The importance of an acceptance angle in the plasma diagnostics with ion energy analyzers is investigated by means of numerical simulations. It is shown that wide acceptance angles result in low energy tails in measured ion distribution functions (IDFx). For flowing plasmas or plasmas with beams, the orientation of the analyzer’s orifice gives different results due to bending of ion trajectories in the vicinity of the analyzer. It is demonstrated that the maximum in the IDFx is at energies lower than the plasma potential. Simulations are done with DIP3D, a three-dimensional particle-in-cell code.