Context :

Aims : 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 .

Methods : 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 .

Results : M68 had 45 previously confirmed variables , including 42 RRL and 2 SX Phoenicis ( SX Phe ) stars .
In this paper we determine new periods , 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 classes of variables , we derived values for the metallicity of the cluster of [ Fe/H ] = -2.07 \pm 0.06 on the ZW scale , or -2.20 \pm 0.10 on the UVES scale , and found true distance moduli \mu _ { 0 } = 15.00 \pm 0.11 mag ( using RR0 stars ) , 15.00 \pm 0.05 mag ( using RR1 stars ) , 14.97 \pm 0.11 mag ( using SX Phe stars ) , and 15.00 \pm 0.07 mag ( using the M _ { V } - [ Fe/H ] relation for RRL stars ) , corresponding to physical distances of 10.00 \pm 0.49 , 9.99 \pm 0.21 , 9.84 \pm 0.50 , and 10.00 \pm 0.30 kpc , respectively .
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 .

Conclusions :