Linear Response
The property of a digital sensor that its output is directly proportional to the number of photons received.
Formula / rule
Signal ∝ photons
In depth
Linearity has a profound consequence for exposure: because each stop doubles the light, the brightest stop of a fourteen-stop capture contains half of all recorded levels and the darkest contains a tiny fraction. Film, by contrast, has a rolling shoulder that compresses highlights gracefully, which is why digital highlight clipping looks so abrupt.
Examples in practice
In a 14-bit file, the top stop occupies 8,192 levels while the sixth stop down has only 256.