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Binoculars
Choosing binoculars isn't always easy, and sometimes advertisements make exaggerated claims. Only you can make the choice, bearing in mind price, performance, size, durability, and your own particular needs.
What do the figures mean?
In all binocular specifications, two sets of figures are supplied (eg 10x40). Sometimes there's a letter as well, such as B or GA. The first figure indicates magnification the number of times the image is enlarged. This is usually 7x, 8x, 9x or 10x, but you can get binoculars of 12x and 16x as well. The second figure indicates the diameter of the 'objective lens' - the lens furthest from your eye. (The measurement is in millimetres.) The larger this lens, the more light reaches your eye. It's this measurement that determines the physical size of the binoculars. The letter B means that the eyepieces (usually with fold-down rubber cups) aresuitable for people who wear glasses. You can use these binoculars while wearing your glasses without losing much field-of-view.
'GA' means that the binoculars are rubber covered to protect them against wear and tear. Some manufacturers use 'BA' to mean that the binoculars are rubber-covered and have fold-down eyepiece cups.
The field-of-view is sometimes given in degrees, rather than, for example, '140m at 1000m'. A rough guide: 1 degree is approximately '17m at 1000m'.
What magnification do I need?
Generally, the lower the magnification the wider the field-of-view the brighter the image the easier to use without a support the closer you can focus. So 7x or 8x are ideal if you are using a telescope as well, or for woodland use, or at sea, or in poor light. They are recommended if you're walking, too, being easier and quicker to use. You get good depth of field, and you don't have to keep refocusing.
10x are the best compromise, however, if you aren't using a telescope as well. They focus down to about 15 feet as a rule, but are still manageable for distance work.
The higher the magnifications the narrower the field-of-view, the duller the image, the more difficult to hold steady. High magnification binoculars are more suitable for hidework, viewing at estuaries and reservoirs, and when you're not using other optical instruments as well.
What type of binoculars do I need?
There are two main body types: the traditional porroprism design, with an angled body shape, and the roofprism (or 'Dach') design, with its 'straight through' look. Roofprisms are more expensive. They have internal focusing which is more robust and protects the optics, and they are more compact.
Both designs are available as fullsize instruments (eg 10x50 or 10x40). You can also get them in a more compact form (eg 8x30) or as a pocket field glass (eg 8x20 or 9x25).
Miniature binoculars are fine as backups to your fullsize ones and when you need them to be small and light. But miniatures don't have the same light gathering power or field-of-view as the larger binoculars do. Undoubtedly the best for birdwatching are the top quality roof prisms.
