Consider the following hypothetical situation: I am shooting a shot of a fly's head with the MP-E 65mm at 4:1 magnification. The fly is dead, so it isn't going anywhere. I am in the studio with constant light, camera on a tripod, etc. I take one shot at f/11 and ISO 100 with the 450D. Then, I take another shot from the exact same position at f/11 and ISO 100 with the 550D. I upload both shots onto my computer and ensure that the settings for everything related to raw file rendering (especially sharpening!) are set exactly the same. I then downsize both files to be 1024 pixels on the long side using DPP. When I view the exported, 1024-pixel-wide photos at 100%, do I see a difference in sharpness?
The sensors in these two cameras have a different pixel density, but theoretically, the engineering quality of the sensors should be very close. The issue at hand is the diffraction limited aperture (see first table in this link for details about these and other current and recent DSLRs: http://www.the-digital-picture.com ...al-SLR-Camera-Review.aspx). In other words, particularly when shooting very high-magnification macro (i.e., greater than 1:1), is it actually better to have a camera with fewer pixels covering the same sensor size when it comes to photos that are printed small or re-sized? Obviously the answer will change if you start changing sensor size (i.e., let's leave the 5D out of this!), and it will also change if you view the two photos at 100% WITHOUT re-sizing them first. Let's set those issues aside. The key question is: is the extreme macro photographer who wishes only to show web-resolution photos actually doing him/herself a disservice by getting the highest density APS-C cameras out there, because he/she will need to shoot at a slightly narrower aperture to get the same pixel-level sharpness after re-sizing to 1024 pixels on the long side?
The answer to this question may come down to something I don't really understand: the algorithm used by most software to re-size photos. In a purely hypothetical example, imagine one camera literally had twice as many pixels covering the same exact sensor size. The DLA would be pretty low in the high-density camera when compared to the low-density camera, but images from both would have to be re-sized significantly to fit into a 1024-pixel width. When downsizing, do two slightly blurry pixels on a super-dense sensor equal the same re-sized quality as one sharp pixel on a less-dense sensor? I would guess that they do, but I feel like anecdotal examples I've seen suggest otherwise.
If this has already been answered, please feel free to point me in the right direction. A quick search did not turn up much. If you don't know the answer but still want to be helpful, and you own multiple APS-C cameras released within a couple years of each other, you could actually run this test and post some photos. Then you would be a hero, and who doesn't like that??