Cameras measure the light reflected back from the subject to the camera. Reflective light meters assume that all scenes have an average distribution of light, medium and dark tones, the meter also assumes that the average of all tones in the scene is 18 percent grey. On the whole this assumption is correct and will produce a properly exposed image. But as we know not all scenes will contain average tonality, e.g. snow or large expanses of water.
The modes on the 400D are as follows :-
This is the default setting and will analyse from virtually the whole viewfinder area, with the meter analysing the point of focus and automatically applying compensation is the surrounding areas are much lighter or darker. To calculate the correct exposure the camera analyses subject position, brightness, background, front and back lighting and camera orientation.
This mode produces correct exposure in average scenes that include a distribution of light, medium and dark tones. If shooting scenes already mentioned IE snow, water it can be useful to use exposure compensation to increase or decrease exposure by one or two stops. Evaluative metering mode in the default for all Basic Zone Modes.
Centre-weighted Average Metering
Centre-weighted Average metering makes use of the AF points but although there are nine AF points only the seven AF points in the centre are used, hence the name. Centre-weighted metering gives more weight to the area of the scene within the seven AF points in the centre of the viewfinder. The camera then averages the exposure for the entire scene. This is useful for backlit subjects and where the tones are not average and require exposure compensation.
This mode meters from a central 9 percent of the viewfinder. Once again this is handy in backlit or side-lit situations when you want to make sure the subject is properly exposed.
You can use exposure compensation to override the camera's default exposure. EC is set in 1/3 or 1/2 stop increments up to +/- 2 f-stops. It can be combined with Auto Exposure Bracketing and be used in all Creative Zone modes except for Manual.
Auto Exposure Bracketing
AEB ensures at least one exposure in a series of three will be acceptable, one exposure will taken at the standard exposure with one lighter and one darker. AEB can also be set at 1/3 or 1/2 increments up to +/- 2 f-stops. However AEB cannot be used in some circumstances, i.e. with flash or Bulb settings. AEB settings are cancelled if you change the lens, replace a cf card or battery or turn off the camera.
Another handy feature if you don't want to set the exposure on the area where you set the focus, for example if shooting a portrait and the brightest spot is the persons forehead, in this situation you may wish to expose for the highlight but focus on the eyes. This can be achieved by using AE Lock.Once again this function is not available in Manual mode.
When thinking about exposure the issue of flash rears it's ugly head. To flash or not to flash that is the question! Built in flash can produce harsh or unnatural illumination on the subject and more often than not harsh shadows behind. Overexposure and red-eye are also a common effect. The 400D's built in flash and capability to use external flash units offer a greater control and help to avoid these problems.
There is a table in the Digital Rebel Field Guide which gives the Built in Flash Range with the 18-55mm lens
1- 3.7 m (3.3-12.1 ft)
1- 2.3 m (3.3- 7.5 ft)
0.7 - 5.3m (3.3- 17.4 ft)
1 – 3.3, (3.3- 10.8 ft)
0.7 - 7.4m (3.3 – 24.3 ft)
1- 4.6m (3.3- 15.1ft)
0.7- 10.5m (3.3 – 34.4 ft)
1- 6.6m (3.3 – 21.7 ft)
0.7- 14.9m (3.3- 48.9 ft)
1- 9.2M (3.3 – 30.5 ft)
which made interesting reading. Not that I expect to remember all of it just like that ;o) Handy to note that whatever the focal length or ISO the closest you should be is approx 1m and using the highest ISO at 18mm you can reach up to 14.9m (though my brain still likes to operate in feet) Obviously upping the ISO will increase the range, I'll have to find a simple way to remember the increments but in general the longer the focal length the shorter the distance the flash covers and the higher the ISO the farther the distance the flash illumination covers.
The 400D also offers a red-eye reduction option and a Flash Exposure lock and flash exposure compensation. Now I have investigated FEC I will probably use it more :o).
Canon EOS 400D Rebel XTi Digital Camera Flash - Full Review - The Imaging Resource!