Introduction

Do you want to choose the binoculars that are right for you? Would you like to understand the choices better? This guide explains some of the terminology and the advantages and disadvantages of different types. Advice on using and caring for your binoculars is also included.

What do the numbers mean?

Binoculars are designated by two numbers separated by an 'x', for example 8x42. This is usually pronounced 'eight by forty-two' and means that the binoculars have a magnification of eight times and an objective (front) lens of 42mm in diameter.

The magnification tells you how much bigger something will appear; another way of looking at it is that it tells you how much nearer it will appear. For example, binoculars with a magnification of ten times will make something 100 metres away appear to be 10 metres away.

What are Porro Prisms and Roof Prisms?

All binoculars contain prisms to give you an image that is the right way up. Two types of prism are commonly used and these make the two types of binocular quite different shapes. Porro prisms give the traditional binocular shape with the objective lenses being a different distance apart than the eyepieces.

Roof Prisms produce binoculars with straight barrels.The internal focusing used with most roof prism binoculars is less prone to damage. A small number of more expensive porro prism binoculars use internal focusing.

Some people prefer the shape of porro prisms, others prefer roof. In general, we would recommend roof prisms but porro have been used successfully for a long time and provide good value for money.

Should I choose Roof Prisms or Porro Prisms? Does it really matter?

Roof prisms are more expensive to produce so roof prism binoculars will generally be more expensive when comparing equal quality. The price differential is reducing as they become more popular. Roof prism binoculars tend to be less bulky than porro prisms. They also are easier to seal against moisture and dust, and often will focus much closer. Most porro prism binoculars focus by moving the eyepieces in and out and this mechanism is prone to wear and misalignment, especially in cheaper models.

What magnification should I choose?

8x and 10x are the two most common magnifications and 10x is the highest magnification that most people can use without hand shake becoming a problem. So, unless you plan to mount your binoculars on a tripod we recommend choosing 8x or 10x.

8x binoculars generally give a wider field of view which makes it easier and quicker to locate objects such as birds. 10x brings them a bit nearer and may reveal a little more detail. 8x is probably the more popular magnification for bird watching.

Should I choose compact binoculars or full-size ones?

This depends on how and where you plan to use them. Compact binoculars are easier and lighter to carry. In good lighting conditions such as a sunny day they work very well and many people find them perfectly adequate for most of the time. Full-size binoculars collect more light and give a brighter image in dull conditions. This is particlularly noticeable in twilight, at night, or in a dark wood.

What do you mean by 'compact' and 'full-size'?

Compact binoculars usually have 20 or 25mm objective lenses whereas full-size usually have 40 to 50mm. Some manufacturers make intermediate sizes, especially 32mm objectives, but there are fewer of these to choose from. Many people have a full-size pair and a compact pair. The compact pair is carried most of the time, 'just in case', and the full-size are carried when they want to do something specific such as visit a bird reserve. Some people find that an intermediate size covers both requirements.

I've heard people talk about the exit pupil. What is it?

This is effectively the diameter of the light beam entering your eye from the binoculars, when your eye is the correct distance from the eyepiece. You can calculate its diameter by dividing the size of the objective by the magnification. For example, a pair of 10x50 binoculars has an exit pupil of 5mm (fifty divided by ten). A pair of 10x25 binoculars will have an exit pupil of 2.5mm.

The bright circles in the centre of the eyepieces in the picture are the exit pupils. You can see these for yourself by holding the binoculars at arms length and towards the light.

What does the exit pupil size mean in practice?

The bigger the exit pupil the better the binoculars will be for dull conditions. On a bright day when the pupil in your eye is small it may only let in a light beam of 2 or 3mm in diameter. A lot of the light from a pair of 10x50s will thus be blocked by your iris and wasted.

However, as the light deteriorates and your iris opens up, you will benefit from the extra light from the larger exit pupil given by the larger pair of binoculars. If you hold a pair of binoculars away from your eyes and point them towards the light you will see a bright circle on the eyepiece. This is the exit pupil and you can see how it varies in size with different formats of binocular.

What is the field of view?

