  Multimode fibers (MMF) are used in many applications from telecomunications
to minimally invasive micro-endoscopic imaging. However, the numerous modes and
their coupling make light-beam control and imaging a delicate task. To
circumvent this difficulty, recent methods exploit priors about the
transmission of the system, such as the so-called optical memory effect. Here,
we quantitatively characterize a chromato-axial memory effect in step-index
MMF, characterized through its slope $\delta z/\delta \lambda$ and its spectral
and axial widths. We propose a theoretical model and numerical simulations in
good agreement with experimental observations.
