Can you elaborate, please? I mean, if you were to take a hypothetical 25.6 MP FF sensor and use scissors to trim it by a factor of 1.6 in each dimension, you'd end up with a 10 MP APS-C sensor. Using the same lens, there should be no difference how it resolves before and after trimming, right?
P-Mpix isn't a measure of resolution, although resolution is a contributing factor. Resolution ≠ sharpness. A true measure of spatial resolution involves a physical distance. Usual units are line pairs / mm (LP/mm). For a spatially normalized measure in LP/mm, the higher density sensor will outresolve the lower density sensor.
However, that's a per-unit basis - and that's not how we look at images. MTF50
, a commonly used measure of sharpness, is reported in line pairs / picture height (LP/PH). In that case, the greater 'height' of a FF sensor means higher values. You can see that on photozone.de - when you compare a lens on FF vs. APS-C, the MTF50
values will be higher for the 5DII tests than the 50D tests, despite the higher pixel density of the 50D. This isn't just a numerical phenomenon - take a look at the TDP comparison of two 18 MP sensors
, the 1D X vs. the 7D (same lens, the 200/2L IS at f/4). The 1D X is producing a noticeably sharper image.
P-Mpix isn't exactly measuring sharpness, either. It's basically a measure derived from subjective quality factor (SQF), which simply put is an MTF measurement that's adjusted to match human perception (the psychophysical basis is that humans percieve some spatial frequencies better than others, and viewing distance is relevant to perception of sharpness, too).
That difference you see in TDP's ISO 12233 crops can be measured by SQF, and that's basically what P-Mpix is telling you. For example, the Canon 300mm f/2.8L IS II that delivers 22 P-Mpix on the 5DIII achieves only 14 P-Mpix on the 7D. In fact, the 300mm f/4L IS at $1400 delivers better perceived sharpness on the 5DIII than the $7000 supertele lens on the 7D.