The same but different: Nikon D800E
At 36.3MP the D800 offers resolution that in theory, rivals medium format cameras, but its sister model the D800E presents a more serious challenge, boasting the same pixel count but with the effect of the optical low-pass filter (commonly known as an 'anti-aliasing' or AA filter) 'cancelled out'.
Almost all digital cameras employ an optical low-pass filter over their sensors to slightly blur the image at a pixel level in order to avoid moiré patterning. This gives more usable images for general photography (moiré is annoying and can be time-consuming to correct) but comes at the expense of a slight decrease in critical sharpness. Removing the effect of this filter, as Nikon has done in the D800E, should theoretically result in higher resolution. Although the difference might not be critical to the average enthusiast, it could be of some importance to studio and landscape professionals (many of whom will be used to working with medium format cameras, which similarly avoid AA filters).
Of course, the pressing question for prospective buyers is just how much more detail the D800E offers over the stock D800. You can see how the D800E performs on our resolution page and also view real world comparison images from both cameras later in this review.
'Cancellation' of anti-aliasing filter
Nikon uses slightly curious wording about the D800E - that the anti-aliasing effect of the optical filter stack in front of the sensor is 'cancelled' - and the diagram below shows what this means (courtesy of Nikon). Anti-aliasing filters work by using a birefringent material to split light rays into two based upon the polarization of the light; a first layer splits it horizontally, the second vertically. The practical result is a slight blurring of the image, to avoid moiré and aliasing artefacts. In the case of the D800E, the first filter is instead directly counteracted by the second, resulting in no overall blurring.
This seems like an odd way of doing things; why not just remove the filter altogether? Our best guess is that it simply makes manufacturing the two models side-by-side easier: instead of having to make an entirely different filter stack for the D800E, Nikon just needs to change the first low-pass filter in the overall assembly. Crucially, this also maintains the same infrared blocking and anti-reflective properties across the two cameras.