40 mp would be a bad idea in an expensive pro body. especially when you have the best sensor ever tested (DxoMark D800e) available for $26-2800. It's only 4 mp better which is really peanuts at that resolution, and this article right here (written by people smarter than me) has a few interesting points -
Do Sensors
They provide A LOT of info here but it seems 35mm digital does have its limits -
You have all the data at hand, but take the green-yellow light and f/8-f/11 aperture values as a reference. It represents a realistic, not too demanding case. Consider a 35mm system with a lens at f/11. At best, the maximum resolution you will get is equivalent to 16 MP, even if your camera has 22 or 25 MP. In the case of an APS-C based system the limit goes to 7 MP, and 4 MP considering a Four Thirds format. Stopping down to f/22 the limit of the effective resolution of the 35mm based system goes to 4 MP!See again the
Figure 2: the lens limits the resolution of the 5 microns pixel based system with an aperture of f/22, but it is also the case for f/16, f/11 or even f/8. That pixel pitch leads to a 10 MP Four Thirds sensor, a 15 APS-C sensor, a 35 MP sensor of 35mm format and a 70 MP sensor of 36x48mm dimension. Compare now those numbers with the values presented in
Table 3. Only for highly corrected lenses (with better performance at f/5.6 than f/8) do higher sensor resolutions make sense. For instance, you can put 60 million of pixels into a 35mm sensor, but only a diffraction-limited lens at f/5.6 would take advantage of it. The price to pay is in the form of huge files, and comparatively low signal to noise ratios (which translates to noise, narrower dynamic range, poorer tonal variability… see, for instance, the Olympus E-3 reviews at
dpreview.com and at
The Luminous Landscape). The only alternative way for more detail is more capture surface, this is, a larger format, but aberrations are harder to control for larger light circles
