Spectral density mapping provides direct access to protein dynamics with no assumptions as to the nature of the molecule or its dynamic behaviour. Reduced spectral density mapping characterises a protein's motions at a lower experimental burden, assuming that the spectral density function J(ω) is flat around ωH. This introduces little error for 15N relaxation data but is less valid for 13C studies, perturbing J(ωC) considerably to an extent that depends on the nature of the molecule's motions. We propose the fitting of spectral density at high frequencies to a single Lorentzian and show that the true values of the spectral density lie between those determined by the two approximations.