Ask an optician about a lens and you won't hear a focal length — you'll hear a power, in diopters (D): $P = \dfrac{1}{f}$ (read aloud: P equals one over f). A SHORT focal length means a strong lens that bends light hard, so flipping $f$ makes bigger mean stronger — which is exactly how a label should behave. Reading glasses run about +1 to +3 D; a magnifying glass, +4 or more.

The whole trap of this lesson fits in one sentence: the diopter is defined on metres — $f$ must be in metres before the flip. Focal lengths love to arrive in centimetres, and 25 put in raw gives 0.04 D, a hundred times too feeble; convert first (25 cm = 0.25 m) and the honest $P = 4\ \mathrm{D}$ appears. Convert, THEN flip — in that order, every time.