The Irish Bomfords

Binoculars

A. G. BOMFORD

Abstract

This article sets out the factors to consider in selecting a pair binoculars, and the points to check when inspecting a pair before purchase.

The young man, who was studying the country, took his glasses from the pocket of his faded, khaki flannel shirt, wiped the lenses with a handkerchief, screwed the eyepieces around until the boards of the mill showed suddenly clearly and he saw the wooden bench beside the door; the huge pile of sawdust that rose behind the open shed where the circular saw was, and a stretch of the flume that brought the logs down from the mountainside on the other bank of the stream. The stream showed clear and smooth‑looking in the glasses and, below the curl of the falling water, the spray from the dam was blowing in the wind.

For Whom the Bell Tolls: Ernest Hemingway.

The young man, whose name was Robert Jordan, was an amateur soldier. A professional soldier would have had his glasses pre‑set, and would not have exposed his head from behind cover until he knew he was going to see perfectly. Through a pair of binoculars set at infinite focus, most people can see perfectly from infinity to 100 metres, and there is no need to change the focus to observe objects at greater distances.

There are other myths about binoculars ‑ that the field of view depends on the diameter of the object lens, and that the higher the magnification the better one sees. This paper may help to dispel them.

Optical Properties

Object Lens Diameter

Most readers will know that a pair of 8 x 30 binoculars have magnification of 8, and object lenses 30 mm in diameter. It is the size of the object lens that determines the amount of light collected and passed to the user's eye, and, just as with astronomical telescopes, it is a better measure of the "power" of a pair of binoculars than the magnification.

Having collected a lot of light with a big object lens and concentrated it into a brilliant image, one can either magnify the image a great deal, reducing the luminance to what it was originally or even less, or not magnify it so much, and give the user the benefit of the increased luminance. Binoculars of the latter sort have their uses. They are, of course, useless in pitch darkness ‑ down a coal mine with the lights out - but they are useful at dusk and even in moonlight, for a yachtsman, say, approaching a harbour and trying to make out unlit navigation beacons. There are few more dramatic demonstrations of the value of large object lenses than to take out two pairs of binoculars, with, say 20 mm and 50 mm object lenses, and to look at the Milky Way. Through both pairs, the brighter stars will be readily visible, much further apart than with the naked eye; but the background through the 20 mm glasses will be almost black, while through the 50 mm it will be a blazing vista of small stars.

Small object lenses and high magnification make the distant scene appear hazy. This is readily demonstrated. The converse, that large object lenses help one see better in haze, is less easy to believe. Without using infra‑red light, one cannot really see better through water droplets or smoke particles, yet the increased luminance obtained by using large object lenses with moderate magnification usually increases the contrast in the distant scene, and, like distant sunshine, improves visibility. This attribute is particularly important to surveyors on reconnaissance for primary triangulation or traverse, and to fire watchers.

Like every other attribute, large object lenses have disadvantages: an increase in weight and bulk, and to some extent in price, though this is not now as severe as it was.

Magnification

To many people, magnification seems the most valuable attribute. but high magnification is seldom a help. One will probably read the name of a ship at sea (an excellent practical test) more readily with 7x glasses than with 16x glasses, especially in a poor light, or if one is in a boat oneself. Good visibility depends not only on the size of the object being viewed, but also on its contrast with its background, and its stability. High magnification dilutes the luminance of the image, reducing contrast, and making the scene appear hazy. It also accentuates the angular movement of the field of view, and at 16x and over, a tripod is as essential for binoculars as it is for a theodolite. It is well known to be almost impossible to use binoculars in a moving vehicle. High magnification may be necessary to see very small, relatively close objects, such as bullet holes in a target on the 500 metre range; but for most purposes 8x is a useful limit, and 6x and 7x are sometimes better.

Exit Pupil Diameter

If one divides the object glass diameter by the magnification, one obtains the diameter of the exit pupil, the circle of light which can be seen by looking into the eyepiece from a distance of 0.3 metres or more. Through the exit pupil passes all the light gathered by the object lens.

The only light which improves the observer's vision, however, is that which enters his eye pupil, which varies in size, being as little as 2.5 mm in bright sunlight increasing to 7.5 mm in the dark. Binoculars with an exit pupil less than 2.5 mm. would degrade the observer's vision even in the brightest light, and they are seldom made. Similarly, there is no point in making binoculars with an exit pupil greater than 7.5 mm‑the designer would do better to increase the magnification. Small 8 x 30 binoculars have an exit pupil of 3.75 mm, and are as satisfactory as bigger ones in bright light; but in poor light when the user's pupil is enlarged, glasses such as 7 x 50 with exit pupils of 7 mm are better.

