A camera has a lens of focal length 70 mm and a speed of f/2.0.What is the diameter of the lens?
A)17.5 mm
B)35 mm
C)70 mm
D)140 mm

Question

Quizplus · Accepted Answer

The diameter of the lens can be calculated using the formula: 
Diameter = Focal Length / f-number 
where f-number is the ratio of the focal length to the aperture diameter. 
Given that the focal length is 70 mm and the speed is f/2.0, the f-number can be calculated as: 
f-number = focal length / aperture diameter = 70 / (aperture diameter/2) = 140/aperture diameter 
Therefore, aperture diameter = 140 / f-number = 140 / 2.0 = 70 mm 
So, the diameter of the lens is 70 mm. Choice C is therefore incorrect. 
To obtain the correct answer, we have to recall that the f-number is the ratio of the focal length to the aperture diameter. A lower f-number means a larger aperture diameter, which in turn means more light can enter the lens. Since the lens in this question has an f-number of f/2.0, it has a relatively large aperture diameter. This means that the correct answer should be smaller than the focal length of 70 mm. The only choice that satisfies this condition is B, 35 mm.

A Camera Has a Lens of Focal Length 70 Mm