Choosing binoculars

This is a basic introduction. For more detail and specific binocular recommendations at various prices, see the Powerpoint file at the bottom of this page - by Dimitrios Kechagias.

Use your eyes!

·       Before you think about choosing a pair of binoculars or a telescope, don’t forget that you’ve got an excellent pair of light collecting devices in your head.  Take some time to go outside and just look up. This is the best way to familiarise yourself with the constellations, the movements of the planets and the phases of the moon.

·       If you can, find a dark sky site or at least a place out of direct view of street lights. Give your eyes at least 20 minutes to adapt to the dark – you will be surprised how much more you can see.

·       There is no better way to view the annual meteor showers or the Milky Way in all its glory than just using your eyes.

·       To find your way around the night sky you can use the sky maps in the astronomy magazines, a planisphere, a smartphone app like Google Sky, or download free planetarium software like Stellarium.

Choosing binoculars

·       Sir Patrick Moore used to say before you think about buying a telescope you should consider binoculars first. Binoculars have a larger field of view than a telescope making them easier to use and are ideal for scanning the star fields of the Milky Way and viewing larger objects like The Pleiades star cluster, the Andromeda Galaxy and the brighter comets.

·       There are two designs of binocular – porro prism and roof prism. Porro prism binoculars are the traditional ‘binocular-shaped’ binoculars. Roof prism binoculars are the more streamlined looking ones. Choosing one design or the other isn’t important other than to know that roof prism binoculars, because of their design, have to be manufactured to much finer tolerances. This means that for equivalent optical quality roof prism binoculars will be more expensive than porro prism.

·       Binoculars are described by two numbers, e.g. 8x40 (pronounced 8 by 40). The first number, 8 in this case, is the magnification. The second number, 40, is the diameter of each of the objectives (the front lenses).

·       The most common magnifications are 7, 8 and 10 for normal-sized binoculars and 15, 20 and 25 for big binoculars. Higher magnifications may be useful for easily making out the moons of Jupiter for example, but binoculars with a magnification of 15 are very difficult to hand-hold steadily, and higher magnifications are worse. Also, the higher the magnification the smaller the field of view and the dimmer the image.

·       The diameter of the objectives determines the light gathering power of the binoculars, so for astronomy larger objectives are better. However, binoculars with large lenses are heavy and difficult to hand-hold for any length of time.

·       The field of view depends on the magnification and on the design of the binoculars. Field of view ranges from around 6 to 8° for low magnification down to 3 to 4° for high magnification.

·       So what to choose? First, for a given budget, you will probably get better quality porro prism binoculars than roof prism binoculars. 8x40 binoculars are light in weight and fairly low magnification so are easy to hand-hold. 15x70 binoculars will show you far fainter objects but are very heavy and really should be mounted on a tripod or other means of support. The best compromise for all-round astronomical use are 10x50 binoculars.

·       Recognised brands include Celestron, Helios, Strathspey, Olympus, Nikon, Pentax and Opticron. At the top end of the market are Leica, Zeiss and Swarovski.

·       As with most optical equipment, you tend to get what you pay for. Higher priced binoculars will usually have better quality prisms and anti-reflection lens coatings and better general build quality. You should be able to get a reasonable porro prism 10x50 for about £50 and upwards, e.g. Helios Fieldmaster.

·       Be aware that with cheaper binoculars that the manufacturers’ quality control can be poor or non-existent – they hope that inexperienced users don’t spot the defects. On the other hand you may be lucky and get a decent pair.

·       For much more on choosing and using binoculars see Stephen Tonkin’s excellent website Steve writes the binocular observing column for Sky at Night magazine.

Pete Collins,
25 Nov 2016, 07:25