An 108-mm-focal-length lens is used to focus an image on the sensor of a camera.

Question

An 108-mm-focal-length lens is used to focus an image on the sensor of a camera. The maximum distance allowed between the lens and the sensor plane is 138mm.

A) How far in front of the sensor should the lens (assumed thin) be if the object to be photographed is 3.0 m away from the lens?

B) How far in front of the sensor should the lens (assumed thin) be if the object to be photographed is 1.0 m away from the lens?

C) What is the closest object this lens could photograph sharply? Follow the sign conventions.

Explanation / Answer

Part A)

Apply 1/f = 1/p + 1/q

1/10.8 = 1/300 + 1/q

q = 11.2 cm (112 mm)

Part B)

1/10.8 = 1/100 + 1/q

q = 12.1 cm (121 mm)

Part C)

1/10.8 = 1/p + 1/13.8

p = 49.7 cm (497 mm)

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