Relative Brightness

Some manufacturers quote a statistic called "relative brightness". The formula varies from one manufacturer to another. In some cases it is simply the square of the exit pupil diameter. Other manufacturers maintain that increased magnification does improve visibility even in poor light, and quote the geometric mean of the object lens diameter and the magnification, calling it the "twilight performance". Neither statistic really gives the purchaser any new information.

Field of View

It is commonly supposed that the field of view is dependent chiefly on the size of the object lens. This is not so. It is difficult to combine a wide field of view with high magnification, but most glasses of between 6x and 9x, no matter what the size of the object lens, are designed with a field of view of about 7', something over 100 metres at 1 km, about 14 times the diameter of the moon. This is amply sufficient for picking up a bird or aeroplane against a blue sky after one has had a little practice and acquired the necessary reflexes. There is no point in seeking anything wider, nor settling for anything less.

Blooming

Modern binoculars have their glass surfaces bloomed to decrease reflection and increase the transmission of light. Reputable manufacturers bloom all the internal surfaces, but may not bloom the two external surfaces, where the blooming might rub off. There is little point in saving a few dollars and buying glasses which are not bloomed on the internal surfaces. Most blooming appears purplish when seen in reflected light, and does not affect the colour of the field of view. Some manufacturers make glasses with so‑called "mercury" blooming, which appears golden in reflected light and gives an unusual whiteness to the field of view, which some people like, and others do not.

Eye Relief

Binoculars are of course so designed that the user's eyeball is a few millimetres behind the eye lens, not in contact with it. It is desirable that the observer's eyelashes should not come in contact with the eye lens when he blinks, else a film of grease is gradually deposited, which does nothing to improve the optics and has to be wiped off from time to time with a risk of scratching. Small 6 x 20 binoculars seem especially prone to this trouble.

Some manufacturers make binoculars especially for spectacle wearers, designed for the eyeball to be about 20 mm from the eyepiece, enabling the observer's spectacles or dark glasses to intervene. This is essential for people who suffer from complaints like astigmatism which cannot be compensated by focusing the binoculars, and many people with normal eyesight may find they prefer it.

Optical Systems

All binoculars consist essentially of an object lens, which collects the light and concentrates it into a small but brilliant image, and an eye lens which magnifies the image. The eye lens can enlarge but cannot otherwise improve the image, whose quality depends solely on the size and quality of the object lens.

Just as in a pin hole camera, the image formed by the object lens is inverted and reversed. Surveyors do not find this a serious inconvenience in theodolites, but most users of binoculars prefer to see things the right way up.

In prismatic binoculars using the Porro system, the light from the object lens is reversed and erected by two 90° prisms before it forms the image. This makes the binoculars shorter, but it does not reduce weight or cost, as the prisms have to be rigidly fixed in a strong shell.

Other inverting systems are possible. Zeiss also makes "Dialyt" binoculars, in which the light is inverted by a single pentagonal prism inside a straight tube joining the object lens to the eye lens. They are longer but less bulky, and the shell is simpler and stronger for a given weight. Erecting telescopes commonly use an extra inverting lens.

Opera glasses require no inverting system. The converging rays from the object lens are intercepted by a weak diverging eye lens before they form an image. This system is much simpler and cheaper, but the quality of the optics is less good, and the magnification is usually only about 3x. It is suitable for toys, which can be made with a plastic shell and clear plastic lenses, and for the opera, where the colour fringes may even add to the gaiety of the scene; but it is not recommended for field use.

Mechanical Properties

Centre Focusing

Many binoculars for the civilian market have a central knob which adjusts the focus of both oculars simultaneously. Centre focusing is necessary for people who wish to observe objects at rapidly varying distances at close range, such as birds. For those who can see all they want with the naked eye at ranges up to 100 metres, centre focusing is unnecessary, as one can take full advantage of the flexibility of the human eye which can normally focus at any distance from infinity to 0.3 metres. Through binoculars set at infinite focus, this range is reduced, but most people will find that they can focus without strain on objects as close as 100 metres. People with long sight or short sight need a different basic setting, which they should memorize.

Unlike a theodolite, binoculars have no cross hairs, so there is no parallax which makes refocusing on objects at different distances obligatory, and centre focusing has disadvantages: mechanical strength is reduced, and since the two eyepieces have to slide in and out, dust, water and fungus find it easy to get inside. Without centre focusing, the only way for moisture or fungus to get inside is down a long, well greased screw thread.

