In a dark room, lighting a single candle may feel like a drastic change. But in a room already lit by ten candles, adding one more might not feel as significant. Or, while the intensity of sound is determined by the amplitude of the sound wave, doubling the amplitude does not make it feel twice as loud to humans.

This idea can apply to graphic design, for example, typographic scale or making contrast of sizing between visual elements in general.

Compare two type scale examples below. The left increases the font size by 6 points each step, while the right increases the size by the ratio of 1.2x (each step is 20% larger than the previous). You can see the change between each step appear to decrease as the size increases in the left example, while the change feel more consistent in the right example.

$fontsize = b \cdot r^{(step - 1)}$

$\begin{aligned} step = \log_{1.2} \frac{fontsize}{b} + 1\end{aligned}$