Binoculars have become very popular in recent years. They have moved beyond military, police and surveillance use and become a common piece of equipment for recreational purposes. Beyond bird watching, they are increasingly being bought for use in hunting, fishing, boating, camping, hiking, in fact they can add to the pleasure of most outdoor pursuits.

Most of these activities take place not just in daylight but at dawn and dusk or even in the nighttime. You see many adverts these days for night vision binoculars and they might seem like an attractive proposition, but not all night vision binoculars are made the same. You need to be aware of the differences or you could end buying something that doesn't live up to your expectations or paying a lot of money for functionality that you don't need.

If your usage will be mainly at dawn or dusk, you might choose low light binoculars. How can you tell what's good for low light? We need to look at Exit Pupil.

This is the diameter of the light beam leaving the eyepieces. Hold the binoculars at a short distance and on each eyepiece, you will see a small circle of light, the exit pupil. On binoculars that are 12×50, you divide 50 by 12 to get just over 4mm diameter for these. A higher figure equals more light, but that is not the only factor.

For use in real darkness, your binoculars must have an infrared light source. The useful range of these varies a lot. Choose wisely for your application. For hunting, you need to be able to see your prey at a reasonable distance.

Different optical devices have different uses. A monocular, sometimes known as a spotting scope, does just that. Do a quick sweep of the terrain or the sea. Spot movements in the trees.

A compromise is a bi-ocular, with two eyepieces but only one front lens. A true night binocular, with two lenses, allows your eyes to be adjusted equally to the night vision viewfinder. This is preferable for extended periods of use.

Before buying night vision binoculars, think about how you will use them. Will it be mainly in true darkness and for extended periods? Will you use them often? A true night vision binocular can run to hundreds of dollars and more. Be sure that it is what you really need.