Ratio of actual image data to digital noise — higher SNR means cleaner shadows and ISO flexibility. Critical in night scenes: better SNR = easier color grading without artifacts.
You're standing in front of a dimly lit facade at night, the actor is in shadow, and the sun has just set. Your camera has to go up to ISO 3200, maybe even 6400. This is where it's decided whether your image stays clean afterward or if the colorist has to fight a pixel storm in grading — that's the signal-to-noise ratio. It describes how much real image information (the signal) wins against digital errors (the noise). A high SNR means the camera distinguishes between what's actually happening in front of the lens and the electronic garbage the sensor produces in low light.
In practice, you notice this immediately. An Alexa Mini LF with good SNR values still delivers a grainy but controllable image at ISO 1600 — the grain is almost organic because the real signal is strong enough. A weaker camera with poorer SNR will turn into a mush of colored pixels at the same ISO. The problem: The sensor can't distinguish whether a green dot is actual light or just electronic noise. In the DI, this becomes a nightmare — you can't just throw a noise filter over it without destroying the real image.
The SNR depends on several factors. Sensor design plays the main role — larger photosites (pixels), better architecture = higher SNR. The electronics behind it: How clean is the preamplifier? How stable is the power supply? Even poor cable routing can degrade the SNR. On set, you control it through your exposure. If you expose too dark, the sensor forces the amplifier to ramp up — the SNR drops. Exposure is your first weapon against noise.
In practical workflow: Always shoot as bright as possible without clipping. Use available light, incorporate reflectors, give lighting more time. If you have to push it — ND filters are not your enemy, but your friend. A camera with an SNR of 65 dB has significantly less latitude than one with 70+ dB. That's a four-stop difference in practical grading. You only see the difference in the DCP under projector light or on the big screen — and then it's too late to reshoot the takes.