Everyone goes on about the D800 - excluding mps it is barely to the 2007 1Ds3 standard as an all-round camera
You made my day!
Resolution and DR have direct impact on IQ and yet you compare D800 with 1Ds Mark III.
Even my 2004 1Ds2 is still churning out impressive pictures at 16x12.
My 2001 Fiat Coupé still runs very smooth, you think I should compare it to a 2012 Mercedes-Benz?
Resolution beyond those pixels needed have no impact on IQ. a 10mp picture when printed contains no extra detail whether it comes from a 21mp camera or a 36mp camera. It is the colour rendition and clarity that really counts.
DR on the 1DS3 is very close that on the D800 - particularly in normal shooting where DR requirements rarely get above 10. The 1Ds3 matches the noise of the D800 at 50-200iso
I would be very surprised indeed if there was any visible improvement on a 16x12 from a D800 rather than the 1Ds3 - particularly when the Canons superior lens are taken into account.
Have to start off saying that I completely agree with you regarding Canon lenses. In recent years, the quality of Canon optics has shot to the top of the charts. They are sharper, with better lens coating (SWC, or SubWavelength Coating, is a nanocoating technology not related to multicoating), and lighter in weight than pretty much anything from the competition. Not even Zeiss compares to a Canon lens these days when you factor in lens flare, which is FAR better controlled on Canon lenses (many Zeiss lenses flare very badly, despite the use of their much-vaunted T* multicoating.)
Lenses aside, I think we need to clear up the facts behind some of your other arguments. First off, color rendition. This is a VERY OLD argument that dates well back to the early days of color film, and color film in general. It meant a lot when you found a color film that produced the kind of color you really liked. I am not a big film shooter myself, however I've seen quite a few photos shot with film. Most of my favorite landscape photographers who still use or have heavily used film in the past all seem to use Velvia 50, and for good reason. A few have chosen other types of film, such as Provia, for the types of work they do...such as winter landscapes taken at the poles...the cooler tone of Provia suits those regions better than Velvia in many cases.
The idea that a digital sensor and particular camera can produce better digital color than any other camera is an odd idea. For one, digital sensors don't see in color at all, for all intents and purposes. The pixel values read out of any given photodiode are simply a scalar value representing a luminance reading. In a bayer sensor, each pixel has a color filter over it to restrict the wavelengths of light that actually reach the photodiode, but it doesn't make the raw value read out of that pixel more blue, red, or green by any measure of the word...its still just a luminance value, albeit linked to a certain (relatively broad) wavelength range. What makes those raw pixel values have color is the algorithmic demosaicing process performed by a computer. A key component of such algorithms is the RGB tone curve applied to each channel during demosaicing. Apply the same exact tone curves of the 1Ds III to any other Canon camera, and the results will be nearly identical. If there are any discrepancies, its not a difficult process to tweak some RGB curves in ACR, Lightroom 4.1, or any variety of other RAW processor to normalize the results. You could even save those tweaks as import or camera profiles that are then automatically applied to every photo you import from any number of cameras, and achieve the exact color rendition you want every time you import any photos from any camera.
To put it in simpler terms: Color rendition is a SOFTWARE MATTER, not a hardware matter.
The notion that any particular camera is solely capable of producing the kind of final output color you want is archaic, and really needs to be eliminated from discussions about digital photography and camera gear. It applied to film because of the largely immutable nature of any given film (excluding what you could do with chemicals and projectors in a dark room)...but just about EVERYTHING is mutable with a RAW file.
The next point about the downscaled results of a D800 not looking any better than the native results from a 1Ds III, I strongly dispute. Printed at a high PPI and at maximum DPI, such as 600ppi at 4800x2400 or 720ppi at 2880x1440/5760x1440 will easily demonstrate either the inferiority of the 1D III or the superiority of the D800. Downscaling such a large native image by so much is a far better approach to sharpening the photo, as you have more source data to work with than if you simply apply a sharpen filter to the 1Ds III. As someone who has spent about three years printing my own work, I can attest to the benefits of printing at 600ppi rather than 300ppi, even for photos that you wouldn't think could benefit from it (See my article here: http://photo.stackexchange.com/a/2737/124
). The improvements are subtle but meaningful to anything more than a basic cursory glance. In my experience, at least for fans of my work, a high quality, 600ppi print will draw most viewers in closer than a 300ppi print is suitable for, and I always get exclamations about the fine, sharp detail. And these are 8x10, 8.5x11, 12x18, 13x19 prints...nothing exceptionally large. For the larger prints I do have, such as 24x36" and 30x40" canvas prints, the difference between a 150ppi print and a 300ppi print are also noticeable, even to eyes less trained than my own. The more source data you have to work with, regardless of whether you are printing huge or printing small, can and does have an impact on the final results. I'd take a 36mp sensor any day for my 8x10 and 13x19 inch prints, if solely to provide the crisp, sharp, clear results I want without the need to apply a halo-creating sharpening filter that inevitably shows up in print regardless my efforts.
To say that the D800's resolution advantage has NOTHING to offer whatsover over the 1DsIII is somewhat naive. If nothing than for the intrinsic "sharpening" you gain when downscaling a huge image to nearly half its size, and the ability to avoid having to apply a halo-generating sharpening filter, is valueable. Not to mention the quality of those sharper edges...sharpening tends to enhance undesirable artifacts, increases microcontrast in undesirable ways at times, etc. Downscaling absorbs undesirable artifacts and doesn't unduly increase microcontrast where it doesn't belong, nor overdo acutance along strong edges, producing a cleaner result. Printed at high PPI (600/720), I'd take the D800 (or a high-MP EOS) over the 1DsIII every time.