Suiter's definition of EER (SR) is in the spatial domain in terms of energy in the Airy disc.
SR may also be described in the frequency domain as "The amplitude of the 0 frequency term in the MTF"
The use of SR is always accompanied with a caution on its use - the optical system must be Aplanatic.
BTW the use of corrector plates can introduce an insidious chromatic aberration due to oblique (off axis) rays and the inherent spherical aberration, this aberration goes under the grand name of "Oblique spherro-chromatic Aberration"
Cheers,
Jerry.
