Based on the principle of thin lens imaging, the relationship between focal length and object distance is deduced (1/f =1/u+1/v), which reveals the basic law of lens imaging, that is, when light passes through the lens, there is a certain mathematical relationship among object distance, image distance and focal length, and it can also help people to predict and calculate the position and size of images at different object distances.

Focal length, object distance and image distance are three key parameters in optical imaging. The focal length f is the distance from the lens center to the focus, the object distance u is the distance from the object to the lens center, and the image distance v is the distance from the image to the lens center.