Geometrical optics. Photometry. Quantum nature of light > Optical systems > Problem 13.3.4

Statement

$13.3.4.$ At what distance from the lens is the object located, if the distance between the object and its actual image is minimal? The focal length of the lens is $f$.

Solution

For a thin len,
$\frac{1}{s}+\frac{1}{s'} = \frac{1}{f}$
as the distance between the object and its actual image is minimal, $s = s'$,
$\frac{2}{s} = \frac{1}{f}$
$s = 2f$

Answer

$s = 2f$