The brightness distribution for the plasma tail of the comet C/2009 R1 (McNaught) is determined on the basis of photometric observations made with a small Newtonian reflector (200/1000) from 9 to 12 June 2010. Images of the comet were detected using the Canon CMOS APS-C camera with short exposures. Brightness distribution is simulated and diffusion model parameters for the plasma tail of the comet are calculated. The induction of magnetic field in the comet’s plasma tail is estimated. Estimates for light particle lifetimes and for coefficients of lengthwise and transverse diffusion of ions are calculated.