The Pentax 645D is substantially larger than the Nikon D800E, but this isn't necessarily a bad thing! Its buttons are very big and easy to press, and it features two huge tripod mounts for easy studio and landscape shooting.
In comparison, the D800E's buttons are clustered much more closely together, and unlike on the 645D, not all of them are easy to reach while shooting, such as the ISO button. In general, the Pentax user interface lets you change primary shooting settings, such as mode, ISO, shutter speed, or aperture, much more quickly than the Nikon user interface. Nikon's interface certainly has a steeper learning curve, and can be confusing at times because certain buttons behave differently depending on shooting mode or what other buttons are pressed. The Nikon does have several innovative features, however, and it makes drive modes very easy to cycle through by having a dedicated switch for this on the left side of the camera. There are also two customizable buttons on the right side of the camera, near the lens mount. One neat feature which the Nikon offers is a two-dimensional electronic level superimposed in the viewfinder, which can be toggled using one of the customizable buttons. The Pentax requires you to look at the electronic level on the LCD if you'd like to see both dimensions.
We also feel obliged to mention that Nikon lenses mount "the wrong way" compared to other brands, meaning that you turn left to tighten and right to loosen. This can take a while to get accustomed to. Because of the relatively small size of the D800E body, its lens release button can be difficult to hold down while in the field and shooting with large lenses such as the 24-70mm.
While the 645D's viewfinder is very large, it only offers 98% coverage, which means that there's no way for you to see exactly what you'll get via the viewfinder. The Nikon, on the other hand, has a darker and smaller viewfinder, but it offers 100% coverage and the camera also features live view.
Compared to other Pentax DSLRs, the 645D is unique in that it offers a dedicated mirror lock up mode, using the red switch on the front of the camera. This is almost a necessity given the size of its mirror. The Nikon also offers mirror lock up via its drive mode menu.
The 645D continues Pentax' tradition of providing helpful hints on the screen as well as indicating with setting is controlled with which e-dail - something that can be different between shooting modes and hence slow you down without these hints:
Both cameras offer a status screen which is particularly useful when shooting on a fully extended tripod because it lets you check the settings on the back of the camera.
Inside the menu, the D800E lets you configure multiple custom setting presets and quickly switch between them, while the 645D only supports a single preset. Apart from this, however, the two menu systems are about just as easy to use.
In general, though, even through the Nikon has a more complex interface, in the hands of a seasoned shooter, it'll be just as easy to use as the Pentax. Directly comparing the ways these two cameras handle is like comparing apples to oranges, because the 645D is clearly not meant for fast-paced photography, and its large size makes it beg never to be used without a tripod.
We'll let the pictures do the talking for the remainder of this section.
Next, let's take a look at the lenses and the general image quality of each camera.