An older camera has a lens with a focal length of 50 mm and uses 34-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?
I can't even figure out what equations to use for this. I didn't know width of the detector even played a role? Can someone explain how width affects any of this?