After the light emitted by an object point is focused through the lens, all the light forms a conical beam. The apex angle of a cone is related to the aperture: the larger the aperture, the greater the apex angle. The apex of the cone contacts the negative to form an image point. If the negative moves forward or backward a little for a fixed distance, the cutting beam forms a circular spot. The size of the circular spot is related to the vertex angle: if the vertex angle is large, the circular spot is also large.

In other words: if the film deviates from the same distance, the aperture is large and the circular spot is large. Now instead of moving the negative, we move the object point so that the vertex of the beam moves. If the spots formed are the same, the object point of the thinner beam (meaning smaller aperture) can move a larger distance, which means that the depth of field of the smaller aperture is larger.

With the increase of object distance and depth of field, the depth of field will decrease with the increase of focal length. Generally speaking, the focal length of the standard lens of 35mm camera is about 28-70mm. Therefore, if the focal length is higher than 70mm, it means that it supports the telescopic effect. If it is lower than 28mm, it means that it has wide-angle shooting ability.