The Pentax 645D is a rock-solid camera, though given its size and weight, it's certainly not meant to be hand-held for most practical purposes. If you expect to get decent results hand-held, you had better shoot using a fast shutter speed! With the 55mm kit lens, the 645D feels fairly well-balanced in your hands, but if you mount anything bigger, the length of the whole setup may end up being a bit overwhelming (see the lens specification page for a photo of the 645D with a 400mm lens). Stabilization is currently only offered in one lens (the 90mm macro), which should start shipping in early 2013.
When you have a tripod, though, things change a bit. The two immense tripod mounts on the bottom and the side of the camera, as well as its symmetry, make switching between horizontal and vertical tripod shooting a breeze. We recommend that you keep two tripod brackets on your camera at all times, be it in the field or studio.
The 645D has so many external controls and buttons - it's rarely necessary to go into the menus. The buttons are nice and large, and there are no real quirks are far as ergonomics go. The buttons and the menu systems are similar in function to those found on the Pentax K-7/K-5 DSLRs, which makes the camera quite intuitive to use. For detailed information on all of the 645D's external controls, please take a look at the construction and handling page of our 645D vs D3x review. One feature of note is Pentax's Hyper Program mode, which lets you adjust the aperture or shutter speed on-the-fly in P mode simply by turning one of the e-dials. This will lock in a particular shutter speed or aperture until cancelled by the user (you cancel by pressing the green button or by exiting P mode). This feature is very handy, as it means you don't have to switch to Tv or Av mode every time you want to set the shutter speed or aperture yourself.
Another thing we love about the 645D is its huge top LCD, which is almost as wide as the main LCD on the back.
Left: 645D top LCD Right: D800E top LCD
This comes at a price, though; if you look at the photo below, you'll notice that the sensor plane lies in front of the top LCD, meaning that the 645D could have been made much, much smaller if Pentax had wanted to do so. Unlike film 645 cameras, the 645D doesn't need to have any extra room for a film carrier.
The 645D body features weather-sealing, as do all current 645D lenses (the DA 25mm, D FA 55mm, and D FA 90mm Macro), so you'll be able to use the camera outdoors without having to worry too much about potential damage. Weather-sealed lenses bear the "AW" designation in their names, which stands for "All Weather".
Whenever you mount legacy 67 lenses (via the 67-to-645 adapter) on the 645D, the camera can be used in M and Av mode. The adapter makes use of the 645 aperture coupler to transmit to the camera the position of the aperture ring. Thus, the 645D can automatically set the shutter speed for you (without having to stop-down meter).
Overall, the 645D doesn't have perfect handling, mostly due to its large and irregular size. Fortunately, though, it's easy to change just about anything other than shooting mode when looking through the viewfinder, which makes it a little bit easier to use than the Nikon in the studio.
Refer to the photos below to see the 645D's external features and button layout.
There is a front and a rear IR port for convenient shutter release using the Pentax remote control F. A traditional wired release is also supported. The 645D supports GPS functionality via the O-GPS 1 accessory.