The use of chemodynamical simulations of disc galaxies calibrated in Band K bands allows us to compare the location of Ultra HarmonicResonance (UHR) and corotation (CR) with photometrically estimated barlengths. We apply several observational criteria to determine barlengths. We show that the minimum of ellipticity located just outsidethe bulk of the bar is correlated with CR whereas the location of UHRcan be approximated using the end of the position-angle plateau of barisophotes. We give evidence that the classification of slow/fastbars, based on the ratio ${\cal R} = RCR/Rbarcould increase from 1 (fast bar) to 1.4 (slow bar) just by a change ofmethod.