Why is it necessary to adjust the optical elements to the same height in the experiment of measuring the focal length of convex lens?

The basis of this experiment is based on geometric optics. If the center of the element is not adjusted to be coaxial, the measured image will have a big error with the real focal length on the plane deviating from the optical axis.

Because when we take pictures, the distance between the object and the camera (lens) is not always the same, such as taking pictures of people. Sometimes we have to shoot the whole body, we will be far away, and we will be very close when we take a bust.

Extended data:

Focal length in photography:

The lens of a camera is a set of lenses. When the light parallel to the main optical axis passes through the lens, the light converges to a point called the focal point, and the distance from the focal point to the center of the lens (that is, the optical center) is called the focal length. A lens with a fixed focal length, that is, a fixed focus lens; The lens whose focal length can be adjusted and changed is a zoom lens.

(When a beam of light parallel to the main axis of the convex lens passes through the convex lens, it will converge to a point on the other side of the convex lens, which is called the focal point, and the distance from the focal point to the optical center of the convex lens is called the focal length of the convex lens. There is a focal point on each side of the convex lens. )

Baidu encyclopedia-focal length