Two particle image velocimetry (PIV) softwares applied to turbulent flows are compared. One is based on a standard crosscorrelation (CC) algorithm and the other is based on an iterative multi-pyramid optical flow (OF) algorithm. First, still particle images are used to evaluate the cutoff frequency induced by each method. Then a step response analysis highlights the capabilities of each method to minimise the effect of unresolved velocity gradients. Two different benchmarks with various turbulent length-scales, down to the Taylor microscale, are then used to analyse the velocity spectra and the turbulent kinetic energy dissipation estimation. First, a synthetic PIV dataset of homogeneous isotropic turbulence is processed and compared with direct numerical simulation (DNS) results. Then a grid turbulence wind tunnel experimental dataset is used to calculate velocity spectra and second-order structure functions, which are compared to laser Doppler velocimetry spectra. All these results point to the fact that, although OF is more diffusive and up to 5% less accurate than cross-correlation, the numerical diffusion improves the calculation of sub-window unresolved gradients and allows for direct and more robust measurement of the onset of the viscous subrange in experimental turbulent flows.