Friday, August 27, 2010

#12 Optics Tip: What do the Numbers Mean?

Every pair of binoculars is described by a set of numbers, for instance 8 x 42, as above. The first number refers to the magnification – the object will appear 8 times closer when looking through the binoculars. The second number is the effective diameter of the objective (front) lens in millimeters, meaning the size of the lens through which the image enters the binocular – 42 mm. The larger the lens size, the greater the field of view, and the brighter the image will appear.

In the case of binoculars, bigger is not always better! If you choose to use a higher magnification (i.e. 10 power) you may see the bird closer and larger, but the field of view will be smaller, and you must hold your hands very steady to keep it in focus. And a large lens size (42-50+ mm) offers a wide field of view, but the optics will be much heavier for it. Choose a pair that is most appropriate for your needs.

The number 7.7 degrees references the field of view, which in itself is not usually a decision-making factor for most birders.  And speaking of numbers, Nikon also includes the 7-digit serial number of the individual equipment on their optics, which is useful if you need to reference them.
photo © adrian binns

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