The distance from the principal point of the lens to the focal position at which the lenses connect the image.
In the case of a single lens, the principal point often comes to the center of the lens as described above. However, normally the CCTV lens is made up of multiple lens.
In that case, it becomes the synthesized main point. (The principal point may be outside the lens.)
If it is a C mount len, the distance from the lens to the focal position is fixed. (This is called flange back, becomes 17.526 mm.)
Therefore, the f value (focal length) is an index showing the field of view of the lens.
Even with lenses of the same shape, the principal point is closer to the image pickup device if the f value is small.
In other words, the field of view gets wider. (Blue line in the figure below)
If the f value is large, the principal point becomes farther from the image sensor. The field of view gets narrower. (Red line in the figure below)
