An 81 mm focal-length lens is used to focus an image on the film of a camera. Th

Question

An 81 mm 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 140 mm.
a. How far ahead of the film should the lens be if the object to be photographed is 10.0m away?
b.How far ahead of the film should the lens be if the object to be photographed is 2.8 m away?
c.How far ahead of the film should the lens be if the object to be photographed is 1.2 m away?
d.What is the closest object this lens could photograph sharply?

Explanation / Answer

Formula used is : 1/v - 1/u = 1/f ---->> v = uf / (u+f)

a) u = -10m ; f = 0.081 m

v = -10*0.081 / (-9.919) = 81.66 mm

b) u = -2.8 m

v = -2.8*0.081 / (-2.8 + 0.081) = 83.413 mm

c) u = -1.2 m

v = -1.2*0.081 / (-1.2+0.081) = 86.86 mm

d) v = 140 mm = 0.14 m

u = vf/ (f-v) = 0.14*0.081 / (0.081+0.14) = 51.31 mm

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