The foot of the perpendicular on the picture plane through the optical centre of the camera lens, is known as A. Isocenter B. Principal point C. Perspective centre D. Plumb line

The correct answer is B. Principal point.

The principal point is the foot of the perpendicular from the optical center of the camera lens to the image plane. It is a fixed point on the camera, and it does not change with the focus or zoom settings. The principal point is used to calculate the perspective distortion of an image.

The isocenter is the point where the three principal axes of a camera intersect. It is a virtual point, and it does not exist in the real world. The isocenter is used to calculate the perspective distortion of an image, but it is not as

The perspective center is the point where the optical axis of the camera intersects the image plane. It is a virtual point, and it does not exist in the real world. The perspective center is used to calculate the perspective distortion of an image, but it is not as important as the principal point.

The plumb line is a line that is perpendicular to the ground. It is used to measure the angle of elevation or depression of an object. The plumb line is not related to the principal point, and it is not used to calculate the perspective distortion of an image.