It is common to see optical components like camera lenses or telescopes specifie

Question

It is common to see optical components like camera lenses or telescopes specified in terms of an aperture and an f ratio or f stop . For example, a lens for a 35mm camera could be described as 50 mm, f/2.2. This notation means the diameter of the lens is 50 mm, and that the focal length of the lens is 2.2 times this diameter. Small f-ratios are called "fast", while larger f-ratios are "slow"; faster lenses are harder to design and build. What is the focal length the lens specified above? What is the focal length and radius of curvature of a telescope mirror described as 3.0m, f/5.0?

Explanation / Answer

50 mm, f/2.2 means diameter = 50 mm and f= 2.2*50 = 110 mm

so

3.0m, f/5.0 = diameter = 3.0 m and f= 5*3 = 15 m

and radius of curvature = 2*f = 2*15 = 30 m

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