A reading lens is marked +2.50 D. A person adds a weak lens of -1.25 D, held in contact with it, to…

The power is $P = \dfrac{1}{f}$, with $f$ in metre and $P$ in dioptre. Lenses in contact: $P = P_1 + P_2$. Lenses a distance $d$ apart: $P = P_1 + P_2 - dP_1P_2$, with $d$ in metre. The back focal distance is $F\left(1 - \dfrac{d}{f_1}\right)$. For two lenses one after the other, the image of the first is the object of the second, and $m = m_1m_2$.