The focal length of a thin lens is 40 mm and its aperture diameter is 10 mm.What is the f-number of this lens?
A) f/4.0
B) f/2.0
C) f/5.6
D) f/0.30
E) f/0.40

Question

Quizplus · Accepted Answer

The f-number (N) of a lens is defined as the ratio of the focal length (f) to the diameter of the aperture (D), so N = f/D. Given a focal length of 40 mm and an aperture diameter of 10 mm, the f-number is 40 mm / 10 mm = 4.0, which is represented as f/4.0.

The Focal Length of a Thin Lens Is 40 Mm