Question:

How can we change a camera from F/4 to F/5.6?

Increase the aperture to 2 time keeping the focal distance constant

Increase the aperture to √(2) time keeping the focal distance constant

Increase the aperture to 12 time keeping the focal distance constant

Increase the aperture to 1/√(2) time keeping the focal distance constant

Solution:

Correct option is D. Increase the aperture to 1/√(2) time keeping the focal distance constant.
F - number of a camera = f/diameter of aperture
For F/4 camera, F number is 4, so
4 = f/D_1
For F/5.6 camera, F number is 5.6, so
5.6 = f/D_2
⇒D_2/D_1 = 4/5.6 = 1/√(2)
⇒ D_2 = D_1/√(2).
So increase the aperture to 1/√(2) time keeping the focal distance constant.