When an object 1.6 m (160 cm) away is being photographed, how far from the film should the lens be placed?

Question

A camera is equipped with a lens with a focal length of 22 cm. When an object 1.6 m (160 cm) away is being photographed, how far from the film should the lens be placed?

Answer 1

Use the thin lens equation, 1/f = 1/p + 1/q where f is the focal length, p is the object distance, and q is the image distance. Rearranging, 1/q = 1/f - 1/p = 1/22 - 1/160 = 3.92 x 10^-2. Taking the reciprocal gives 25.5 cm for the image distance (the distance from the lens that the film should be placed to be in focus).