Perceptual Megapixels, P-MPix for short, can be described as the “equivalent” number of megapixels when using a particular lens. Just as lenses offer different equivalent focal lengths when paired with different sensor sizes, sensors can have different equivalent megapixels when paired with lenses of various optical qualities.
If you stick a horrible lens onto a high-megapixel camera, your photographs may pop out with a large megapixel “size”, but the sharpness of the photo may be equivalent to the sharpness of a photo taken with a much smaller sensor and a “perfect” lens.
For example, say you’re shooting with a 24-megapixel camera, but are using a lens rated at 18 P-MPix. This means that the resulting photos are equal in sharpness to an 18-megapixel camera shooting with a optically perfect lens.
I browsed DxOMark’s lens ratings with the new scores. Make sure you’re on the Optical Metric Scores tab. Pick your mount type to browse all. Even better – select your camera on the right side and you’ll only see lenses that have been tested with that camera. Good stuff.
Biggest surprise was how well some of the cheap Samyang lenses scored. Some of the Sigma and Tamron lenses did well, too.