  Exceptional points are non-Hermitian degeneracies in open quantum and wave
systems at which not only eigenenergies but also the corresponding eigenstates
coalesce. This is in strong contrast to degeneracies known from conservative
systems, so-called diabolic points, at which only eigenenergies degenerate.
Here we connect these two kinds of degeneracies by introducing the concept of
the distance of a given exceptional point in matrix space to the set of
diabolic points. We prove that this distance determines an upper bound for the
response strength of a non-Hermitian system with this exceptional point. A
small distance therefore implies a weak spectral response to perturbations and
a weak intensity response to excitations. This finding has profound
consequences for physical realizations of exceptional points that rely on
perturbing a diabolic point. Moreover, we exploit this concept to analyze the
limitations of the spectral response strength in passive systems. A number of
optical and photonics systems are investigated to illustrate the theory.
