Constraints on the rms lensing deflection angle ⟨ d² ⟩ ^{1 / 2} from Planck TT+lowP CMB power spectra in the base ΛCDM model (samples, colour coded by the value of the Hubble constant), showing that the inferred lens deflection amplitude is strongly correlated with the parameter combination . The solid black contour is the constraint from CMB lensing only, which prefers lower values of the lensing amplitude, giving the joint constraint shown by the red contours. Since the lensing reconstruction mainly constrains scales smaller than the peak of the deflection angle power spectrum, there is an additional degeneracy that makes ⟨ d² ⟩ ^{1 / 2} slightly anti-correlated with the measurement of lensing power on smaller scales, which is approximately proportional to (the square of)