How to calculate focal length?

Question

Lenses

There is a camera raspberry pi hq, with a fixed lens.
In order to focus, you need to "twist" or "unscrew" such a lens from the camera. I understand that by doing this, I am changing the distance from the last lens of the objective to the sensor.
How (what formulas) to calculate the distance by which you need to "twist" the lens so that the focus is on an object of 5 meters?

Answer 1

For example , formulas 116.8 and 116.10.
You know the focal length, but you don't know the position of the principal planes of the lens. Therefore, it is necessary to determine them experimentally (more precisely, to exclude them from the formulas). For example, you focus the lens to infinity, while the rear main point will be at a distance F from the matrix - mark the corresponding position of the lens (it doesn’t matter what exactly, it can be the back surface of the lens, you can just some kind of mark on the lens). To focus on L = 5m, you need to move the lens further from the matrix by a distance d, while the distance from the matrix to the main point will become F + d, from where (neglecting the position of the front main point compared to L):

-1/L - 1/(F+d) = -1/F
d = F^2/(L-F)