spherical aberration
- This results in a blurring or softness of the image

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

o 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. This concept is illustrated in the figured above.
(It may be noted here that the Fujinon 'ÔF' range of binoculars use `flat field' technology which virtually eliminates this problem).

o Chromatic aberration
- This is due to the different wavelengths of the different colours of light. An image with chromatic aberration will appear blurred and suffer colour 'flaring' around the fringes of objects. This is because the different colours are not all focussed at the same point as they should be.

When the image viewed is geometrically different to the object, it is said to be distorted. There are two types of distortion; pincushion and barrel. Both types are caused by different parts of the image being magnified by varying amounts, instead of it all being magnified uniformly. The effects of distortion are illustrated below.

Astronomy