an article from photonotes.org - it mentions about nikon is better than nikon in low light conditions..
i also read about the specs between cmos and ccd sensor which explains ccd is better in low light condition from a link @ cs.com..
Here goes the article..
Autofocus doesnt work very well when its dark. What can I do?
Low-light autofocus is quite a challenge for any camera that uses a passive autofocus system, like all EOS cameras. Passive autofocus means that the camera simply looks through the lens and reads whatevers there - it doesnt send out infrared or sound or light or radar beams or whatever to determine the correct focus distance. (the AF assist systems used by most Canon cameras and flash units dont change this, since the lights are optional assist systems and are not required by the autofocus system)
Generally speaking the more expensive the camera the better the autofocus. There are exceptions to this, but its a reasonably accurate predictor of how good AF is going to be when light levels are lower. So a top of the line EOS 1V is going to have wildly better low-light autofocus than an old EOS 1000 consumer camera, for example. This doesnt quite apply with digital cameras, though - the D30 and D60 are both more expensive than roughly comparable film cameras, but have very poor low-light autofocus.
One way to get a sense of the cameras low-light AF performance is to look at its specifications. Light levels for AF systems are measured in exposure values (EV), and autofocus systems have their light sensitivity levels expressed as a range of EV. A good camera can autofocus from 0-20 EV. A consumer camera can usually only manage 2-20EV, which means that it needs more light at the dimmer end of the scale to work properly.
Unfortunately low-light AF performance is one area where Nikon products are generally better than Canon products. Many Nikon products autofocus down to -1 EV, for example. Nonetheless there are several things you can do to help your camera with its low-light AF performance.
The majority of EOS cameras with multiple focus sensors have a cross-type sensor as the central sensor. As noted above, cross-type sensors are more accurate than linear sensors because they can detect both horizontal and vertical lines rather than just lines in one direction. So always switch manually to your cameras centre focus point for improved accuracy.
There is one significant drawback to this approach which you should keep in mind, however. Canon E-TTL flash biases flash exposure to the selected autofocus point. So if you focus on something with the central focus point, recompose the image by moving the camera and then take a photo theres a good chance that the flash metering will be out. In this case you should lock flash by using FEL prior to recomposing.
Passive autofocus systems try to detect changes in contrast, so help your camera out by giving it a sharp line to focus on. Look for a wall or the edge of something where one side is brighter than the other and the two areas are sharply delineated. This, more than anything else, will make a tremendous difference. Dont try to focus on a blank wall - AF systems have difficulty with featureless surfaces in bright light, let alone when its dark.
Use an AF assist system. Many EOS cameras contain some sort of light-producing lamp which helps the autofocus system in low light conditions. The best kind of AF assist light is a bright red LED which projects a series of patterned lines. Its relatively unobtrusive as AF assist goes, and the striped lines help the AF system lock focus. A number of cameras - notably the 10/10s, Elan/100, Elan II/50, 5/A2/A2E, Rebel S II/EOS 1000FN and others - have such red lights. Other cameras have white incandescent flashlight-type AF assist lights which work, though not as well. Others, such as the Rebel 2000/EOS 300, Elan 7/EOS 30/33/7, have no dedicated lights and simply emit stuttering pulses of light from the popup flash, which is extremely annoying. Whichever system your camera uses, however, see if it helps with autofocus. Note that some large lenses or lens hoods can block the light from body-integral AF assist lamps.
Attach an external Speedlite flash unit. All Canon Speedlite flash units contain red AF assist lamps which can help autofocus considerably, especially if your camera hasnt got one. There are a couple of limitations to keep in mind, however. First, two cameras - the 5/A2/A2E and the 10/10s - will not activate external Speedlite AF assist lights. Second, many flash units are not capable of a wide enough spread of their AF assist light beams and so cannot illuminate the outer autofocus points of multiple focus point cameras. Even those with the ability to cover all the focus points of your camera generally illuminate the centre point with lots of light and the outer points with less light. So again its best to stick with the centre AF point.
Use a faster lens (ie: a lens with a smaller maximum aperture value). A lens with a maximum aperture of f/1.8 is going to autofocus in low light much more reliably than one with a maximum aperture of f/4.5, for the simple reason that the faster lens will let in many times more light.
Try to focus manually. This is difficult if your camera has a simple matte focus screen. But if youre lucky enough to have a focus screen with a split circle focus assist aid (see below) then its probably most reliable to focus manually.