I am considering to to buy 5d3 but there is one thing that I am not sure of. Is 5d3 pixel count good enough to print large size prints? When I say large size i mean something along the size of 4 by 2 feet. As you probably guessed i am between d800 and 5d3.
I am not sure what else comes to play when printing large size prints but I believe ISO has to do with it. Can somebody give me some guidelines when it comes to that.
The D800 has absolutely no
resolution advantage over the 5D Mark III when using zooms and low end primes this has been proven by many sources ranging from DXO to lensrentals to many minor reviews. The D800 only has a resolution when shooting with the best primes and only between f/4.0-f/8.0. That advantage is also at absolute best only an 11% increase in linear resolution, despite having more than 60% more megapixels.
The reason why both cameras will show such similar resolution is that most lenses cannot outresolve the 5D Mark III and when they do it's not by much. Canon has actually stated that it did not push the megapixels of the 5D Mark III because the lenses cannot handle it.
If you want more resolution 20 something megapixels you need to shoot with a medium format camera and with a medium format lens, the d800 is not a replacement for a medium format camera.
If you don't beleive me here's a comparison between the 5D Mark III resized to 36 megapixels and the D800 at 3350 LP/PH, this is shot with the same Sigma 70mm f/2.8 Macro, and heavily sharpened to show any difference, there is none.
Many lenses acheived more than 3400 lp/ph in the center but will show no difference slightly off center across over 85% of the image area such as the famed Zeiss 21mm. That's why to see any real difference you need a prime that acheives over 3400 lp/ph near the mid-frame and edges, which means you need a truely stelar lens - most primes do not acheive those numbers, you'll want lenses like the Nikon 85mm G, 24mm PC-E, 60mm Macro and the super telephotos on your D800 if you want more resolution than a 5D Mark III.
If you don't plan on shooting under carefully controlled circumstances with carefully selected lenses it will make no difference to you if you chose the 5D3 or D800. Either way a medium format will blow both cameras out othe water with nearly twice the resolution.
A 4 fott wide print with the 5D Mark III should be fine though.
If you're curious here's what the difference will look like if you use a high quality prime from the D800 to the 5D Mark III, the images are randomized, 5D Mark III images are resized to 36 megapixels versus D800 images.http://www3.picturepush.com/photo/a/7801636/img/Picture-Box/crops5d3vsd800.jpg
Hope that helps.