**1. Objective lens diameter**

The diameter of the telescope is the diameter of the lens of the telescope. The telescope model generally clearly indicates the aperture and focal length of the telescope, for example: 102/1000 refracting telescope, the first group of numbers 102 represents the aperture (mm) of the telescope, the latter group of numbers is the focal length of 1000mm (millimeters). The larger the diameter of the objective lens of the telescope, the greater the light-gathering power (light-gathering power affects the brightness of the telescope’s field of view, etc.). The larger the diameter, the greater the field of view and brightness, which is conducive to the observation under dim light, but the larger the diameter, the greater the volume and weight, and the higher the cost. Generally, it can be selected between 30-50mm according to needs.

**2. Magnification**

The magnification of the telescope is not the bigger the better! Once the magnification exceeds the limit magnification of the telescope (effective magnification), the image quality will become blurred and the details will be lost due to the ineffective magnification. Generally, 6 times or less is low magnification, 6-10 times is medium magnification, and 10 times or more is high magnification. Many people always think that the higher the multiplier, the better. Some manufacturers also use false high magnifications to attract consumers. In fact, the reasonable multiplier of a telescope is related to the telescope’s aperture and observation method: the larger the aperture, the higher the multiplier can be. , The fixed observation with a tripod can be higher than the handheld observation. If you buy hand-held observation binoculars, the range of 7-10 times is sufficient, and the maximum is no more than 12 times. Otherwise, the higher the multiple, the smaller and darker the observation field will be, and the observation effect will decrease, especially when it is brought by high magnification. The jitter is also greatly increased, so that the observed scene cannot be stabilized and it is difficult to observe normally.

**3. Field of view**

Field of view is the range in which we observe the scene. This value refers to the range seen at a distance of 1000 meters. The larger the field of view, the larger the observation range. For example, 1000/93M means that the telescope can observe a wide range of 93 meters at a distance of one kilometer, and it can also be converted into “degrees” to represent 5°30′. The comparison of the field of view size must be carried out under the conditions of similar aperture and the same multiple. The key to the size of the field of view lies in the design of the eyepiece part of the prism system. The high-end varieties and military telescopes of general famous manufacturers adopt the wide-angle and large-field design. The large field of view telescope will give people a wide, comfortable and relaxed feeling.

**4. Eyepiece diameter**

A telescope with a small eyepiece will have a small field of view, and the other has a poor light transmission rate. In order to improve the field of view of the telescope, the manufacturer also made a big fuss on the internal structure of the telescope.

**5. Closest distance**

The shortest focusing distance of the telescope indicates the shortest distance at which the observation object can be focused when using the telescope. For example, 0.5 on the scale of the telescope means that the telescope can observe the details of objects within 0.5 meters and within a few meters, on the scale of the telescope. The mark 5-∞ means that you can observe a target from 5 meters away to infinity. Generally speaking, most outdoor enthusiasts use telescopes to observe distant scenery, so the closest focusing distance of the telescope is not very important, but in some specific situations (such as viewing cultural relics that cannot be approached in museums/ Artwork, viewing murals, etc.) only need a telescope with a shorter focusing distance. At this time, the closest focusing distance of the telescope in the parameters is particularly important. Normally, the focusing distance of the telescope is related to the depth of field of the telescope. The longer the focal length and the closer the focusing distance, the shallower the depth of field.

**6. Focusing method**

There are two kinds of external focusing and internal focusing. The eyepiece moves relative to the objective lens along the optical axis, which is called external focusing. The advantages of external focusing are simple and good image quality; but the appearance of the instrument The size is large and the sealing is poor. The external focusing is often used in refracting astronomical telescopes. Internal focusing is to obtain a clear object image by moving one or a group of lenses in the objective lens group. At present, most binoculars use external focusing, and a few high-end products use internal focusing. Many binoculars (marked CF) use a central focus knob to adjust the focal length of both lens barrels at the same time. Other double lenses (marked with IF) are mostly waterproof binoculars. Each lens is equipped with a separate focusing device. The CF binoculars are easy to use. The mechanical structure of IF binoculars is simple.