How to choose a binocular?
It is easier to choose a binocular than a telescope but you have to consider many details. The important technical specification is - the correlation between "magnification - aperture". It is a feature that is indicated in a binocular certificate and on its carcass (for example 7x50). The aperture - it is a diameter of the objective indicated in millimeters. The bigger the aperture the brighter the image will be. For astronomical observations it is necessary to have a binocular with an aperture at least of 50 mm, otherwise the image will be dim. The following procedure will help you in the selection of good binocular. Divide the aperture by the measure of magnification. The resulting value known as "exit pupil" is equal to the image size visible to the eyepiece. It should be approximated to the size of darkness adapted eye pupil - according to rough calculations it is equal to 7 millimeters - the size of mydriatic pupil. For the binocular with specifications of 7x50 the value of exit pupil is approximately equal to 7 mm and such kind of a binocular is optimal for most of observers. The 10x50 binocular is also good especially for people with weak vision and foe those who has to make observations in well lightened places. The eye pupil increases to the size of 5 or 6 millimeters in this case. If you prefer to hold a binocular in hands during observations and not to use mount pillar then you should buy a binocular with magnification no more than 10 and with maximum aperture of 50 millimeters. With too big magnification the image will tremble and a binocular with high aperture is too heavy to hold it in arms.
Some important tips of binoculars
It is very good if a binocular is equipped with multiply antireflected lenses i.e. with multilayer coating lenses which increase the contrast of an image. The cheap models have one-layer coating and not all optical coatings are treated but it doesn't mean that the binocular is bed. You should also consider such a parameter as the remoteness of exit pupil - the distance between an eyepiece and an eye at which the diameter of an entrance pupil is equal to the diameter of an eye pupil. If you wear eyeglasses and you aren't going to take off them then you need a binocular with the highest possible remoteness of an exit pupil - approximately 15 millimeters. Before buying a binocular examine several models and choose the most proper for you.