How to choose photo lens?

Focal length, in short, is the distance from the center of light to the focal point where light is concentrated when parallel rays pass through a lens. It has a crucial impact on the monitoring range and clarity of the camera. So, in practical applications, how should we choose the appropriate focal length based on different monitoring needs?
The length of the lens focal length has a significant impact on the captured image, determining the size of the image, the size of the field of view, the depth of field, and the perspective strength of the image. Among them, the field of view is the angle formed by the lens as the vertex and the object image of the measured target within the maximum range that the lens can capture. Depth of field refers to the range of distance measured in front and behind the subject within the clear imaging range of the camera lens.
What effects will lenses with different focal lengths produce, and what are the differences between these effects?
▶  As the focal length decreases, the visible distance gradually narrows, and the field of view angle increases accordingly.
▶  On the contrary, as the focal length increases, the visible distance becomes farther and the field of view angle decreases accordingly.
When choosing the focal length of a camera, appropriate choices should be made based on different usage scenarios. For situations with large field of view and relatively small range of object distance variation, such as corridors, halls, etc., it is recommended to use 2.8mm or 4mm lenses.
▶  In situations where the field of view is large and the range of object distance changes is relatively large, such as outdoor squares, parking lots, etc., it is recommended to choose a lens of 6-12mm.