Hi,
Having smaller pixels would be beneficial, as they make for a cleaner image.
For a better view:
http://echophoto.dnsalias.net/ekr/Articles/Aliasing2/Dollars.jpg
The images here were shot with a P45+ that has 39 MP, with 6.8 micron pixels. The other camera used was a Sony Alpha 77 SLT with an APS-C sensor, with 3.9 micron pixels. The P45+ images were shot with an 80/2.8 Planar CF while the Sony images were shot with a 70-400 zoom at 85 mm. Shooting distance was 4 m.
As you can see, the large pixels of the P45+ don't deliver a clean image at f/5.6 and f/11, stopping down to f/22 results in a clean image, but sharpness is lost.
This is not really about the number of megapixels but about the size of the pixels. Any detail a digital sensor can not resolve it will yield back as lower frequency artefacts. The sharper the lens is the more artefacts. This is simply a fact. It seems that the resulting fake detail is not very offensive, unless it shows up as color moiré.
The detail here is a good demonstration:
http://echophoto.dnsalias.net/ekr/Articles/Aliasing2/Nyquist1_cropped.jpg
Both the small pixel A77 and the large pixel P45+ show aliasing, but the aliasing on the P45+ is offensive.
Why don't we see this so often in practice? Many subjects don't have high contrast, high frequency detail. Also, workflow issues like less than pin point accurate focus, small apertures and camera vibration can all reduce the transferred amplitude at Nyquist.
As a example, Diglloyd (Lloyd Chambers) has noticed that the Leica S had a lot of "christmas tree" artefacts, but stopping down to f/11 reduced that problem pretty effectively, by diffraction.
Now, raw converters are doing heroic efforts to hide these artefacts, but it may be interesting what is really fed to the raw converter:
http://echophoto.dnsalias.net/ekr/Articles/Temp/Capture.PNG
As you can see, in this case the raw image is full of colour artefacts. As I say, raw converters do a heroic effort to reconstruct the image, but it may be better to feed them with a clean image.
In a sense, it would make a lot of sense to go with APS-C if you don't need more than 24 MP. It also makes a lot of sense to go 100 MP on 44x33, if you have good lenses. From aliasing side it will behave as 24 MP on APS-C, but obviously, you could print much larger, like printing A0 instead of A2.
Best regards
Erik
While I have yet to feel constrained by the mere 37.5MP of the Leica S, I do look in awe at my old 60MP files from a borrowed IQ160. Would I print larger than 30x40 if I had the pixels? Maybe 1 print in 10 years. I have regular access to a 44" printer and I still haven't printed anything bigger than 30x40.
Of course, if the ~65MP S(008) comes out, I'm getting it!
On topic, I sure hope reasonably priced Hassy and Fuji 100MP bodies come out to put pressure on Leica!
Best,
Matt