Whatever the P-Mpix is, it isn't simply a report on sharpness/res/acutance.
As I understand, sharpness and acutance as it relates to perceived detail is always a combination of lens resolution and sensor resolution, assuming adequate lighting and shutter speed.
The P-Mpix ratings from DXO appear to basically combine these two variables, lens resolution and sensor resolution, to provide an overall score for general, perceived sharpness, which I assume would apply at any print size (always comparing equal size prints of course).
I don't often bother using prime lenses because of the inconvenience of a fixed focal length, but I was seduced by the glowing reports of the exceptional quality of the Sigma Art 35/1.4, so I decided to get one.
Now, what I find interesting about the P-Mpix ratings for the Sigma 35/1.4, and my most used Nikkor lens, the 24-120/F4, is that a tripling of pixel count (from the 12mp of the D3 to the 36mp of the D800) more than offsets the lower resolution of the 24-120 zoom, compared with the Sigma prime. The rating for the zoom used with the D800 is 12 P-Mpix, and that for the Sigma Art used with the D3, is slightly less at 11 P-Mpix.
What's also interesting is that the rating for the Sigma 35mm when used with the 16mp D7000 is the same as that for the 12mp D3, ie, 11 P-Mpix.
Since I've hardly used my Sigma 35mm so far, this would seem to be an appropriate time to do a bit of testing in preparation for future decision-making as I take photos on my travels. So I took the following shots from the balcony of my hotel room, using the 24-120 zoom with my D800E and the Sigma Art 35/1.4 on the D7100.
Now, DXO haven't tested either of these lenses on the D7100. I'm making a reasonable assumption that the P-Mpix would jump from 11 to around 13 or 14 as a result of the 50% increase in pixel count. I would therefore expect to see a slight, but perhaps barely noticeable improvement from the Sigma on the D7100, compared with the 24-120 zoom on the D800.
The equivalent focal length on the full-frame format is around 50mm, and to achieve an equivalent DoF as well as FoV I used F5.6 at 50mm, and F4 with the Sigma 35mm.
It so happens that Photozone rate these lenses on the Nikon D3X as being sharpest at F4, in the case of the Sigma, and sharpest at F5.6 in the case of the Nikkor zoom at 50mm. This fact, if it applies to my particular lenses, which might be different due to quality control issues, should make my test more legitimate.
I have to say that I'm very surprised at the results, shown in attached 100% crops. Exactly the same sharpening, clarity and vibrance has been applied to both images in ACR, yet I cannot discern any significant or meaningful difference in resolution and detail in any part of the images.
I would have expected the Sigma to at least be sharper at the edges of the cropped frame, yet it isn't. In fact, the zoom at 50mm appears very slightly sharper in the bottom left corner. Perhaps my copy of the Sigma is below standard, or perhaps my copy of the Nikkor 24-120/F4 is above standard. Looks like I'll need to do more testing.
If the color or contrast of these test images doesn't look quite right, it's because the images have been processed on an uncalibrated laptop.
Any explanations as to what's going on here would be welcome.