The use of chemodynamical simulations of disc galaxies calibrated in B and K bands allows us to compare the location of Ultra Harmonic Resonance (UHR) and corotation (CR) with photometrically estimated bar lengths. We apply several observational criteria to determine bar lengths. We show that the minimum of ellipticity located just outside the bulk of the bar is correlated with CR whereas the location of UHR can be approximated using the end of the position-angle plateau of bar isophotes. We give evidence that the classification of slow/fast bars, based on the ratio ${\cal R} = RCR/Rbar could increase from 1 (fast bar) to 1.4 (slow bar) just by a change of method.