It’s true that when observing the night sky with your binoculars, you will want the biggest possible amount of light to reach your eyes and not reflect back into space from the objectives. Because of this, modern lenses are designed with anti-reflection optical coatings on at least one of the air-to-glass surfaces. The more sophisticated models will have all glass surfaces coated, but they are usually more expensive.
A single-layer of magnesium fluoride (MgF) coating is the least expensive coating, but there are also modern broadband multi-coatings. Some optics manufacturers in a bid to save money, coat only some of the air-to-glass surfaces, and here we can talk about the four levels of coatings used on binoculars.
The lowest quality are the coated lenses, with a single-layer MgF coating on some of the optical surfaces. To be fully coated means that all the air- to-glass surfaces are coated with a single MgF coating. When multi-layer coatings are applied to on some surfaces, that is called Multi-coated, and finally when all the surfaces have multi-layer coatings applied to it, that is fully multicoated.
There has been a recent invasion of some binoculars into the market that have the so-called ruby coatings, intended to reduce shine in bright light and improve the contrast between objects that green or brown in color. Binoculars with such coatings should be avoided, because it will nor perform efficiently for astronomical use.
All binoculars are built with prisms that serve as mirrors for the reflection of the incoming light between the widely spaced objectives and the narrowly spaced eyepieces. Another role they have is inverting the image that objective lenses project, in a right side up and not reversed left to right view. There are two types of prisms: roof and porro prisms.
Roof prisms are inside the optical tubes lining, allowing the binoculars to be made small and light. Hikers and birders will find this advantageous, but not so with astronomical use, where it will have a low performance.
Binoculars with roof prism design split the light beam in two parts, then recombining them. This process leads to “phase shifting” which means that less light is transmitted in the eyepiece and contrast is decreased. Besides these, roof prism binoculars are mainly much more expensive than porro prism binoculars of the same quality.
In Porro-prism binoculars the objective lenses and eyepieces are aligned in an offset arrangement, with the objective lenses being farther apart than the eyepieces. They are very affordable and offer a wide field of view. However, there is a minor draw back in porro prisms; they are easier to knock out of alignment than the roof prisms.
There are two types of glass with which prisms are made of: BK-7 borosilicate flint glass and BaK-4 barium crown glass. Prisms made of BaK-4 are mostly preferred over the standard BK-7 for most designs, this is because of their higher reflective index and the giving of brighter and well defined images.
If you would want to check the type of prisms in your binocular, point it towards a light source and hold it, take a look at the exit pupils. If it is made of BaK-4 prism glass, the exit pupil will be round and illuminated evenly. But if it is made of BK-7 prisms glass, the exit pupils will show a squarish, gray edges around it.