Optical rangefinder, using observation mirrors at both ends to measure the visual angle of the target, so as to calculate the distance and azimuth. Theoretically, the larger the size, the longer it is, the farther it can be measured and the higher the accuracy, for example, the one on the mast of a battleship is very large.

The principle of optical rangefinder is to use trigonometric function to find the distance of the target.

Give the simplest "monocular combined" rangefinder. Monocular means that there is only one eyepiece, and the observer observes with one eye; However, the optical system is divided into two objective lenses (at both ends of the instrument), and the left and right optical paths image on the upper and lower sides of the eyepiece respectively. That is to say, from the eyepiece, the upper half sees the image of the right objective lens, while the lower half sees the image of the left objective lens.

When observing the target through the range finder, the left and right sight lines will form an included angle. The included angle will be different with different distances. As long as the included angle is known, the target distance can be calculated by trigonometric function.

Because there is an included angle between the left and right sight lines, the vision of the upper and lower halves is different when viewed from the eyepiece; Then we have to adjust the angle of the objective lens to make the upper and lower images overlap, which means that the two lines of sight intersect exactly at the target position. As long as we know the angle between the left and right objective lenses at this time, we can calculate the distance. This is the combined rangefinder.