An 80-mm-focal-length lens is used to focus an image on the film of a camera. The maximum

Question:

An 80-mm-focal-length lens is used to focus an image on the film of a camera. The maximum distance allowed between the lens and the film plane is 120 mm.
(a) How far ahead of the film should the lens be if the object to be photographed is 10.0m away?
(b) 3.0 m away?
(c) 1.0m away?
(d) What is the closest object this lens could photograph sharply?