An older camera has a lens with a focal length of 50 mm and

An older camera has a lens with a focal length of 50 mm and uses 36 mm wide film to record its images. Using this camera, a photographer takes a picture of the Golden Gate Bridge that completely spans the width of the film Now he wants to take a picture of the bridge using his digital camera with its 12 mm wide CCD detector. What focal length should this camera\'s lens have for the image of the bridge to cover the entire detector?

Solution

Assuming f-number to be constant:

f/D is same for both, we have:

50/36 = f/12

or f = 16.66mm