Light is both the enemy and the best friend of photographers. Even when you are shooting with a high-quality piece of equipment, you rely on light to get a good, crisp shot. With point-and-shoot cameras, low-light is difficult to overcome, but even a DSLR like the Canon Rebel XSi (or EOS 450D), low-light can pose problems to photographers. A lot depends on your lens and just how little light there is.
Focus in Low-Light
On any camera, low-light focusing is difficult, especially on auto-focus mode. The Rebel XSi, like most DSLR cameras, uses an autofocus sensor light to sense contrast in your image. In daylight, this is easy enough for the camera, since objects have a lot of definition and the Rebel can detect foreground and background based on light. But in low-light this becomes really difficult, since two objects may appear close to you the camera can't distinguish between them. This usually creates a whining sound as the focus motor spins back and forth; it doesn't indicate a problem with the camera, however.
Blur
To capture images in low-light, your camera uses slower shutter speeds than when shooting in daylight. On auto or programmer mode, the Rebel XSi automatically computes how long to keep the shutter open. For shutter speeds faster than about 1/15 of a second, you need to have a steady hand to eliminate blur, since the shutter will stay open longer and cause your pictures to be blurry if the camera moves at all. For longer exposures, use a tripod and remote control or the self timer, so that the motion caused by tapping the shutter button is removed from the equation.
Lens
With low-light photography, the lens is the key. The Rebel XSi has a wide range of shutter speeds and exposure compensation settings, so you'll be able to find settings to shoot in low-light, but you are only as good as your lens. When looking at lenses for low-light photography, maximum aperture plays a big role. The wider the aperture blades can open on your lens, the more light that can reach your sensor; this allows for shorter exposures and less blur. Lenses under f/2.8 are good for low-light photography, as you can use fast shutter speeds and still have well-exposed shots.
Tricks to Overcome Blur and Focus Problems
When shooting in low-light (outside after dark or indoors with incandescent lighting), a tripod helps create clear shots of still objects. Attach a good low-light lens and switch the focus mode to "M" for manual, especially when shooting in really dark areas. Use a flashlight or similar to focus the camera on the tripod, and then fire the shutter button on a timer or with the remote control. This eliminates all shake and keeps your shots clear, but if anything moves in the image, expect blur and try again.
Tags: shutter speeds, low-light photography, your lens, Canon Rebel, Focus Problems