There are many binoculars in the market. They are different in appearance, size and price, and have different uses. Some are used to observe birds, sports games and concerts, and some are used to appreciate the magical beauty of the night sky. If you want to choose and buy your own binoculars, you must know the following points.
The meaning of the numbers：
Every binoculars sold on the market has a number similar to this: “7×35”, “10×50” or “12×80”, etc. The number before the “×” represents the magnification, the above three pairs The magnifications of the binoculars are 7, 10, and 12 respectively. The number after the multiplication sign represents the diameter of the main mirror of the telescope (the mirror on the side of the observation object), in millimeters. The apertures of the above three binoculars are 35mm, respectively. 50mm and 80mm.
The importance of numbers：
Most people believe that the higher the magnification of the telescope, the better the viewing effect. This is not the case, but the higher the magnification, the lower the image quality. Observing with a low-power lens, the image will be brighter and more stable, and the observation range will be larger. If you use a high-power lens to observe, you will find that the image becomes larger, but the field of view becomes smaller. In addition, high magnification binoculars require high stability. If the stability is not good, the image will tremble so badly. It is difficult for ordinary people to hold binoculars of 10 times or more with their hands for a long time. If it is more than double, then you must equip the binoculars with a stable tripod.
Objective lens aperture：
The larger the aperture of the objective lens (which is basically equivalent to the diameter of the objective lens), the stronger the light collection ability, but the mirror will be heavier. If you need to use binoculars often in bright places. It doesn’t matter if the caliber is larger and smaller, but if you want to observe with binoculars in the dark, such as watching celestial bodies, then a larger caliber is very important, generally speaking. It is better to choose binoculars whose size, weight, and caliber are suitable for your observation activities.
What affects brightness?
Exit pupil: If you divide the objective lens by the magnification factor, such as “35/7” or “50/10”, then you can get the diameter of the light beam that hits the eye through the telescope in millimeters. The larger the value, the more light or celestial information your eyes receive. This value is called the exit pupil of the telescope. What’s the point of it?
Let us suppose that you are going to buy a pair of binoculars for observing birds. You want to use it to observe birds at dawn or in the evening. At that time, birds often fall in the bushes and hide in the shadows. If you buy 10×25 binoculars, then the exit pupil diameter is 25/10 = 2.5 (mm). The pupil diameter of our eyes ranges from 2mm to 7mm, which varies regardless of the dimness of the light. The darker the light, the larger the pupil diameter. If you plan to use binoculars to observe in a dark place, you should choose binoculars whose exit pupil is similar to the pupil of your eyes in the dark, so as to make the most effective use of the information received by the telescope. So what about the “7×50” binoculars? Its exit pupil is 50/7=7.14mm, which is almost the same as the diameter of the pupil in the darkest part of the human eye. When used in the dark, the light it collects can be efficiently received by your eyes. So it is also an ideal choice.
If you wear glasses, how should you choose a binocular?
If you wear glasses, then you should pay attention to the distance between the eye and the eyepiece when you can see the entire field of view or the stars in the field of view. Different telescopes have different distances. Generally between 5-20mm. The rubber eye mask on the eyepiece is to keep the eyes at a suitable distance and feel comfortable when observing. If you need to wear your eyes to watch with binoculars, then the distance between the eyepiece and the eye becomes larger, so choose a larger distance between the eyepiece and the eye.
Which type of binoculars are suitable for starry sky observation？
If you choose binoculars to observe the starry sky, the aperture of the objective lens is the most critical. If you want hand-held binoculars. The best binocular model that meets the two conditions of hand-held and used to observe the starry sky is 7×50 (exit pupil is 7.14mm) or 8×56. If you plan to use the binoculars on a fixed tripod, the best choice is 10×70 or 12×80.