A camera lens has a fixed focal length of 35 mm. How far must the lens be moved, relative to the image sensor, in order to change the focus from an object that is 20 m away to one that is 3 m away?
A) 0.34 mm
B) 0.41 mm
C) 0.35 mm
D) 0.47 mm

Question

Quizplus · Accepted Answer

The focal length of the lens is 35 mm. Using the thin lens formula (1/f = 1/o + 1/i), where f is the focal length, o is the object distance, and i is the image distance, we can calculate the initial image distance as follows:
1/35 = 1/20 + 1/i
i = 28 mm
Similarly, for the final object distance of 3 m, we can calculate the final image distance as:
1/35 = 1/3 + 1/i'
i' = 34.29 mm
The difference in image distance between the initial and final positions is:
Δi = i' - i = 6.29 mm
Since the change in position is relative to the image sensor, and assuming the sensor is at the focal plane of the lens, the lens needs to move by a distance equal to Δi to focus from the initial to final positions. Therefore, the answer is 0.35 mm, which is closest to choice C.

A Camera Lens Has a Fixed Focal Length of 35