Chapter 23, Problem 21Q

To determine

That the camera lens need to move farther away from or closer to the camera film or sensor if a photographer moves closer to his subject and then refocuses

Answer to Problem 21Q

Solution:

The camera lens moves farther from the film when the photographer refocuses.

By moving closer to the object, the object distance has become smaller, which the thin lens equation says increases the image distance (and the image is now behind the film). To bring the image forward (and back to the film), you need to move the lens away from the film.

Explanation of Solution

A camera using a thin converging lens, the ray diagram of the image forming is given by the figure.

The Lens equation of the image formed by the lens is given by

$\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}$

Where,

$f\text{ is the focal length of the lens}$

${\text{d}}_o\text{ is the object distance}$

${\text{d}}_i\text{ is the image distance}$

The lens formula can be used to analyze the given scenario. While refocusing the camera, the camera is brought closer to the object. Hence, from the lens equation, the object distance is reduced.

If the object distance is reduced, the image distance is increased and so the image falls beyond the film position. As the film is immovable, the lens has been moved away the film and thereby increasing the image distance which will satisfy the above equation and producing the image on the film.