Objective: The objective lens is in the front of the sight, is the part of the scope to receive an external light source, the larger the diameter of the objective, the more light sources can be received, the same distance, the larger the diameter of the scope, then the shooter can see the clearer image. Usually, in order to get more daylighting, manufacturers will coat a layer of fluoride on the surface of the objective lens to improve the amount of light transmission and reduce the amount of reflection (see High School Physics Compulsory for details). If you see a purple or yellow reflection on the objective, needless to say, it's a reflection caused by the coating.
The diameter of the objective lens also determines the exit pupil diameter of the sight, what is the exit pupil diameter? We know that the scope will produce an image behind the eyepiece, and the diameter of this image is the diameter of the pupil. Pupil diameter = objective lens diameter / magnification In normal circumstances, the exit pupil diameter is not less than 5mm, because this is the size of the pupil of our human eye, if it is too small, then we cannot see a complete and clear image, and in low-light conditions, the human pupil is magnified to 8mm, so we can obtain a larger exit diameter by adjusting the magnification of the scope.
Adjustment handwheel: The adjustment handwheel is located in the middle of the lens barrel, wherein the upper is the pitch handwheel, there is BDC (ballistic height correction knob) and target correction knob, the difference between the two is only that the target correction knob has a smaller scale indicator; Located on the right is the directional handwheel of the sight, which adjusts the horizontal direction to correct the wind deviation and the advance of the moving target; On the left is the focusing handwheel of the objective lens, which can be adjusted to make the image clearer and reduce the error. Of course, some scopes are through the focus ring.
Inverted image group: We know that the convex lens sees an inverted, magnified real image outside the double focal length, while the eyepiece sees an enlarged virtual image due to the small pupil, so if there is no inverted image group, what we see from the eyepiece is an enlarged, inverted image. This is very inconvenient for aiming and shooting. The inverted image group plays the role of putting the image right.
△ The focal length of the objective lens is actually very short, and the distance to the inverted image group is already greater than 2 times the focal length
Lens barrel: The lens barrel is used to install in the inverted image group and adjust the handwheel and other parts to protect and transmit light. The diameter of the lens barrel is generally about 1 inch, and the lens barrel of the European scope is generally about 30mm. The larger the barrel, the brighter the light and the lower the angle of refraction, resulting in a clearer image.
Eyepiece: The eyepiece is located at the end of the scope, and the function of the eyepiece is to further enlarge the image magnified from the objective lens to the human eye. In general, the diameter of the eyepiece is smaller, only slightly larger than the barrel, which is determined by the action of the eyepiece. The focal length and diameter of the eyepiece determine the exit distance of the sight, that is, the shortest distance from the imaging surface to the eyepiece, and the exit distance is generally 5 to 10cm, which is to avoid the scope hitting the orbit because of recoil when shooting. Since the exit distance is fixed, when installing the scope, the appropriate exit distance should be adjusted, once debugged, when aiming according to the gun, the shooter can quickly obtain the maximum field of view, clear image; Parallax is also minimized.