Crystal ball imaging principle

The answer to your question is as follows:

A crystal ball can be regarded as a spherical lens, a kind of convex lens, just like the lens of a camera. The image on the negative is inverted, so the imaging principle of a crystal ball is equivalent to a convex lens.

Middle school students have learned the imaging principle of convex lens;

When the object distance (u) is more than twice the focal length (f), the image is inverted and reduced.

The focal length (f) of a spherical lens is the distance from the center (0) to the focal point (f).

The following figure is the optical path diagram of light parallel to the main optical axis but not passing through the main optical axis when it enters the spherical lens:

Principle diagram of convex lens imaging;

In the process of photography, the object distance is much larger than twice the focal length, so the inverted and reduced real image of the object is captured through the crystal ball.

I hope my answer is helpful to you.