Magnification in Normal Focusing (Angular Magnification)

In normal focusing, the object is placed exactly at the lens's focus, so the image forms at infinity, and no ordinary linear magnification can be meaningfully defined (a linear ratio $h'/h$ would itself be infinite). Instead, angular magnification is used: the ratio of the angle $\theta_i$ the image subtends at the eye (aided, through the lens) to the angle $\theta_0$ the object subtends at the unaided eye viewing it directly from the near point $D$. For the unaided eye, $\theta_0\approx h/D$; for the aided eye (image at infinity), $\theta_i\approx h/f$. So $m=\theta_i/\theta_0=(h/f)/(h/D)$, giving $\boxed{m=D/f}$ -- one less than the near-point-focusing magnificatio …