Question

A line object 5 mm long is located 50 cm in front of a camera lens. The image is focused on the film plate and is 1 mm long. If the film plate is moved back 1 cm the width of the image blurs to 1 mm wide. What is the F-number of the lens?

• A. 7.33
• B. 8.33
• C. 10
• D. 12

Answer 1

Answer: (B)

8.33

Answer 2

Substituting u = 50 cm and =$\frac { 1 } { 5 }$ in the Gaussian lens formula

+=

Fig.

gives f = 8.33 cm, v = 10 cm. From the similar triangles in Fig. we have

=$\frac { 0.1 } { 1 }$,

or D = 0.1 v = 1 cm. Therefore, F = f/D = 8.33.