Binoculars

Generally, for birdwatching, 3 metres isn’t an issue, but if you’re likely to use your binoculars to look at insects (dragonflies and butterflies, for example) or to get a closer look at inaccessible wildflowers on the far side of a ditch or fence, then close-focus could be more important for you. 

The eye-relief of an optical instrument refers to the maximum distance behind the eyepiece that a user can place his or her eye and still see the full extent of the field of view.

If you wear glasses, pay particular attention to the eye relief quoted in a binocular’s specs. Eye relief measurements of 16 mm should work well for most eyeglass wearers; however, some may find they require an eye relief of up to 18 mm or even more to experience the full field of view.

It’s is important to ensure your binoculars have been purged with an inert gas usually nitrogen but occasionally argon) at the factory. This drives out the air (and the water vapour it contains), eliminating the risk of internal condensation/fogging as your move between dramatically different temperatures (from a warm car, for example, to a freezing lakeshore, or vice versa).

A wider field of view makes it easier to scan large open areas quickly, and makes it much easier to find and follow fast, erratically moving subjects like small birds. Higher magnification reduces the field of view.

Larger objective lenses let more light into the optical system… so binoculars with larger objective lenses have the potential to deliver a brighter image at the eyepiece than those with smaller objectives. There’s a caveat though: as you increase the size of the objective lenses binoculars become steadily bulkier, heavier and more expensive.

In practice, for a binocular you want to carry with you and use in the field, the useful range of objective lens sizes is from 30mm-50mm, 42mm being the most popular.

A quick indication of brightness can be gained by dividing the diameter of the objective lens by the magnification.

For example, an 8x42 binocular would have a brightness value of 5.25 (42 divided by 8).

Every binocular has two numbers printed somewhere on the instrument (10x42, 8x32, etc.) -- the first of these numbers (before the x) signifies how much the binoculars will magnify the view you’re looking at.

While higher magnifications will bring your subject closer, they will also reduce image brightness, decrease depth of focus, reduce your field of view and accentuate hand shake and atmospheric aberrations. The sweet spot for birdwatching and wildlife watching, is 8x or 10x magnification.

Binoculars for birding have to be able to cope with whatever the weather can throw at them. Make sure the binoculars you choose are waterproof and fully sealed. As well as protecting against rain, or accidental immersion, this will also mean your binoculars are impervious to dust and other airborne contaminants.

The weight of a pair of binoculars will, of course, depend on its size, but also on the type of housing/body material used in its construction, and the quality of the optical components it contains. Magnesium alloy is the strongest, lightest and most expensive material currently used for binocular housings.

Images viewed through binoculars that use HD or ED elements in their optical system tend to appear crisper, more defined and have more accurate colour fidelity across the spectrum -- resulting in a more natural, realistic image.

ED glass produces a sharper and clearer image than HD glass. ED glass is short for Extra-low dispersion glass. It significantly reduces chromatic aberration while improving colour reproduction.