  We analyse 20 nights of CCD observations in the V and I bands of the globular
cluster M68 (NGC 4590), using these to detect variable objects. We also
obtained electron-multiplying CCD (EMCCD) observations for this cluster in
order to explore its core with unprecedented spatial resolution from the
ground.
  We reduced our data using difference image analysis, in order to achieve the
best possible photometry in the crowded field of the cluster. In doing so, we
showed that when dealing with identical networked telescopes, a reference image
from any telescope may be used to reduce data from any other telescope, which
facilitates the analysis significantly. We then used our light curves to
estimate the properties of the RR Lyrae (RRL) stars in M68 through Fourier
decomposition and empirical relations. The variable star properties then
allowed us to derive the cluster's metallicity and distance.
  We determine new periods for the variable stars, and search for new
variables, especially in the core of the cluster where our method performs
particularly well. We detect an additional 4 SX Phe stars, and confirm the
variability of another star, bringing the total number of confirmed variable
stars in this cluster to 50. We also used archival data stretching back to 1951
in order to derive period changes for some of the single-mode RRL stars, and
analyse the significant number of double-mode RRL stars in M68. Furthermore, we
find evidence for double-mode pulsation in one of the SX Phe stars in this
cluster. Using the different types of variables, we derived an estimate of the
metallicity, [Fe/H]=$-2.07 \pm 0.06$ on the ZW scale, and 4 independent
estimates of the distance modulus ($\mu_0 \sim$ 15.00 mag) for this cluster.
Thanks to the first use of difference image analysis on time-series
observations of M68, we are now confident that we have a complete census of the
RRL stars in this cluster.
