  We study a proper-time renormalisation group, which is based on an operator
cut-off regularisation of the one-loop effective action. The predictive power
of this approach is constrained because the flow is not an exact one. We
compare it to the Exact Renormalisation Group, which is based on a momentum
regulator in the Wilsonian sense. In contrast to the former, the latter
provides an exact flow. To leading order in a derivative expansion, an explicit
map from the exact to the proper-time renormalisation group is established. The
opposite map does not exist in general. We discuss various implications of
these findings, in particular in view of the predictive power of the
proper-time renormalisation group. As an application, we compute critical
exponents for O(N)-symmetric scalar theories at the Wilson-Fisher fixed point
in 3d from both formalisms.
