All binoculars are described by using a pair of numbers, such as 7×50 or 8×30. The first number, including the x, represents magnification or “power”. This tells the degree to which the object observed is enlarged. For example, a 7x binocular makes an object appear seven times closer than when viewed by the naked eye.
There are some models of binoculars that offer variable magnification, usually in the range of 5x to 8x. They are called zoom binoculars, and in most cases are not very suited for astronomical observations because of the inferior optical quality and fragile mechanics. The best thing to do is to avoid them and stick with the usual fixed-power binoculars.
Magnification is not that important, and in most cases comes within 7x to 12x. If the magnification exceeds these figures, most likely you won’t be able to hold the binoculars steady enough and the images will be blurry and in constant movement. This is especially frustrating when observing faint objects like galaxies and nebulae. A tripod mount or image-stabilized binoculars will get you rid of this problem, but we will talk about this later in the article.
The second number in the two-number code is aperture (from the Latin aperire – “to open”), the most important specification of binoculars if you plan to use them for astronomical observations. It represents the diameter of each of the objective lenses (the lenses furthest from your eye), given in millimeters. Therefore, 7×50 binoculars have objective lenses 50 mm in diameter.
Aperture is so important because it determines the light gathering ability of your binoculars. Most celestial objects glow very dimly, so a large aperture becomes much more important in low light conditions. For example, 35 mm binoculars will do great when you watch a baseball game on a sunny day, but when used to observe the night sky you will find that they are pretty useless compared to typical 50 mm binoculars.
The 15 millimeters difference in lens diameter does not sound that huge, but we need to compare the surface area, not the diameter. The area of a circle is proportional to the length of the diameter times itself, so a small increase in diameter makes a big difference in area. This is why our 7×50 binocular has twice the light gathering ability of the 7×35 binocular.
After all these said, it is clear that when it comes to aperture bigger is better. The larger the light gathering area, the brighter the images will appear. Compared to the naked-eye a 50 mm binocular gathers from 50 to 100 times as much light, translating into a difference of five stellar magnitudes. Therefore, if from your observing site you can see stars to magnitude 5.5 with the naked eye, the binocular will show many more stars down to magnitude 9.5 or even dimmer.
If you divide the objective lens diameter by the magnification, you will get a number approximately between 4 and 8. This number is called the exit pupil, and represents the diameter of the beam of light that leaves the eyepiece when you hold a binocular with the objective pointed towards a light source. For example, a 7×50 binocular has an exit pupil of 50 divided by 7, just over 7 millimeters in diameter.
Ideally, the exit pupil of your binocular should be equal or slightly smaller than the pupil of your dark-adapted eye. In this way the binocular delivers the maximum amount of light and produces the brightest possible images for its aperture.
Average young adults under dark night conditions have pupils that are open to about 7 millimeters. This means that any instrument with an exit pupil larger than 7 millimeters will only waste light, as only the centre of the light beam could enter the eyes. As we get older our eyes dilate less, so the exit pupil size we need decreases to around 5 millimeters.
Field of View
The field of view is the area of sky or land seen through your binoculars, determined by the design of the instrument’s optics. It is expressed in two ways; as the width in feet at 1,000 yards, or in degrees of field. When expressed in feet the field is called linear, and when expressed in degrees it is called angular. Don’t let the terms confuse you, the conversion is easy – divide the liner field by 52.35 and you get the angular field.
In most cases the field is indicated on the outside of the binocular, in degrees. Average values are between 5 and 10 degrees, or roughly 260 to 520 feet. To get an actual idea of how wide this field is, think that in five degrees you can fit almost 10 Full Moon diameters!
For astronomy, a wide field of view is desirable because if offers a more pleasant viewing experience, and you can see more of the sky at a better edge performance compared to a narrower field. However, when increasing the field beyond a certain point images start to exhibit signs of distortion, especially near the edges of the field. Also remember that field of view is related to magnification; the higher the power of your binoculars, the smaller the field will be.
Eye relief is the distance behind the eypice lenses at which the image is in focus, and indicates how far the binoculars can be held from your eyes and still allow you to see the entire apparent field of view. In general, the longer the focal length of an eyepiece, the greater is the eye relief. Standard binoculars have eye relief ranging from only a few millimeters to 25 millimeters or more.
Long eye relief is especially necessary for eyeglass wearers, because glasses increase the distance between the lens and your eye. In case your eyeglasses correct only for near or farsightedness, you can simply take them off and refocus the binocular to compensate.
Interpupillary distance is the distance between each of the adult person’s eyes. Most binoculars are adjustable to accommodate different interpupillary distances, typically within a range of 60 mm to 72 mm. Many children and some women have interpupillary distances too short for standard binoculars, so the only solution is to choose compact binoculars.