  The cyanide radical CN is abundant in protoplanetary disks, with line fluxes
often comparable to those of $^{13}$CO. It is known to be sensitive to UV
irradiation of the upper disk atmosphere, with models predicting ring-shaped
emission. We seek to characterize the CN emission from 94 Class II disks in the
Lupus star forming region, compare it to observations in other regions, and
interpret our observations with a grid of models. The CN emission morphology is
discussed for two primordial disks, Sz 71 and Sz 98, and modeled in more
detail. ALMA observed CN N=3-2 in Lupus disks down to sensitivities better than
previous surveys. Models constructed with the physico-chemical code DALI are
used to study the integrated fluxes of the disks and resolved emission of CN in
disks without (dust) substructures. We find that CN N=3-2 is bright, and
detected in 38% of sources, but its disk-integrated flux is not strongly
correlated to either $^{13}$CO or continuum flux. Compared to pre-ALMA
single-dish surveys, no significant difference in the CN flux distributions in
Lupus and Taurus-Auriga is found, although $\rho$ Ophiuchus disks may be
fainter on average. We find ring-shaped CN emission with peak radii of ~50 AU
in two resolved disks. We conclude that a large fraction of sources is faint in
CN; only exponential gas surface density cutoffs at $R_{c} \leq 15$ AU can
reconcile observations with models. This is the first observational evidence of
such a compact gas disk population in Lupus. Absolute intensities and the
emission morphology of CN are reproduced by DALI models without the need for
any continuum substructure; they are unrelated to the CO snowline location.
These observations and the successful modeling of these rings provide a new
probe of the structure and conditions in disks, and particularly their incident
UV radiation field, if disk size is determined by the data.
