The diameter of the objective lens: the diameter is more suitable between 50-100 mm, and the diameter is closely related to the volume, weight and multiple of the telescope. Too small aperture will lead to low effective multiplier of the telescope, which cannot be used for fine observation. Too large aperture means larger volume and weight. It is not good for bird watchers to carry a big guy to climb mountains and wading an easy job. At present, the caliber of the mainstream bird-watching spotting scope is about 80 mm. It is lighter to choose a caliber of about 65 mm, and the matching tripod and head can also be lighter.

3) Objective lens material: The objective lens of high-end monocular bird-viewing scope will be made of ED glass, and the objective lens of top-level monocular birdwatching spot scope will be made of fluorite. ED glass and fluorite are expensive, and the processing yield is low, so the selling price is also very expensive. Compared with ordinary optical glass, ED glass can effectively reduce chromatic aberration and improve the clarity of imaging, especially at high magnification. The effect of using fluorite is better than that of ED glass
spotting scopes often come with zoom eyepieces (generally 20-60 times), which are easy to operate and easy to use. A good zoom eyepiece will have a larger field of view, less distortion, and a longer exit pupil distance. There are also fixed-focus eyepieces with different focal lengths available as optional accessories for some monocular spotting scopes, which can obtain better observation results, but the operation is cumbersome when switching magnifications. In addition, unlike astronomical telescopes, the eyepieces of various brands of bird watching scopes are basically not universal.
5) Anti-reflection coating: The use of anti-reflection coating can also improve the brightness and contrast of imaging, reduce flare and glare, and make imaging brighter and clearer. Good AR coatings tend to have a dull green reflection. Due to cost considerations, some bird-watching monoculars do not have a coating on the surface of the inner lens (the surface coating of the outer lens is easy to identify, so generally do not cut corners), or only coated with a blue single-layer anti-reflection coating. It is recommended to choose FMC-coated monocular bird-viewing scope first!
6) Mirror body material: The top model will use aluminum-magnesium alloy to make the mirror body, which is firm and light. The common fuselage is made of aluminum alloy, which is cheap and reliable, and its density is only a little higher than that of aluminum-magnesium alloy. The mirror body of the low-end bird watching mirror is often made of plastic, which is very low in cost, but it is easy to age and deform. A stable mirror body structure is the guarantee of optics and durability. It is recommended to choose a monocular bird-watching spotting scope with a metal mirror body!
7) Nitrogen filling and waterproofing: This is a very popular technology, injecting dry and clean nitrogen gas into the sealed inner cavity of the telescope to prevent fogging and mold growth inside the telescope and the intrusion of dust and moisture, which can adapt to the field with variable climate environment, greatly improving the service life of the telescope.