A lens for a 35mm camera has a focal length given by f 71 m

A lens for a 35-mm camera has a focal length given by f = 71 mm.

(a) How close to the film should the lens be placed to form a sharp image of an object that is 7.1 m away?

(b) What is the magnification of the image on the film?

I know part a is 71.717 , but I need help with part B?

Solution

a)

From thin lens formula

1/f = 1/d_i + 1/d_o

1/0.071= 1/d_i +1/7.1

d_i = 0.071717 m =71.717 mm

b)

Magnification

M=-d_i/d_o = -0.071717/7.1

M=-0.010101