Typically, it's not using photons but the other way round, paths are started from the camera primarily. The noise stems from the simple fact that those rendere

Typically, it's not using photons but the other way round, paths are started from the camera primarily. The noise stems from the simple fact that those renderers try to solve an infinte dimensional integral (all light reflected by all surfaces) with stochastic Monte Carlo methods, the most popular being path tracing. There are other methods (finite element radiosity for example) to simulate light transport that do not exhibit stochastic noise, but those have fallen out of favor.