Binoculars: Telephoto lenses for your eyesBy Staff Writer | Tuesday, 29 April 2008 21:04
There are a myriad of applications for binoculars, but for photographers they’re handy for spotting subjects – especially wildlife – and for getting a closer look at what’s going on around you. This guide will help you make sense of the specifications and features.
Like watches, sunglasses and the ubiquitous iPod, binoculars are a must-have ‘personal accessory’. There’s something about being able to traverse great distances with the aid of optics that’s strangely appealing… perhaps it’s something to do with the observer in all of use. We like to watch, and binoculars allow us to watch very closely indeed.
There’s a staggering variety of models available, from pocket-sized pairs to military-spec monsters which allow you to see craters on the Moon. The potential applications are myriad too. There are plenty of sports, both indoor and outdoor, where binoculars will get you right into the thick of the action safely – and from the cheap seats! At concerts binoculars allow you a front-row view without the normal disadvantages of poor sound and a cricked neck. And, of course, if your hobby involves spotting – trains, planes, birds (of the feathered variety), butterflies, wildlife and anything else which runs or flies – then binoculars are essential viewing aids.
Not surprisingly, since they’re in the optics business, many of the world’s major camera makers offer extensive ranges of binoculars – notably Canon, Nikon, Pentax, Olympus and Leica – but there are also companies which specialise in binoculars and telescopes such as Bushnell, Gerber, Swarovski and Vixen. We can’t help you with your buying decisions as far as brand is concerned, but we can get you to the point where it’s basically the only choice that’s left to make.
Size And Style
There are two basic designs of binoculars. Porro-prism models have the classic ‘stepped’ look which is the result of using a series of prisms to bend the light through 90 degrees. In general, the porro-prism optical design results in a bulkier and heavier pair of binoculars although some models are still quite compact. The slimmer-looking binoculars with ‘straightthrough’ lens barrels employ a roof-prism design. This is generally a more expensive design, but it does permit extremely compact, even pocketable, binoculars.
These smaller binoculars are less powerful and typically offer magnifications of between 7x and 10x, although there are some mid-sized models with 12x power.
The first number quoted in a binocular’s model designation is always the magnification, while the second indicates the diameter, in millimetres, of the front or objective lens. Thus, a pair of 10x25 binoculars have a magnification of 10x and an objective lens diameter of 25 mm. Simply explained, a 10x magnification value means that if you are viewing an object that is 1000 metres away it will appear as though it is only 100 metres distant.
Power And Light What you can also derive from these numbers is the brightness of the optics, and this is done by dividing the objective lens diameter by the magnification. This little bit of arithmetic gives you the exit pupil size, which is then squared to give the relative brightness index.
So, for example, a pair of 10x50 binoculars has an exit pupil diameter of five (i.e. 50÷10) and a relative brightness of 25 (i.e. 5x5). However, a more powerful 16x50 pair has an exit pupil size of 3.1 (50÷16) and a brightness of 9.6 (3.1x3.1). A higher number is more desirable here if you want a brighter image. An exit pupil size of two or three millimetres is sufficient for comfortable viewing in bright light conditions, but if you want good low light viewing then you’ll need binoculars with an exit pupil of between five and seven millimetres.
To recap; a higher first number indicates more powerful binoculars, while a larger second number means more light is admitted. Accordingly, 7x50 binoculars will be extremely bright, but not as powerful as 16x50s, while the latter won’t be so good in dull lighting conditions.
By the nature of their compact design – specifically smaller objective lens diameters of typically between 20 and 30 mm – pocketsized binoculars are not as bright as the mid- or full-sized models, although better quality optics can make a small difference. Consequently, not all binoculars with a given magnification and objective lens diameter are created equal.
Optics And Construction
As with camera lenses, the optical quality of binoculars varies with price, as do other important features such as the lens multi-coating and the general ruggedness of the construction.
If optical glass with a high refractive index is used for both the lenses and the prisms, then the binoculars will transmit light more efficiently, which will really help improve brightness. Multi-coatings on the lenses (and also the prisms) perform the same duties as they do on camera lenses – namely minimising flare, haze and reflections to give a sharper, brighter and more contrasty image. Some binoculars have special coatings – usually coloured red, green or gold – to absorb harmful ultraviolet (UV) radiation, which is an important consideration in a country like Australia.
If you’re going to be using your binoculars very frequently and for long periods at a time, good UV protection is well worth considering for the sake of your eyes.
If you’re going to spend a lot of time in the great outdoors then it will also be worth considering binoculars which are at least moisture-proofed, if not fully waterproofed. All-weather capabilities can be further extended by fully sealing the binoculars and filling them with nitrogen gas, which prevents internal fogging (otherwise condensation would occur when the barrel temperature dropped).
Rubberised coatings on the barrels are now common, but they don’t necessarily mean the binoculars are weather-proofed. However, these cushioned or armoured coatings do provide a better grip and also some degree of protection against scuffs, scratches and even minor shocks. If you’re hard on your equipment, it will be worth paying extra for well-made binoculars which are rugged and durable.
The main controls on a pair of binoculars are the centrally-located focusing wheel or rocker, and a dioptric adjustment, usually on the right eyepiece. You adjust the latter first so that the focusing is then calibrated to your eyesight strength. Some binoculars have additional features such as zooming, rangefinders (which generate distance read-outs), a built-in compass and optical image stabilisation. These create an extra control or two, but no pair of binoculars is exactly complicated to use. Nevertheless, it still pays to thoroughly try before you buy.
What To Look For
While it’s tempting to opt for the highest magnification your money can buy, you may not end up with binoculars entirely suited to your requirements. Size and weight considerations aside, very powerful binoculars generally have mediocre low-light capabilities and so aren’t so easy to use in dull conditions. Additionally, the higher magnification makes it harder to maintain a steady image when hand-holding so, if you do need lots of power, make sure the binoculars have a tripod mount. Finally, because they’re primarily designed for long-distance work, more powerful binoculars don’t generally have very good close-up focusing capabilities.
If you want a more compact pair of binoculars to fit, for example, into a spare pocket in your camera bag, it’s worth comparing both roof prism and porro-prism designs. While the latter may be marginally bulkier, they’re generally brighter and deliver a more three-dimensional image, especially at longer distances (this is because the two objective lenses are further apart).
A binocular’s field-of-view (FOV) is another important consideration. This indicates just how much of the subject you’ll be able to see (many specifications also quote an angle-of-view). So, for example, if the field-of-view is 114 metres at 1000 metres, this means that what you see in the viewfinder spans 114 metres when viewing a subject from a distance of a kilometre. Obviously a wider field-of-view is useful if a lot of your viewing involves tracking moving subjects such as birds or aircraft in flight.
When trying out a pair of binoculars make sure the eyepieces are comfortable to look through, the eyecups fit well, the grips feel right and the focusing control is both easily reached and easily operated. The ergonomics are very important, especially if you’re going to be using your binoculars frequently.