Explanation of common terms of telescope

Magnification: refers to the magnification of the telescope.
For example, for a 7x50 telescope, the magnification of the telescope is 7x. 7x means that when you look at a 700 meter target with a telescope, it is equivalent to looking at a 100 meter target with the naked eye. The higher the magnification, the larger the scene seen. However, high magnification will reduce the brightness, making the field of view darker, and the jitter of the field of view will increase. Generally speaking 10x is the limit of handheld binoculars. 10x or more requires the installation of a tripod to observe.

Objective lens aperture: the diameter of the objective lens size
For example, for an 8x42 telescope, the objective lens is 42MM.
The larger the aperture, the stronger the light, the brighter the field of view, but the weight is also greater, and the production process of large lenses is complex, will increase production costs. Therefore, the aperture of handheld optics usually does not exceed 50mm. Moreover, telescopes over 50mm are too heavy to carry.

Field of view
Field of view is the field of view through the telescope, the larger the field of view, the larger the observation field, easier to observe the target. The most important factor affecting the field of view of a telescope is the magnification, the higher the magnification, the smaller the field of view. Therefore, you should choose the right magnification with your own needs, and not blindly seek high magnification

Eye Relief
When you can just see the full field of view, the distance between the eye and the eyepiece is the pupillary distance. The size of the distance depends on the design of the eyepiece. If the pupil distance is too short, your eyes will be close to the eyepiece glass, which is not suitable for eyeglass wearers; if the pupil distance is too long, the field of vision will be easily affected by black shadows and stray light.
Eyeglass wearers are recommended to choose binoculars with a pupil distance of 14mm or more.

Exit Pupil diameter
When the light passes through the telescope, you will see a circular light spot in the center of the eyepiece, and the rest of the place is black. The diameter of the light spot is the diameter of the exit pupil. The diameter of the exit pupil corresponds to the diameter of the human pupil, about 4mm. A good telescope's exit pupil is a perfectly sharp, circular spot of light in the center surrounded by black. The larger the diameter of the exit pupil, the brighter the brightness of the field of view, the higher the definition, and there is enough light entering the pupil, the more comfortable the human eye observes. If the exit pupil diameter is too small, it will make it difficult to observe the target, and the eyes are prone to fatigue, but the exit pupil diameter is not as large as possible. If the exit pupil diameter exceeds 7mm, part of the light cannot enter the pupil, resulting in waste.

The formula for calculating the exit pupil diameter is objective lens diameter (mm) / magnification.