Question

What is the speed (f-number) of a lens with a diameter of $5.2\mathrm{cm}$and a focal length of $15.6\mathrm{cm}$?

Short answer

The speed of lens is$3$.

Step-by-step solution

Given information

Diameter$,d=5.2\mathrm{cm}$

Focal length $,f=15.6\mathrm{cm}$

Derivation

As the formula is

$$\begin{gathered} F=\frac{f}{d} \\ F=\frac{15.6\mathrm{cm}}{5.2\mathrm{cm}} \\ F=3 \end{gathered}$$