First a word on nomenclature. We often speak of `a pair of binoculars' or just `binoculars', although technically speaking one instrument is `a binocular'. When talking I must admit that I generally refer to the plural, although in this article I will refer to the singular. The dictionary seems to approve of both but not of using the word `pair'.

I don't consider binoculars to be essential equipment for dinghy cruising; nevertheless they can be very useful at times and always have the capacity to increase your pleasure in the surrounding seascape, animate and inanimate.

There are three basic types of binoculars. Galilean or `opera glasses' consist of a pair of plain tubes with lenses at either end. The lens at the far end is called the objective lens and the near one is the eyepiece lens. For technical reasons, neither reasonable magnification nor a wide field of view is possible with this design. The other two types both include prisms between the lenses. This gives two advantages: there is a longer light path between the two lenses - and the prisms can be used to correct the inverted image produced by the objective glass instead of using a concave lens in the eyepiece. These features allow the prismatic binocular to have certain optical advantages. The two types of prisms used are porro prisms and the newer design of roof prisms. Binoculars incorporating the latter have a similar shape to opera glasses in that they appear to consist of two plain barrels. They are lighter in weight than the porro prism type - the original reason for their development.

A binocular has to be focused on the object you want to examine. There are `focus free' types, but they are generally focused permanently on infinity and have low power. The higher power magnification that a binocular has, the more carefully it has to be focused if you want a sharp image. There are two types of focusing. Individual focusing or I/F in which the eyepieces are focused separately for each eye, and centre focusing or C/F in which both eyepiece lenses are focused by one knob between the barrels. The C/F type should always have an additional knob or ring for focusing one eyepiece separately to allow for the different strengths of your eyes. This normally acts on the right eyepiece. Traditionally, binoculars used at sea have been I/F types. Although they are slower to focus, which is not a problem when used for looking at mainly distant objects, they are easier to make water resistant. The sliding barrels of the C/F type are obviously more difficult to make watertight. Nevertheless, waterproof C/F binoculars are available if you are prepared to pay the prices that Leitz and the West Germany Zeiss company ask for their products.

All binoculars have two numbers stamped on the body such as 8 x 32 or 7 x 50. The first number represents the magnification of the binocular. The second number is the diameter of the objective lens in millimetres. It would appear that the higher magnification the better, but this is far from the case. When used on dry land it is generally considered that the maximum power that can be held steady freehand is 8x. Although binoculars are commonly manufactured up to 15x, one cannot get the benefit of the higher power unless some form of support, such as a tripod is used. Another disadvantage of high power comes from the fact that the `exit pupil' diameter, the size of the pencil of light rays emerging from the eyepiece lens, is calculated by dividing the size of the objective by the power. In order to see clearly you have to line up the pupil of your eye with the exit pupil. Although it varies with your age and the quantity of light available, the average diameter of your eye pupil is about 3 mm. For this reason the smallest exit pupil for comfort measures some 4 mm. Hence the most popular specification for general use is 8 x 30 or 8 x 32. For bird watching 10 x 40 is popular but requires some support or you won't see more detail than with the 8x model. Traditionally, 7x is the power that is used for marine use as the movement or the boat makes a steady hold difficult. In my experience, 7x is perhaps still too high for small boats as the common use of this power is based on the comparative steadiness of large ships.

The larger the size of the objective lens, the more light is gathered and the brighter the image. Whilst magnification is cheap to incorporate, large accurately ground objective lenses are expensive. It was thought at one time that the ability of a binocular to make out detail in bad light was dependent entirely on the size of the objective. It is now realised that magnification also enters into it. The `twilight factor' can be calculated by taking the square root of the product of the objective diameter and the power. You can thus compare different binoculars in this respect. Unfortunately for marine use you cannot use too high a magnification so if you require performance of this type there is no substitute for expensive pieces of glass! Another disadvantage of large objectives in a binocular is the weight - the large diameter of the objective has to be matched by large prisms. Weight can greatly spoil the pleasure of using binoculars as holding your hands up in front of your face is not a natural posture.

OTHER FEATURES

Although not important for marine use, a wide angle of view can be useful - for following or picking up birds in flight for example. As this requires a larger eyepiece lens, you usually have to pay extra for this benefit.

Close focusing can be useful if you are going to use it for watching birds on your bird table. Many binoculars, even expensive ones, have only a minimum focus of 20 feet or more. If you wear spectacles occasionally, then the ordinary binocular is OK. If you wear them all the time then it is a nuisance to have to keep on taking them off and putting them on every time you use the binocular. Special binoculars for spectacle wearers are made with greater eye relief. They have rubber eye cups that can be rolled back, or left protruding for ordinary use.

Some binoculars have variable power - zoom capacity. They are not usually very robust however and cannot be made water resistant. There are binoculars that incorporate a compass. Some people like them for taking navigational bearings.

