Another metric, the Perceptual MegaPixel (P-MPix), defined as 'the unit of a sharpness measurement', is used to rate the resolution a camera produces when paired to a particular lens.[11] DxOMark claims that P-MPix is a more accurate and relevant value for photographers to consider than alternate measures of sharpness when evaluating camera and lens image quality.[11][12][13][14][15][unreliable source] As of December 2015, the Canon EF 300mm f/2.8L IS II USM lens mounted on a Canon EOS 5DS R has the highest measured P-MPix (45 P-MPix), followed by the Carl Zeiss APO Sonnar T* 2/135 ZE (41 P-MPix on Canon EOS 5DS R and 36 P-MPix on Nikon D800E).[16]