A camera with a focal length of 0.0500 m (a 50-mm lens)is focused for an object at infinity.To focus the camera on a subject which is 4.00 m away,how should the lens be moved?
A)1.0 cm closer to the film
B)0.06 cm closer to the film
C)4.94 cm closer to the film
D)0.06 cm farther from the film
E)4.94 cm farther from the film

Question

Quizplus · Accepted Answer

To focus on an object closer than infinity, the lens must be moved farther from the film. The lens equation is 1/f = 1/do + 1/di, where f is the focal length, do is the object distance, and di is the image distance. When the object is at infinity, di = f. For an object at 4.00 m, rearranging the lens equation gives di = (f*do) / (do - f). Substituting f = 0.0500 m and do = 4.00 m gives di = 0.05025 m. The difference between di and f is 0.00025 m or 0.06 cm, indicating the lens must move 0.06 cm farther from the film to focus on the object 4.00 m away.

A Camera with a Focal Length of 0