Chapter 25, Problem 3P

To determine

The diameter range of the lens diaphragm.

Answer to Problem 3P

Solution:

The diameter range of the lens diaphragm is 3mm to 46mm.

Explanation of Solution

$f-\text{number}$, defined as the ratio of focal length (f)of the lens and diameter (D)of the lens opening, i.e. $f-number=\frac{f}{D}$

By knowing focal length and $f-\text{number}$, the diameter(D)can be calculated.

Given:

Focal length $f=\text{ 65mm},f-\text{number }=\text{ 1}.\text{4 }–\text{ 22}$

Calculation:

$f-number=\frac{f}{D}$

$\therefore D=\frac{f}{f-number}$

$\therefore D=\frac{65}{1.4}=46.4\simeq 46mm$

And $D=\frac{65}{22}=2.95mm\simeq 3mm$