All binoculars have at least one eyepiece with individual focusing, so that if the user's eyes focus differently when relaxed, the difference can be allowed for. Most surveyors will find it satisfactory to have both eyepieces focusing in this manner, and to dispense with centre focusing. If occasionally they need to examine a nearby object, it is easy to set both eyepieces to, say, +1 division for distances between 30 and 100 metres, and to +2 division for distances between 10 and 30 metres, and pre‑set them.

Weight

Binoculars can vary in weight by a factor of twelve, from 120 grams for a pair of 5 x 15 to 1,440 grams for a pair of waterproof, fungus‑proof 7 x 50. If large object lenses are essential, one has to pay the penalty in weight. It is difficult to get rugged 7 x 50 glasses much lighter than 1,000 grams, and even 7 x 40 glasses may weigh over 850 grams if waterproof.

For a submarine commander, weight may be no disadvantage, and binoculars which are proof against seawater and can stand banging against the hatchway in a crash dive may be essential. But for the life‑long bush walker getting into his sixties, every grain of weight may count, and if he uses his glasses mostly in brilliant sunshine, on the summits of mountains, or in snow, large lenses may be unnecessary. On those wonderful Australian days when it is hard to believe that the earth has an atmosphere at all and everyone is wearing dark glasses, one can see beautifully through a pair of 6 x 20 with an exit pupil of 3.3 mm; but they will be less satisfactory on a dull, hazy day.

Ruggedness and Proofing

A rock climber or yachtsman may conceivably bang his binoculars when in use, but most binoculars are relatively safe around the neck, and they are safe when on the shelf. Some care is needed with them in a truck, but the time at which they are most at risk is between the truck and the store, or between the garage and the study. It is easy to walk into the house laden with the relics of a barbecue and drop one's binoculars on the concrete while trying to open the front door. It is a good rule to insist on oneself, one's family and friends always hanging binoculars around the neck, even if they are only to be carried, not used. Re‑reading and memorizing this last sentence can easily save the reader the cost of next year's subscription to the Institution.

In wet weather, moisture condensing inside is a nuisance, but if it is simply condensation, it should evaporate without leaving a stain. If there is any risk of binoculars being immersed in river water or even being sprayed with sea water, it is best to buy waterproof glasses. Some manufacturers now make binoculars encased in rubber, which aids both the waterproofing and shock proofing. They are unlikely to have centre focusing.

Those who are going to reside in the tropics must decide whether to leave their binoculars behind, whether to keep them when not in use in a sealed plastic bag with a dehydrating agent, or whether to buy a pair of fungus‑proof "tropicalized" glasses. Many manufacturers make them, but they are apt to be heavy and expensive.

The cleaning and repair of glasses in Australia is neither cheap nor easy to arrange, and a serious user does well to buy the most rugged binoculars he is prepared to wear around his neck for long periods.

Price

The Japanese have brought the price of binoculars down by about half in the last twenty years. For quite useful glasses the price is now so low that import duty is trivial, and one may as well buy them in Australia as wait until an opportunity occurs to buy them free of duty.

Good glasses are made in Germany, and if one needs a combination of fine optics, ruggedness, waterproofness, and low weight, one may still do best to pay for German glasses: but for most people a pair of Japanese binoculars carefully chosen should suffice.

If you are buying glasses for a boy, who is going to drop them or take them apart, or for an astigmatic aunt who is not going to use them often nor be able to see well anyway, one should buy the cheapest glasses that adequately pass the tests in the next section but it is unwise to buy unheard‑of makes at a surplus store, nor "specials" at a department store, without very careful inspection: these are the ways dealers move unsatisfactory stock.

For a person who is going to use his binoculars frequently for the rest of his life, it is best to decide what type one wants; inspect two or three pairs of this type by reputable manufacturers; choose the pair one likes best; and be prepared to pay for it. The market is at present pleasingly competitive.

Purchase

The following questions may help a prospective purchaser decide what type of binoculars to buy:

(1) Is waterproofing or fungus proofing essential?

(2) Is weight critical ? How big an object lens are you prepared to carry ?

(3) Is high magnification essential?

(4) Is centre focusing essential?

(5) Is price critical?

Having selected the type of binoculars you need, offer a deposit and if possible take two or three pairs by different manufacturers home for the weekend. Compare them thoroughly, checking the following points.

