The rms scatter of the photometry of 2523 stars in the time series consisting of 100 images stacked from 500 0.1 s exposures each. The photon noise limit and the excess noise limit has been plotted for comparison. Due to the stochastic amplification in the EM stages, an EMCCD will, when regarded as a conventional linear amplifier, effectively multiply the photon noise by a factor of 2. The robust rms is estimated with the function biweightScale from the Python module astLib.