The uncertainty of the photometer directional response index f2 is investigated using Monte Carlo simulations and error models derived for the characterization measurements of three photometers. Both random and biased types of errors are used simultaneously to model the measurement noise, drift of the light source, alignment of the photometer, and the errors of the rotary stage. The values of f2 obtained for each azimuth angle of the photometer are sensitive to the asymmetry of the directional response. It is shown by variation of the photometer symmetry that the changes in the value of f2 calculated as an average of all simulated values are small. It is further demonstrated that with typical measurement distances the quality index f2 can be sensitive to the position of the axis of rotation with respect to the receiving plane of the photometer. The simulated f2 values of the three photometers with expanded uncertainties (k = 2) are (4.325 ± 0.035)%, (1.698 ± 0.027)% and (4.315 ± 0.030)%.