ON BINOCULARS

 © Anthony G Williams

With thanks to Walt Bjorneby for comments

Many people own a pair of binoculars. They may want them for a specific purpose, for example horse racing or bird watching, or just because they feel they may be useful. However, unlike cameras there are no magazines devoted to them, no means of obtaining unbiased advice to help in purchasing unless you are fortunate enough to find a retailer who knows more than just their price. This article is meant to give you some basic information about what you should look for to meet your particular needs and to show you how you can evaluate binocular specifications.

Different Shapes

First, the basics of design and designation. The simplest type of field glasses uses the Galilean optical system. This consists of a positive objective (front) lens and a negative eyepiece. The simplicity lends itself to very light and compact glasses, but the drawbacks are severely limited magnification and field of view. Apart from plastic toy binoculars, this system is nowadays only used in opera glasses.

All other binoculars now made are of the prismatic type in which both objective and eyepiece are positive with prisms used to turn the image the right way up. Focus control is nearly always by a central wheel focusing both eyepieces, with individual correction of one eyepiece to allow for eyesight variations. A few models now claim to be "focus free” but this carries certain penalties as will be explained later.

Prismatics come in various shapes and sizes. The standard “conventional” type uses porro prisms, which pushes the objective lenses much wider apart than the eyepieces, and are called the Zeiss or German pattern. The least expensive of the full-sized (as opposed to compact) binoculars are usually of this type. Very similar, but with smoother and more rounded styling, is the Bausch and Lomb or American pattern, normally featured on more expensive models. Of the two main types of porro prisms, BaK4 are better than BK7. In "micro" or "mini" binoculars, compact versions using the same optical design, the layout of the components is rotated so that the objectives are closer together than the eyepieces, in the interests of achieving the minimum possible size.

Roof prism binoculars use a different optical system which has a “straight through” appearance with the objectives the same distance apart as the eyepieces. This system is commonly found in compact binoculars, which often fold up for stowage, but can also be found in the largest sizes. Glasses of this type are almost always more expensive than others of the same size and quality.

Specifications

This is the inevitable technical bit but I’ll try to keep it as digestible as possible! Binocular specifications are normally given in a cryptic series of numbers and letters, for example 8x30 ZWCF. This translates as follows:

8x     =   linear magnification, that is the object viewed appears eight times larger or nearer.

30     = diameter of the objective lens in millimetres.

Z     = Zeiss pattern

W     = wide angle

C     = centre focusing.

Instead of the Z one might see B or BL (for Bausch & Lomb), & or RP (roof prism) M (micro), or C (compact). Other letters sometimes found are B meaning glasses with long eye relief for spectacle wearers (which is confusing, but in this case the letter appears immediately after the numbers without a gap, as in 8x30B) and A or GA meaning the body is rubber armoured so no case is needed.

The numbers are the most useful part of the designation because they tell you most of what you need to know about the optical performance. The larger the objective lens, the brighter and sharper will be the view, other things being equal. Of course, you usually get what you pay for; an expensive, high quality pair can be expected to give a significantly better view than a cheap pair of the same specification.

The potential brightness of the view can be found by dividing the objective size by the magnification in order to obtain the size of the exit pupil, for example an 8x40 pair has an exit pupil of 40/8 = 5mm. Simple sums will reveal that 7x35 and 10x50 also have 5mm exit pupils so the view through them will be equally bright. However, it isn’t quite as simple as that, which is why I said "potential" brightness. The complicating factor is that the size of the pupil of the human eye varies depending on the amount of light. In bright daylight, it is only about 3.5mm wide, but it opens out to about 7mm in darkness. So in bright daylight, the eye will only let in a beam of light 3.5mm wide; anything more is wasted. A pair of 8x30 binoculars, with an exit pupil of 3.75mm, will therefore give a view just as bright as 7x50 night glasses. As light levels fall and the eye pupils open wider, so they can make more use of the large exit pupil of night glasses. At night, 7x50 (or 8x56 or 9x63, all with an exit pupil of around 7mm) give the brightest view possible. Even at night, a pair of 7x63s would be no brighter, so such sizes are not made. Incidentally, as people age their pupils can’t open as wide, so older citizens will find 7x42 or even 7x35 just as bright as 7x50.

