The last year or so I have mostly focused on model/portrait photography. I feel that the K-5 is a very capable all-rounder suitable for practically any kind of photography. A friend of mine shoots cars (not motorsport) and I got the opportunity to try out his D800 + battery grip + Nikon AF-S 70-200mm F2.8 G IF-ED VR II. For my studio work I usually work with the K-5 + D-BG4 and a combination of two lenses, usually the FA77 and the FA43 or DA40. FA77 for headshots and half-body, the 40/43 for full body.
That the image quality was to be better was expected, but I was more interested in the whole experience. Handling, focusing, post-processing... the whole package. Lets start with a few comparing pictures of the cameras.
Weight
The D800 & 70-200/2.8 combo is very much larger and heavier than the K-5. A more fair comparison would be a 85/1.4 on the D800 or a the DA* 50-135 on the K-5, but such combinations was not possible at this test (I have used the 50-135 before and preferred the FA77). A photosession takes around 30 minutes of active shooting, and the weight of the full frame combo was felt, both in handling and on the neck when adjusting light etc.
Handling
Since I have used the K-5 the last couple of years, I know it inside out. The D800 handling was no problem getting used to when it comes to physical switches, focusing etc.
Menu system
While the physical use of the D800 was easy to get used to, I find the Pentax layout and graphics more strait to the point and no-nonsense style. For example, setting the ISO below the base ISO of 100 on the D800 was a bit of a guessing game. The expanded range is called something like "ISO Low1" or "ISO High 2". Why not just tell me what ISO it is using? I had to go for ISO100 on the D800 since I was working with manual flashes and wanted to know the value. There is of course a period of getting used to a new menu system but I find the Pentax menus the best there is.
Post-processing
I was worried that the post processing would be a strain since the files are twice the size. I use Lightroom 4 and CS6 on a i5-2500K@4GHz/16GB RAM system/SSD(for OS, program, catalog). It was surprisingly little difference in post processing, editing the files was about the same speed. The only major difference was the export-speed from lightroom, taking significantly more time (probably twice). That said, it doesn't really matter to me if it takes 3 or 6 minutes to export a batch. The RAW-files where pretty similar too, needing roughly the same amount of sharpening, responded similarly to color changes etc. Other cameras I own or have tested respond very differently (eg. the X100s raw-files are more sharpened OOC and the Panasonic GF2 doesn't respond to color changes very well).
Image quality
This is the big thing. Is it worth the extra cash and weight? Preferences over menu system, lens selection etc may of course also play a part. Keep in mind that in these studio shots there are a solid background and both cameras can work on very low ISO. This comparison is for these conditions, I will try to make a similar comparison outdoors where ISO might get a little higher (mid range) and bokeh/DoF will be a factor.
The amount of detail the D800 produces are very impressive. Lenses are stopped down to around f/5.6 so they are performing very well. The amount of details in full body shots would be similar to half-body on the K-5. When coming in to headshots you simply get tons of fine details, also very clearly the umbrellas from the flashes in the eyes. I don't do any large prints (one a year maybe) so almost all images are being scaled down for the web or small print. Then you don't really need all those details, and the images looks very similar. I see a situation where you might get "lazy" and start cropping much more into the large files too.
Next is a few images so that you may compare the results for yourself. They are linked to the full sized final images. The models name is Lusi Dimitrova, the sets are called
Fashion Animal and
Doll. Note that the two sessions are not edited to be the perfect images for comparing, but as I wanted each set by itself. There are also a bit of fine grain added, images are cleaner strait from each camera.
Nikon D800, Nikon AF-S 70-200mm F2.8 G IF-ED VR II @ 135mm. 1/125 sec at f/5.6, ISO 100. Pentax K-5, SMC Pentax-FA 77mm F1.8 Limited. 1/180 sec at f/5.6, ISO 80. Nikon D800, Nikon AF-S 70-200mm F2.8 G IF-ED VR II @ 70mm. 1/125 sec at f/5.6, ISO 100. Pentax K-5, SMC Pentax-DA 40mm F2.8 Limited. 1/180 sec at f/5.6, ISO 80. Nikon D800, Nikon AF-S 70-200mm F2.8 G IF-ED VR II @ 105mm. 1/125 sec at f/5.6, ISO 100. Pentax K-5, SMC Pentax-FA 77mm F1.8 Limited. 1/180 sec at f/5.6, ISO 80.