The range of apertures available is getting narrow, but the current 20+ megapixels DSLR still offer a very clear advantage over 12MP sensors when everything is done right, wihch is not always easy and requires serious oversizing of some pieces of equipment for consistency accross a variety of conditions.
I wouldn't quite agree with this, Bernard. I found no restriction in choice of apertures after comparing two very high pixel-density cameras, the Canon 40D and 50D, which on full frame would be equivalent to 26mp and 39mp.
It's true that there's a law of diminishing returns at work regarding increased resolution in the plane of focus, as one stops down towards diffraction-limited f stops. However, for landscape work in circumstances where as much DoF as possible is sought, the higher pixel-density sensor has a clear advantage.
For example, if one doesn't want to compromise resolution too much at the plane of focus, I think most users of the APS-C format would be reluctant to stop down beyond F11, maybe F13 at the most. Many lenses are sharpest around F5.6 so there is often a trade-off between degree of DoF and sharpness at the plane of focus.
This is where the higher pixel density camera has the advantage. You get approximately the same
degree of sharpness and detail with a 50D at F16 as you get with a 40D at F11. But there's no doubt whatsoever, that the 50D at F16 produces greater DoF.
Comparing the 50D at F8 with the 40D at F5.6, using a standard 50mm prime (50/1.4), the 50D image not only has an obviously greater DoF, but is also marginally and noticeably sharper at the plane of focus.
The full frame equivalent of the current Canon 7D would be 47mp. I would have no hesitation in buying a future Canon Full Frame DSLR with the pixel density of the 7D, if the price was right .
The bottom line is, the higher pixel-density camera will never produce worse
results at the plane of focus (using the same F stop and lens), but will frequently produce greater
DoF with the same
degree of sharpness at the plane of focus, when stopped down.
I should add, it is assumed one always compares equal size images on monitor or print. That's only sensible.