Question

A camera fitted with a lens of focal length 50 mm is being used to photograph a flower that is 5 cm in diameter. The flower is placed 20 cm in front of the camera lens. 

At what distance from the film should the lens be adjusted to obtain a sharp image of the flower?

Answer 1

Given:
Focal length, f = 50 mm = 5 cm
Object distance, u =-20 cm
Image distance, v = ?
Putting these values in lens formula, we get: 

⇒`1/v-1/u=1/f` 

⇒`1/v-1/-20=1/5` 

⇒`1/v+1/20=1/5` 

⇒`1/v=1/5-1/20` 

⇒`1/v=(20-5)/100` 

⇒`1/v=15/100` 

⇒`v=20/3` 

⇒`v=6.66` cm  

Film should be adjusted at a distance of 6.66 cm behind the lens.