The control layout is basically identical to the EOS 30D, with a large data input dial on the back, a smaller knurled wheel positioned under the right forefinger, and a joystick-like directional control for menu navigation. The main menu has been revised, and now has the multi-page tabbed appearance of the latest EOS 1D cameras. One feature I was not particularly enamoured by is the exposure compensation control. To operate it you have to half-press the shutter release, then turn the rear dial to alter the compensation setting. I found this to be quite fiddly and counter-intuitive. A simple exposure compensation button would have been far simpler. I also agree with some other reviewers that the "print" button is superfluous on a camera of this class, and would be better replaced by a programmable function button.
Another external improvement is the new 3.0-inch LCD monitor, which has a resolution of 230k dots and is brighter and has higher contrast than the unit on the 30D. It also now incorporates live view, a feature first introduced by Olympus that is appearing on more DSLRs. This is a useful feature for studio work, when the camera may be mounted on a tripod in a position that make using the viewfinder difficult, but Canon's implementation of this feature is not without its drawbacks, the main one being the lack of autofocus in live view mode. Nikon and Olympus both include a basic contrast-detection AF option in live view, while Sony's A350 has a separate phase-detection AF sensor for live view and an articulated monitor. By comparison the 40D's live view mode seems quite limited. The new monitor also has a smaller angle of view than the previous model, although at 140 degrees it is still wide enough for most purposes.
The 40D's other improvements are internal. I've already mentioned the 14-bit data path and improved sensor design. The sensor includes a self-cleaning mechanism which vibrates the low-pass filter, shaking off any dust onto a sticky strip positioned below the sensor. This is basically the same type of mechanism used by a number of other manufacturers, including Sony, Nikon, Pentax and Olympus. It's difficult to judge the long-term effectiveness of such a system, but anecdotal reports from users appear to be favourable. However while testing the camera I discovered that dust had somehow managed to get inside the EFS 17-85mm IS lens that was supplied with my review sample, including several large particles between the front and second elements. The 40D includes a dust spot deletion feature, whereby a photo of a plain white surface is used to create a mask to hide the effects of any persistent sensor dust spots, but it proved to be incapable of correcting the effects of the dust inside the lens.
The autofocus system has also seen some improvements, with nine cross-type sensors spread more widely across the frame, and a diagonally-mounted centre sensor with improved low-light sensitivity. While it's no match for the fantastic 51-point AF system in Nikon's D300 it is still exceptionally quick and accurate, and works well in virtually all lighting conditions.