Question

An 90mm focal-length lens is used to focus an image on the film of a camera. The maximum distance allowed between the lens and the film plane is 130mm .

A. How far ahead of the film should the lens be if the object to be photographed is 2.9

m away?

B. How far ahead of the film should the lens be if the object to be photographed is 1.1

m away?

C. What is the closest object this lens could photograph sharply?

Answer 1

A)

1/f   =   1/v   -   1/u

     v   =   image distance from lens

   u   =   object distance from lens

   u   will be taken -ve following sign convention.

   a.   u1   =  - 2.9 m   =   - 2900   mm

   1/90.0   =   1/v1   +   1/(-2900)

   v1   = 90.0 * 2900 / (2900 + 90)

         =   87.29 mm

B)

u3   =  - 1.1   m   =   - 1100   mm

   1/90.0   =   1/v3   +   1/(- 1100)

   v3   = 90.0 * 1100/ (1100 + 90)

         = 83.19 mm

C)

The image formed will be real as long as the object lies beyond the focus of the lens. once the object is at focus, the image will be virtual and can not be taken on the film.

   Minimum distance over which clear photograph could be taken   = 90.0 mm