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627.


Problem 44.40 (RHK)


The focal length of a small camera is 50 mm and the focusing range extends from 1.2 m out to infinity. We have to find the range of movement necessary between lens and film.


Solution:           Click For PDF Version

When light from infinity is focussed on the film, the separation between the lens and the film is the focal length f, which is 50 mm. For focussing an object at a distance of 1.2 m from the lens, let the distance between the lens and the film be . From the thin lens formula, we have

Therefore, the range of movement necessary between lens and film for focussing range from 1.2 m to infinity by a camera of focal length 50 mm will be 52.2 mm to 50 mm.