  The Hubble Frontier Fields (HFF) are six clusters of galaxies, all showing
indications of recent mergers, which have recently been observed for lensed
images. As such they are the natural laboratories to study the merging history
of galaxy clusters. In this work, we explore the 2D power spectrum of the mass
distribution $P_{\rm M}(k)$ as a measure of substructure. We compare $P_{\rm
M}(k)$ of these clusters (obtained using strong gravitational lensing) to that
of $\Lambda$CDM simulated clusters of similar mass. To compute lensing $P_{\rm
M}(k)$, we produced free-form lensing mass reconstructions of HFF clusters,
without any light traces mass (LTM) assumption. The inferred power at small
scales tends to be larger if (i)~the cluster is at lower redshift, and/or
(ii)~there are deeper observations and hence more lensed images. In contrast,
lens reconstructions assuming LTM show higher power at small scales even with
fewer lensed images; it appears the small scale power in the LTM
reconstructions is dominated by light information, rather than the lensing
data. The average lensing derived $P_{\rm M}(k)$ shows lower power at small
scales as compared to that of simulated clusters at redshift zero, both
dark-matter only and hydrodynamical. The possible reasons are: (i)~the
available strong lensing data are limited in their effective spatial resolution
on the mass distribution, (ii)~HFF clusters have yet to build the small scale
power they would have at $z\sim 0$, or (iii)~simulations are somehow
overestimating the small scale power.
