Bulletin 9 - November 1979: Which Binocular?

Which Binocular?

by Bish Brown

Among the camels, the occasional lagoon-side car, and the odd trail bike to be seen on most late afternoons at the Sewage Farm are single men or couples, just here and there. They arrive purposefully and spend long minutes crouched or standing motionless. To the uninitiated they may be identified as birdwatchers primarily because of their binoculars, continually used for scanning the horizon and closer marshy habitats. Again, to the uninitiated, binoculars are binoculars, but to a keen birdwatcher, they are usually the only means of securing a certain identification, and he will make sure that his 'glasses' are suited to his needs.

How do we choose the right binocular for any particular purpose? If we leave price to the individual choice, should we choose 6 x 30 or 10 x 50?

Let it be emphasized that the above description is not quite correct and should be 6 x 30 or 10 x 30. The "x" means and refers to the magnification. Thus a pair of 6 x 30 glasses makes the object six times larger, or brings it six times nearer to you, whichever you prefer. And the "30"? This stands for the diameter of the objective, or front lens, as measured in millimeters (25.4 mm = 1 inch). The second of the two figures used to describe a model is always larger.

Another term often used is "exit pupil". This is the diameter of the light ray that reaches your eye through the eyepiece and is determined by dividing the diameter of the objective lens by the power of magnification. A 7 x 35 binocular has an exit pupil of 5 mm which is about average, while a 7 x 50 binocular has an exit pupil measuring 7.1 mm. The exit pupil is important if you are looking for glasses which produce a bright image in dim light, because the larger the exit pupil the better the ability to see in dull light.

The field of view of a given binocular is usually stated as a certain number of feet at 1000 yards. The binocular itself is engraved with "field 7.5 deg." or "field 11.0 deg.". At a thousand yards each degree is equivalent to 52.5 feet; therefore, a binocular with an 11.0 deg. field of view would cover 577.5 feet at 1000 yards. The wider the field of view, the easier it is to follow fast-moving objects. In practical terms, when buying a pair of glasses, focus on a distant object and check the extent of field of view to the left and right. Do this with a number of pairs outside the shop. Also compare minimum focusing distances - it is very frustrating not to be able to focus on a very close bird.

The larger the glasses, the heavier they are likely to be and this is also a factor to be considered. You will need to be strong and steady to hold a 10 x 50 because of the weight rather than the magnification. The balance of a binocular can be important for comfort of holding and for carrying around the neck. If you wear spectacles it is worth looking for a model with rubber surroundings to the eyepieces. These can be rolled back allowing the binocular lenses to nearly touch the spectacle lenses and so maximize the field of view.

We would suggest that you settle for glasses of a recognized manufacturer (too many models at the cheaper end of the market have a 'color-fringing' fault), that you try as many different models as possible outside the shop, and choose 7 x 35 or 8 x 30.


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