You will sometimes see the term relative brightness used, together with a somewhat incomprehensible number. in fact, this is simply calculated by multiplying the exit pupil by itself, so a 10x50 pair has a relative brightness of 5x5=25, only half the ‘score” of a 7x50. For daylight viewing, therefore, a relative brightness factor of around 15 is adequate.

The next technical issue is the angle or field of view. This is sometimes given as an angle (e.g. 7.5°), sometimes as the equivalent width of the view at a specified distance (in this case, 393 feet at 1,000 yards, or 131 metres at 1,000 metres). The angle of view (AV) is the most useful figure because it can be combined with the magnification to give a comparison of the view to expect from different binoculars. The table below gives a quick conversion aid:

ANGLE OF VIEW

APPROX FIELD OF VIEW

DEGREES

FEET/1,000 YARDS

METRES/l,000M

5

265

88

6

320

107

7

370

123

8

420

140

9

480

160

10

530

177

11

580

193

12

640

214

By and large, the higher the magnification, the narrower the angle of view. A pair of 8x glasses will typically have an AV of about 7°, a 10x pair between 5 and 6°. The more numerate of you will have spotted that in both cases, multiplying the AV by the magnification gives a "score" in the 50s, which is about average. However, these figures can vary considerably depending on the design of the eyepiece and prisms. "Wide Angle" binoculars can be expected to score around 70 (over 80 in exceptional cases), but if the figure is much under 50 the view will look unpleasantly restricted. Achieving a wide AV requires larger prisms, larger and more complex eyepieces, and (of course) more money. Incidentally, it is often believed that large objective lenses confer a wide AV, but that isn't the case; the main determinant is the design of the eyepiece, although the prisms also have to be big enough to cope. In fact, night glasses with large objectives like the 7x50 often have a rather poor AV because they use simple eyepieces to minimise light loss; 7 degrees is typical for them, but some achieve less than 6°.

Finally comes the question of focusing. tf you are interested in wildlife you might find it useful to have glasses which focus very close. Figures for this are rarely quoted so you just have to try them and see. Some focus free binoculars have a compromise optical design so that everything from about 12m/40 feet (which is not very close) is reasonably sharp without any focusing being necessary. In my experience the best of these can work quite well (although they don’t have a very wide angle of view); the worst are not worth bothering with. They are also no good for astronomy. My advice is not to buy a pair of these unless you have tried them out in comparison with conventional glasses and are satisfied that they work well for you; they work best for people with normal eyesight. It is now possible to buy autofocus binoculars using battery-powered systems developed from cameras, at a significant increase in price.

Another feature from the same source is image stabilisation, which makes it easier to use hand-held high-powered binoculars as it stops the image from jerking about so much. This also adds significantly to the price and weight, and requires batteries.

Making the Choice

Now you have a general grasp of the theory, how should you use it to decide what sort of binoculars to look for? There are six basic variables; size, weight, magnifying power, brightness, angle of view and, last but not least, price. The ideal would be a combination of high-quality performance in a light, compact and inexpensive package, but of course these desirables are mutually exclusive so some compromises will have to be made, whatever your budget.

Size and weight are particularly important to hikers. If a pair of glasses feels a bit heavier than another in the shop, you can be sure that the difference will feel enormous after you have been walking for a few miles, and a heavy pair of 10x50 will be a veritable millstone around your neck. However, even casual users may find themselves much readier to take binoculars with them if they are small and light, although of course if they are too small they will have a relatively dim image with a restricted field of view. Roof prism binoculars are more compact than porro prism types, the downside being that they lack the pleasant stereoscopic or 3D effect obtained by setting the objective lenses wide apart, and may also be less comfortable to hold. A separate case can also be a nuisance, so a rubber armoured pair has advantages (but note that all rubber-covered types are not necessarily waterproof). "B" models with rubber, fold-down eyecups have eyepieces designed to provide a full field of view when the eye is held some distance away, so they are ideal for spectacle wearers. However, they tend not to have a wide angle of view.

