Question

A simple camera with a converging lens of 60 mm focal length is focused on very far objects. To focus the camera on a nearby object 1.5 m away, the distance between the film and lens will have to be

• A. decreased by 2.5 mm
• B. increased by 2.5 mm
• C. kept fixed as before. but aperture increased by a factor of 2.5
• D. kept fixed as before, hut aperture decreased by a factor of 2.5

Answer 1

The correct option is B increased by 2.5 mm

Answer is B.
When the camera is focused at infinity, the image is formed when $q=f=60mm=0.06m$.
When focused at $1.5m$ image is formed is calculated as follows.
$q=\frac{fp}{p-f}=\frac{0.06m\times 1.5m}{1.5m-0.06m}=\frac{0.09m}{1.44m}=0.0625m$.
Therefore, when the camera is focused at $1.5m$ image is formed when $q=0.0625m=62.5mm.$
Hence, the distance between the film and lens will have to be 62.5 mm. That is, it is increased by 2.5 mm.