There are two focal lengths at the same time: the front focal length and the back focal length.

The front focal length is the distance from the front main surface to the front focal point, and the back focal length is the distance from the back main surface to the back focal point.

What is the main surface? The main surface is the surface with vertical axis magnification of 1. Those who take part in the front imaging are called the front main surface, and those who take part in the back imaging are called the back main surface.

What is vertical axis magnification?

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Is the landlord a little confused?

Don't worry, you will understand when the landlord learns the course of applied optics.