Magnification is frequently overrated. The higher the magnification, the more limited the field of view, the dimmer the image and the harder it is to hold the image steady, particularly in a strong wind or when you’re puffed after a hard climb. An image which keeps jerking around can cause nausea, so if you must have high power it's a good idea to mount them on a tripod if possible (look for a tripod bush on the binoculars). Dedicated (and presumably less energetic) naturalists use powers of around 10x, but I recommend no more than 7 or 8x for general use. One way of keeping the binoculars as steady as possible is to cup the barrels between the last three fingers and the heel of the palm of each hand. Place your index fingers against your temples and your thumbs upon your cheekbones. This locks the binoculars to your head and will minimise jiggling.

At one time I owned a pair of Russian 6x24 wide-angles which were optically superb for walkers, but the old-fashioned design was unfortunately rather bulky and heavy. Zoom binoculars are available, typically with specifications like 7-15x35 but I can’t recommend them. The high power is of little use unless you can rest them on something firm or clamp them to a tripod, while at low magnifications the field of view is poor due to the necessary compromises in the eyepiece design. On top of that, they are bigger, heavier and more expensive than fixed power designs. Image stabilisation adds to cost, complication and weight but does provide a genuine benefit if you really do need a high magnification, or have problems in holding binoculars steady.

The amount of brightness you need depends on how important it is for you to see in poor light. Sailors and astronomers are the traditional users of night glasses, usually in 7x50 size, but for normal use they are unnecessarily big for their performance. Much wildlife is most active at dawn or dusk, so if you’re a naturalist you should aim for an exit pupil of around 5mm, that is 6x30 (a rare size these days), 7x35 or 8x40. The latter two sizes are also probably the best for all-round use unless size and weight are at a premium. If you only want to use them in daylight, an exit pupil of around 3.5mm will be as bright as anything else; this means sizes of 7x25 or 8x30. If maximum compactness is your priority, you will have to sacrifice some brightness. A pair of 8x20s has an exit pupil of 2.5 giving a relative brightness score of 6 – less than half as bright as 8x30s even on a sunny day.

The same sort of compromise has to be made over field of view. The view through a pair of wide angle binoculars can be spectacular and is particularly valuable to bird watchers or plane spotters as it is much easier to pick up targets in flight. However, the more complex eyepieces required can impose a significant size and weight penalty, so most compact binoculars don’t feature this. They are also more expensive.

Finally, price. As you would expect, more expensive binoculars usually give a clearer and sharper image across the field of view, due to better quality lenses, prisms and lens coatings. This can be useful to travellers as it means that you can buy a small, high-quality pair which give as good a view as larger but cheaper glasses. They may also be more weatherproof and, in particular, feature fog-proof nitrogen filling, which is worth having. Cheap binoculars will often feature a fall-off in sharpness towards the edges of the view. An important point to watch with inexpensive and, especially, second-hand models is that the two halves of the binoculars are properly aligned. If you can’t immediately combine the two images as one when looking through them, don’t part with your money however cheap they are. Buy a monocular instead!

The important rules to follow are; decide on the specification you want and the amount you are prepared to pay, then go and look through as many different types as possible before making your decision. Remember that unless you have specialist needs a pair giving a moderate view, but compact enough to be carried without trouble, will be more useful than binoculars which are optically brilliant but so big and heavy that you can’t be bothered to carry them. Choose with care; unlike most modern possessions, a good pair of binoculars can last you for life.

So what do I use? Two rather different pairs: Swift Audubon 8.5x44 (the "American" porro prism type) which match the image quality of anything else I've tried, have an extra wide field of view and focus very close; and Minolta 6x18 which are the shape of a flat box so they can slip into a shirt pocket yet give a remarkably good view. The Swifts are great for serious work but the Minoltas get more use!

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