In 1993, the result of the multiplication of aperture and magnification was proposed as an indication of the performance of binoculars. An 8 × 30 would thus have a value of 240 and a 7 × 50 would have a value of 350. That would be slightly worse than the slightly smaller 10 × 40 with 400 and a 10 × 50 would provide 500. The higher the value, the better is the suitability for astronomy. Zoom binoculars are in theory a good thing, but usually they pay a very high price for the various possible magnifications, since image quality and field of view suffer – or remain acceptable only for very expensive models.
Besides aperture and magnification, it is also very important which objective lenses and prisms are used. To compensate for chromatic aberrations, the objectives of modern binoculars consist of several lenses. There are also the prisms which invert the image – an astronomical telescope without prisms shows an upside-down image. Since there are reflections on each glass surface, brightness and contrast are reduced. To minimize these reflections the surfaces can be coated. With low cost models, only some of the lenses may be coated, while good binoculars have coating on each lens. In vague statements such as „coated optics“ caution is recommended – this may also mean that only the outer sides of the front lenses or the eyepiece are coated. Here, you have to trust the manufacturer‘s declaration – the dealers often only know what is printed on the packaging.
The color of the lenses gives a first indication of the coating. If you hold a pair of binoculars at an angle to the light, usually a reddish or greenish tinge appears. A good coating of magnesium fluoride shimmers uniformly purple. If the coating is too thick, it looks greenish, while a pink shimmer may indicate a too thin coating layer.
A green shimmer can also be caused by very good, multi-coated lenses. With these lenses, the light loss of 1.5% can be reduced to less than 0.5% per surface. Unfortunately, low cost models can seem like multi-coated lenses because of a much too thick coating on the lenses. However, they provide an image with a much poorer contrast and image brightness than good binoculars.
The prisms are the most important difference between a pair of binoculars and a telescope. The older design of binoculars use Porro prisms. Binoculars with Porro prisms are somewhat bulkier than those with the more elegant roof prisms and the distance between the objective and eyepiece is longer. Roof prisms allow a straight and thus more compact design with lighter weight. Porro prisms are a bit more useful for astronomical use, as they are not only of lower cost but also provide brighter images for the same amount of money.
If you hold the binoculars looking at a bright object (never towards the Sun!), and look through them at arm‘s length, you can learn a lot about the type of glass used for the prisms. If the exit pupils appear as clear, round circles against a dark background, higher quality BaK-4 glass was used. Diamond-shaped distortions on the edge or non-uniform illumination reflections are the result of less expensive BK-7 glass.