An 50.0 mm focal length lens is used to focus an image on the film of a camera.

Question

An 50.0 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 122 mm. How far ahead of the film should the lens be if the object to be photographed is 10.5 m away?

How far ahead of the film should the lens be if the object to be photographed is 3.22 m away?

How far ahead of the film should the lens be if the object to be photographed is 0.630 m away?

What is the closest object this lens could photograph sharply?

Explanation / Answer

We know that the
1/u+1/v = 1/f where u is object distance , v is the image distance and f is focal length
so
f= 50 mm
u = 10.5 m
On applying formula
1/u+1/v = 1/f
v = 50.23 mm (hence the film should be 50.23 mm away)
Similarly when u = 3.22 m
v= 50.7886 mm
When u = 0.630 m
then v = 54.31 mm
we know that the maximum distance of image is 122mm
therefore v= 122 mm then u will be
u = 0.0847 m
this will be the closest distance which camera can take sharply.

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