Lens cap - this covers just the eyepiece lenses when the binocular is hanging from the neck strap and protects the lenses from rain etc., whilst still allowing instant use.

Coated lenses - all but the cheapest binoculars nowadays will have `coated' lenses. It is that purple or bluish bloom on the surface of the glass and it allows more light to pass through the lenses rather than be reflected from the surface. In low priced glasses however, it could well be that the internal surfaces are not coated.

Rubber armour - useful protection for a binocular subject to rough handling.

FINAL CHOICE

First choose your specification. For marine use, if you have lots of dough, buying new, and don't mind weight, then you could well choose a 7 x 50. A more than adequate alternative and much lighter is a 7 x 42 or even 7 x 35. These both have a good sized exit pupil which makes them comfortable and easy to use. If however you are strapped for cash you can often find bargains around as not many people know much about the subject. I have equipped my three children with good binoculars without spending a fortune. If you are buying new then 7x is the lowest power commonly available. If you are hunting for a bargain then look out for 6x glasses, perhaps ex service ones. I have a pair of 6 x 30 I/F glasses by `The Universal Camera Corp. New York U.S.A.' that I got from a junk shop when I was a teenager. Although not coated they are robust and still in alignment after nearly 50 years. They are always included in my dinghy cruising gear. If you buy a pair of old binoculars that are otherwise in good shape it is possible to have them coated. Unless you are buying really expensive glasses that are specified by the manufacturers as water resistant or waterproof, always go for I/F type, as even without a guarantee they will always be splash proof.

Having decided on the specification go and try out as many pairs of binoculars as possible. Carefully adjust each binocular before you compare. If it is of the C/F type, put one hand over the objective that is on the side of the adjustable eyepiece and focus on a distant object with fine detail like a wall or tree using the centre knob. Then put your other hand over the other objective and focus the other eye using the separate eyepiece adjustment. You will now be able to focus at other distances using only the centre knob. If the binocular is of the I/F type then you merely have to focus each eye on a distant object in turn. Don't be tempted to close one eye as this often distorts the vision; cover each objective. Having done this, with practice you will be able to turn the adjustable eyepieces in unison when you want to change the viewing distance. When you have adjusted individual focusing for your eyes, then adjust the angle of the barrels of the binocular to suit the distance apart of your eyes - you should be seeing one circular view not two. Before you do anything else look through the binocular from the objective end - any dirt, loose parts etc. can be easily seen. Yes, you can find them even in new instruments!

Now you can test them seriously. Concentrate on distant objects to start with. The most common fault with binoculars, both new and used, is alignment of the two sets of optics - they should both aim at the same point. If you feel a sense of strain then it is usually due to this fault. It is amazing what a difference this makes - a cheap binocular, if it is well set up, will outperform more expensive models. Alignment is more important, and noticeable, the greater the magnification. Now focus on an object about 50 feet away - you will probably be looking at the object in the centre of the field of view. Now move the binocular slightly to one side so you view the object at the edge of the field. The object will go out of focus to a greater or lesser extent depending upon the quality of the instrument. If you refocus you should get it sharp again, except on cheap jobs. Try this test again with an object at infinity, somewhere near the horizon. If you were searching for a navigation buoy you could miss it if the binocular were really bad. However, unless you are spending a lot of money, it is not worth making too much of this as you will invariably concentrate on the centre of the field. The final test is to compare one binocular with another. If you use the same test object to look at you will be able to compare the definition of the instruments. Very slight differences are not too important if you otherwise like a particular binocular for other reasons.

FINAL THOUGHTS

The more you use binoculars the less you will worship magnification. A binocular allows you to see more detail through several features including its light gathering power which allows higher definition - not just size of image. That is why the accepted range of magnification by reputable manufacturers lies only between 6x and 16x. When I first started studying birds I tested good quality binoculars of 15x and 16x power and with no less than 56mm objective lenses, against good telescopes of 40x and above. Although it might not be true for other people, I found that plumage detail could be ascertained more readily with the binocular in spite of the handicap of lower power - much of this comes from being able to use both eyes at once. The advantage of the telescope is only that it is cheaper. For general bird watching a 10 x 40 is OK, but if you can afford more than one instrument go for lower power with larger objectives as your main instrument, with the advantage of larger exit pupil and if possible wider angle. Needless to say the C/F type with its greater speed of focusing is preferable where bird watching is concerned.

….AND CARE

Every binocular should have a neck strap - and every user should put it over his head immediately he picks the instrument up before he looks through it. This habit avoids you dropping it to the ground or in the water with expensive results.

Salt or grit on the lenses should be washed off with a gentle flow of water. Then dry with soft tissue or well washed cotton handkerchief. Greasy finger marks are best dealt with using a proprietary spectacle cleaning fluid as the traditional carbon tetrachloride is now deemed hazardous to health and in any case virtually impossible to obtain.