The focal length of your Nikon camera lens is 70 mm.It is used for taking pictures of your friend who is 1.5 m tall.What is your friend's distance from the lens if the image just fills the 24-mm vertical dimension of the film?
A)4.45 m
B)5.45 m
C)6.92 m
D)9.1 m

Question

Quizplus · Accepted Answer

Using the lens formula and the magnification concept, the height of the object (friend's height) divided by the height of the image (film's vertical dimension) equals the object distance (friend's distance from the lens) divided by the focal length. So, $1.5\,m / 0.024\,m = d / 0.07\,m$, solving for $d$ gives approximately $4.45\,m$.

The Focal Length of Your Nikon Camera Lens Is 70