Signal-to-noise Ratio
The ratio between the useful signal recorded and the noise accompanying it, which determines apparent image cleanliness.
Formula / rule
SNR = signal / noise
Measured in
dB or ratio
In depth
Signal-to-noise ratio is the single figure that best predicts how an image will look, and it improves with the square root of the light collected — so gathering four times the light halves the visible noise. This is why exposing generously, using a larger sensor, or averaging multiple frames all produce the same kind of improvement.
Examples in practice
Stacking four identical frames doubles the signal-to-noise ratio, equivalent to a one-stop improvement.