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 night time. 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.

There is a difference between binoculars designed to make the best use of very low light conditions and a true night binocular. If you will be operating at dawn or dusk, low light binoculars could suit your purpose. You can get an idea of how well a pair of binoculars will perform in low light from its exit pupil. This is the diameter of the beam of light as it leaves the eye pieces. If you hold the binoculars at arms length, you can see these small circles of light in each eye piece. To find the actual size of the exit pupil, you divide the diameter of the lenses by the magnification. e.g. The binoculars are 12x50. 50mm divided by 12 is just over 4mm. A bigger exit pupil will do better in low light, but there are other considerations.
 

Source: Compact Porro Binoculars,    Monocular Telescope