< \mu_B < 1.23$. In Fig. (\[histo\]) we compare this classification with the actual mass ratio of bulge and disk, $M_{bulge}/M_{disk}$. With this classification, we find that the Sc types are well represented by galaxies with small bulges (less than $\sim 20 \% $ of the disk mass). On the other hand, Sb types show a wide variety of bulge-to-disk ratios (peaking between 0.2 and 0.7), and Sa galaxies are rare below 1, where they show a very flat distribution. Both Sb and Sa types leak into the S0 and elliptical regimes, defined in the model for values $M_{bulge}/M_{disk}>1$. It is not difficult to imagine that such a scenario is present in observed galaxies too, especially if a band-pass is used where the luminosity evolves rapidly with the age of the stellar populations, and is moreover subject to dust extinction, like the B band. The photometric definition of Sb and Sa model galaxies select objects that do not belong to a uniform population in terms of physical parameters and formation history.

In general photometric classifications may rely on more than one band, and the accuracy of the classification increases with the number of bands available. However, the photometric classification relies 