Question

A lens for a 35-mm camera has a focal length given by f = 40mm .

Part A

How close to the film should the lens be placed to form a sharp image of an object that is 5.7m away?

Express your answer using two significant figures.

di =

mm

Part B

What is the magnification of the image on the film?

Express your answer using two significant figures.

m =

Answer 1

Given a 40mm lens is mounted and the correct exposure is f/5.6 @ 1/250 second, then if a 56mm is mounted, and ISO and light level remain unchanged, the same setting i.e. f/5.6 @ 1/250 second will yield the same results. This is because the focal ratio (f/#) is universal , it takes into account the focal length and the working lens diameter. In other words, f/5.6 delivers the same light energy to the film or digital chip regardless of focal length. The focal ratio (f/#) transcends camera format and focal length that is why we use this system to set our lens aperture.

That being said, your question is: If f/5.6 @ 1/250 second is correct, what will be the equivalent exposure if we change the shutter speed to 1/60 second? Since the difference between 1/250 and 1/60 is 2 f/stops, we stop down 2 f/stops thus the revised exposure is f/11 @ 1/60 second.