Repair: Learn how to repair binoculars.
Magnification: The most common magnification sizes are between 7X and 10X. 10X is generally regarded as the largest magnification which is practical for hand held binoculars. 7X may seem to a bit weak, but is easy to use, and combined with a large objective lens, provides good performance in low light conditions.
Size of Objective Lenses. The objective or "light gathering" lenses are important as they determine the amount of light that is received by the binoculars. 7X32 means that the objective lens is 32mm in diameter. The amount of light the binocular receives is a function of the area of the lens rather than diameter. This means that 7X50 objectives gathers more than twice as much light as 7X35 lenses. The size of the objective lenses affects both image resolution and performance in poor light conditions. All things being equal a 10X50 will provide better resolution than a 10 X40.
Field of view. The wider the field of view, the easier it is to scan the terrain.
Depth of field. The depth of field describes how much of the field is "in focus" at any one time. Binoculars with a poor depth of field require a great deal of attention to focusing in order to keep things in sharp focus.
Eye Relief. Eye relief is particularly important if you use spectacles, and describes the minimum distance between the exit lens and your eye. A long eye relief means that you can hold the binoculars further from the eye (with spectactles in between) and still get the full benefit of the binoculars. A good eye relief figure is from 16mm upwards.
Size of exit pupil. The exit pupil size is the size of the pupil through which the light reaches your eye. In good light a 4mm exit pupil produces the same brightness as a larger exit pupil, this however changes in dim light, where the larger exit pupil provides a real advantage. A less obvious advantage of a slightly larger exit pupil is the ease of use. A small exit pupil requires precise alignment with your eye, and tends to be more strenuous to use.
Type of lenses and lens coatings. Special coatings on the various lenses found in binoculars are aimed at reducing reflection, and it is here that the greatest advances in image quality have been made. Cheaper binoculars usually have coatings only on the external lenses, while better binoculars have coatings on all lenses. Single coatings on a lens reduce light loss to 1.5 % from about 4% for uncoated lenses. The best multicoatings achieve a figure of less than 0.5% per lens. Considering that a pair of binoculars typically has more than 10 lens surfaces the difference between "fully multicoated" and "coated" can be quite substantial.