An 80.0 mm focal length lens is used to focus an image on the film of a camera.

Question

An 80.0 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 112 mm.

a) How far ahead of the film should the lens be if the object to be photographed is 16.8 m away?

b) How far ahead of the film should the lens be if the object to be photographed is 3.00 m away?

c) How far ahead of the film should the lens be if the object to be photographed is 0.520 m away?

d) What is the closest object this lens could photograph sharply?

Explanation / Answer

Lens equation is (1/u)+(1/v) = (1/f)

measure all lengths in mm.
(a)
f=80mm
u=1680mm

(1/v) = (1/f)-(1/u) = 0.01381 so v = 84mm

(b) (1/v) = (1/f)-(1/u) = 0.01357 so v = 82.19mm

(c) (1/v) = (1/f)-(1/u) = 0.01306 so v = 94.54mm

(d) located at the principal focus, so 82.19mm

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