Fechner’s logarithmic law and Weber’s ratio law are one and the same.

Evidence

Suppose we could just barely tell the difference between 10 and 11 units of light intensity, a 10% difference and an absolute difference of one unit. Weber’s prediction would be that we could also just barely tell the difference between 1 million and 1,100,000 units, a difference again of 10%, but an absolute difference of 100,000 units. In fact, Fechner called his law Weber’s law, and he was correct, because Fechner’s S = k log R implies ΔS = kΔR/R (for small changes Δ; this is proved in calculus, but can be easily checked on a calculator). So Fechner’s law is really Weber’s law, since to keep ΔS fixed at the threshold for perception, then ΔR must rise and fall as R. — Eric J. Heller, Why You Hear What You Hear- An…, 2001

Categorization

• XREF: Links to the decibel scale in acoustics and to Fechner–Weber’s law in psychophysics textbooks.