This is effectively the size of area you can see when holding the binoculars steady. It is usually expressed as an angle or a width at a specific distance, often 1000 metres. One degree is approximately 17.45m at 1000m.

The field of view is dependent on the overall design of the binoculars but is largely dictated by the magnification and eyepiece. Producing a wide field of view with excellent images to the very edge is expensive so expect to compromise between field of view, image quality and price.

I wear glasses. Should I keep them on or take them off when using binoculars?

This depends on the reason for wearing glasses. If you are simply long- or short-sighted then the binoculars may have enough focus range to accommodate you in which case you may find it more comfortable to remove your glasses.

If you have astigmatism the binoculars will not compensate for this and it is better to keep your glasses on. If you have to keep your glasses on (as I do) or prefer to keep them on then you should choose binoculars with long eye relief. Because more and more people are wearing glasses, manufacturers have responded by producing more binoculars with long eye relief.

What is 'eye relief'?

This is the distance from the eyepiece to the optimum position for the pupil of your eye. Long eye relief leaves room for your glasses between your eye and eyepiece. Many binoculars have pop-up eyecups that are pushed down when wearing glasses or popped up when not. This allows you to position your eye in the optimum position whether or not you are wearing glasses.

What is the dipotre adjustment for?

Dioptre adjustment allows the binoculars to compensate for small differences between your eyes. One eye may be shorter sighted than the other for example. The dioptre adjustment is usually on the right eyepiece which rotates to make the adjustment. Look for a small scale and an index mark that shows the amount of adjustment. This usually has a plus and minus sign but models vary. Not all binoculars have the adjustment on the right eyepiece so you should consult your instruction book if you can't find it.

How do I set up and use my binoculars?

Set the eyecups down if you are wearing glasses, or up if not. Set the dioptre adjustment to be in the neutral position. Look through the binoculars and adjust the separation of the two halves until you see a single round picture. Most binoculars have a single hinge between the two barrels; just twist around this hinge to adjust the separation. A few compact models have a double hinge and you should try to keep these symmetrical as you adjust the separation.

Choose a target to look at and focus the binoculars (usually done by rotating the central focusing knob). Now close your right eye and fine tune the focus with the same control while looking only through your left eye at the same target. Change eyes and fine tune the focus using the dipotre adjustment while looking only through your right eye. Note the position of the dioptre adjustment for future reference.

Your binoculars are now set for use and you just need to use the main focus knob to view objects at different distances.

(For a more accurate dioptre adjsutment it is better to cover each eye as you look through the other, rather than close it, but this is tricky and is easier with help from a second person.)

Do your binoculars come with cases?

At the time of writing, all our binoculars come with cases and carrying straps. (Some years ago Leica charged extra for compact cases but these are now included in the price.)

How do I look after my binoculars?

Binoculars contain precision optics whose performance relies on maintaining accurate alignment. Whilst they are reasonably robust they do not take kindly to being dropped or abused. Treat them with care and they will give good service for many years.

Avoid getting finger marks on the lenses and restrict cleaning to removing dust with a soft brush or puffer bottle where possible. If greasy marks get on the lenses clean them gently using lens cleaning fluid and cloths designed for the purpose. Lens cleaning kits are available from Green Witch and will help to keep your binoculars in good condition.

Try to avoid getting them wet even if they are waterproof; water may leave marks on the lenses. If the binoculars do get wet then let them dry out naturally before putting them away. In particular, do not put on the lens caps if the binoculars are wet. Avoid extremes of temperature such as leaving them in a car in direct sunlight.

Is there anything I shouldn't look at through my binoculars?

Never look at the Sun through binoculars. Instantaneous and irreversible damage will be done to your retina unless the binoculars are correctly fitted with suitable filter material such as Astrosolar film. Looking at the full Moon will be dazzling but not harmful.

And finally...

Binoculars are designed to be used so make the most of them. Try them at sports events, in the theatre, on the night sky, or anywhere where you want to get closer to the action. Take them with you when looking round churches and other old buildings; it makes it so much easier to see detail on the ceiling and on high walls.