What's new / changed (key points)

10 megapixel CMOS sensor
As expected Canon has introduced a ten megapixel sensor in the 400D, the same two megapixel step made in the last generation, interestingly however Canon are also making some strong claims; less distance between microlenses and improved fill factor (combined mean better relative sensitivity) and improved output amplifiers (lower noise). All of this is claimed to result in the same signal-to-noise ratio as the 350D, despite smaller photosites.

EOS Integrated Cleaning System
Despite talking down the issue of sensor dust for several years Canon appear to have decided it's time to implement their own dust reduction system (following from Olympus and Sony). The Canon system is implemented in three ways; firstly sensor surfaces have anti-static coatings, next one part of the low pass filter has a piezo element which vibrates it shaking off dust (similar to the Olympus system), lastly you can manually take 'dust reference' shots (of a white wall for instance) which can be used to remove 'stubborn dust marks' from images later.

Nine-point Auto Focus sensor
One criticism of the EOS 300D/350D was auto-focus, Canon has decided to bite the bullet and simply installed the considerably better nine point AF sensor from the EOS 30D in the EOS 400D. This affords both faster, more accurate focusing as well as center point AF which is compatible with F2.8 lenses.
Continuous shooting burst increase
Thanks to faster processing and an increased internal buffer the EOS 400D can shoot a burst of 27 JPEG or 10 RAW images at three frames per second before the shooting rate slows. This is almost double the performance of the EOS 350D.

Single 2.5" LCD with a wider viewing angle
Gone is the double LCD layout of the EOS 350D, now replaced with a larger 2.5" LCD monitor which serves the purposes of camera settings display (and adjustment), review / play and menus. This new monitor is brighter and has a considerably wider viewing angle of 160° horizontal and vertical (compared to the 350D's 80° horizontal and vertical).

Camera settings display on LCD monitor
Because the status LCD has gone the LCD monitor now displays a camera settings screen. Kudos too for the slightly off-white background, it is much easier to read at a glance than other status screens on other digital SLRs. You can disable this display by pressing the DISP button and even choose to not have it display at power on.
Proximity sensor below viewfinder
Just below the viewfinder eyepiece is a proximity sensor which disables the LCD monitor when you raise the camera to your eye, this to avoid light from the LCD being distracting during composition.
Changes to the viewfinder view
The 400D's viewfinder status bar appears to be borrowed from the EOS 30D and now carries the white balance adjustment indicator and flash exposure lock indicator. Also of course the AF point pattern reflects the new nine-point AF.
Picture Styles
As expected the EOS 400D gets Canon's now "across the range" Picture Style feature which provides five preset image 'looks' (combinations of color, tone mapping and sharpness) as well as three user definable presets. Frankly Picture Styles make much more sense on the EOS 400D than they ever did on the EOS 1D or 5D level of camera.
Improved user interface
In use it's clear that Canon has worked to make the EOS 400D easier to use with simple setting adjustments via the 4-way buttons as well as single page menus, the first of which has just four options.
Image magnification in record review
The EOS 400D now offers the option (via a custom function) to magnify in record review, this requires a two-button press during the review itself. It would have been nicer to implement the same kind of automatic magnified record review seen on the Panasonic DMC-L1.
Histogram as Brightness or RGB
You can now choose between a Brightness (luminance) histogram or RGB histogram display in play and record review (as record review display is the same as the last used play mode).
Auto rotation with three options
The Auto Rotate feature now has three options: you can choose not to record orientation information at all, to record it but not rotate the image on the LCD or to record it and always rotate the image when shown on the LCD.
Three options long exposure NR
In addition to the On / Off options for long exposure (one second or more) noise reduction there is now a third, Auto, which only applies noise reduction if it detects pattern noise in the image.
Additional custom functions
There are two additional custom functions; (10) Magnified view (play or play/record review) and (11) LCD display when power ON (always on or remember status when powered off).
Up to 9999 images per folder on card
One issue which annoyed some EOS 350D owners was the creation of a new folder every 100 images, thankfully the EOS 400D has been updated in the same manner as the EOS 30D supporting up to 9999 images per folder.
Setting adjustments automatically stored
One of our biggest bugbears with the EOS 350Ds control system was the fact that after making a setting change on the LCD monitor you had to remember to press SET otherwise the change was lost. We're happy to report that this has now been resolved and that changes to settings are stored as you make them, so a half press on the shutter release returns you to shooting mode with the new setting.
New Direct Print features
For those who may use it there are now a range of new Direct Print features such as printing with EXIF information, red-eye reduction, face brightening and two types of contact sheet (35 or 20 image).