Spherical and Chromatic Aberrations

Look through the glasses at a straight edge, such as the roof of a building. Across the centre of the field of view it will appear straight and colourless. Across the edge of the field of view, in all except the most expensive binoculars, it will appear slightly curved and coloured. This is a very simple test for comparing optical quality. Since the human eye only sees clearly over a relatively small area, there is no point in paying a lot of money for hand‑held binoculars which are optically perfect to the extreme edge of the field of view. If lines across the centre two‑thirds of the field are not visibly bent or coloured, that should suffice for most purposes. Then look at a distant view, not the sky, and see if it all appears in focus simultaneously. Some cheap glasses used to have rings or haloes which were not quite in the same focal plane as the rest of the field. All modern glasses should he free from defects of this sort.

Dirt and Fungus

Look through the object tens towards the eyepiece and ensure that the lenses and prisms are completely free from fungus, smudges, dirt, and bubbles in the glass.

Collimation, Eye Relief and Blooming.

The two optical axes must be parallel. Even new glasses may have been knocked in the shop. Slight non‑parallelism may not be readily apparent but will cause eyestrain with prolonged use. With a greater degree of non‑parallelism, the user may see properly if he squints or is accustomed to it, but the glasses may be troublesome to his friends. Set the eyepiece at infinite focus, raise the binoculars rapidly to the eyes, and ensure that you immediately see straight through them with both eyes without having to squint. At the same time, check that the eye relief is adequate, and the blooming satisfactory.

The Case and the Strap

Binoculars are no use when in their case and its quality is of little importance. However, it is worth enquiring whether the binoculars come with a case and, if so, what one is paying for it. The strap by which the binoculars hang around the neck is important, particularly with second-hand glasses: if the strap breaks the first time the glasses are used and they fall on a concrete floor, the purchase money has been wasted. A good strap will last many years, but it is cheap insurance to check the strap on purchase and at least once a year.

Waterproofness

If a salesman claims his binoculars are waterproof, it is a good plan to ask him to immerse them in a bucket of water for a few minutes, before you leave his shop. It is not really reasonable to expect him to wet his stock until you prove that you are a serious purchaser; but if on immersion reputedly waterproof glasses emerge full of water, you can reasonably expect to get your money back.

Maintenance

There are many binoculars locked up in stores for issue to soldiers, fire watchers and surveyors, through which one can see little better than with the naked eye. They have probably received plenty of hard knocks but no maintenance since the day they were made. If the lenses get dirty or the collimation gets out of adjustment, an interesting and enjoyable hour can be spent taking binoculars apart and cleaning the lenses and prisms; but without collimating equipment, it is virtually impossible to put them together again satisfactorily, and this is how most binoculars end their useful days. There is no practical alternative to sending binoculars for cleaning and re‑collimation to a firm which specializes in this work. A decade ago it was difficult to find anyone in Australia who could collimate binoculars, but surveyors can now seek advice from their instrument firms.

Cleaning and collimation is not cheap, but if you can keep water and fungus out, and avoid dropping them, binoculars should not need maintenance more than once in a decade.

Conclusion

I have owned four pairs of binoculars since I was sixteen. The first were cheap, second‑hand 8 x 30 made in France before the war. They were dropped, taken apart and never satisfactorily put together again. They were my only pair with centre focusing. The second pair were even cheaper, 1947 Voigtlander 6 x 30, which served me well for five years until I filled them with water crossing a river. They were taken apart to be cleaned and never put properly together again either. The third were a beautiful pair of Ross Tropical 7 x 42, heavy (860 grams), expensive (£46 sterling, before the Japanese entered the market), but rugged, fungus proof and with a reasonable 6 mm exit pupil. I have had them sixteen years. They have been re‑collimated once by Ross and are today as good as new. On long journeys I now find them heavy round the neck and have lately supplemented them with a pair of Zeiss 6 x 20 with Dialyt straight‑through optics and eye relief. They weigh 145 grams and will slip into the breast pocket of a suit. In bright sunshine, they are as good as the Ross up to about 5 km, but the Ross are better in distant haze, in poor light, in the rain, at sea and in the tropics.

Hemingway's Robert Jordan was so fascinated by what he could see through his binoculars that he forgot for a moment about the disposition of the sentries and the structure of the bridge he had come to examine; and many people are amazed when they look for the first time through binoculars that are in good condition and clearly focused. Of man's five senses, seeing is the only one that can be readily enhanced in an aesthetically pleasing manner, and binoculars can do much to enhance our appreciation of the world around us. At today's prices, there must be relatively few people in Australia who cannot afford to own a pair, and the author hopes that this article will help people choose wisely and get a pair that will give them life‑long satisfaction.

A. G. Bomford, M.A., F.R.I.C.S., F.I.S.Aust., M.A.I.C., is a Vice‑President of The Institution of Surveyors, Australia and an Assistant Director in the Division of National Mapping. He has written this article in response to the Editor's request for articles of more general interest.