<p>Digitally zooming doesn't change DOF or focal length, it merely narrows the field of view and uses less of the file. Nikon does provide for digitally changing the sensor's effective size on its D800, with the added advantage of a smaller file, allowing more frames per second. This is not a problem with the 5D MkIII while the 6D might gain from this, if firmware were updated to allow it.</p>
<p>Cropping (no different from digitally zooming in-camera) does not necessarily magnify the imperfections of a lens, so long as your using the middle of the image, where most lenses actually perform better. If you crop to show a corner of an image, then the lens's imperfections will be amplified. </p>
<p>I crop a high percentage of my wildlife shots, even those shot with 1,000mm, and see little ill effect until I start nearing pixel-levels. A 20% crop is no big deal, but 50% and above can be problematic when the image is viewed full-screen. Any focus imperfection is amplified, as is noise and other pixel-problems. Optical focal length is always superior to digital zoom, or cropping. </p>