  The resonant electric quadrupole amplitude in the transition $\gamma
N\leftrightarrow\Delta(1232)$ is of great interest for the understanding of
baryon structure. Various dynamical models have been developed to extract it
from the corresponding photoproduction multipole of pions on nucleons. It is
shown that once such a model is specified, a whole class of unitarily
equivalent models can be constructed, all of them providing exactly the same
fit to the experimental data. However, they may predict quite different
resonant amplitudes. Therefore, the extraction of the E2/M1($\gamma
N\leftrightarrow\Delta$) ratio (bare or dressed) which is based on a dynamical
model using a largely phenomenological $\pi N$ interaction is not unique.
