A camera with interchangeable lenses is used to photograph a

A camera with interchangeable lenses is used to photograph a distant object. The angular size of the object is 14 degrees. What is the longest focal length lens that can be used on the camera in order for the image of the object to fit within the width of the film frame, 36 mm?

Solution

w h\'/h = q/p, so h/p = .331.55 = 36/q

q = 108.5mm

1/f = 1/p + 1/q

1/f = 1/infinity + 1/q (1/infinity is zero)

1/f = 1/q

f = q

f = 108.5 mm

or 90 mm