Canon 4050D
The twelve megapixel EOS 450D (Digital Rebel XSi) is a very similar camera to the EOS 400D, however there are a range of significant improvements; twelve megapixel CMOS sensor, 3.0" LCD monitor, Live View with both AF modes (contrast-detect and passive), 14-bit processing and RAW, spot metering, improved AF, 3.5 fps continuous shooting, a larger viewfinder, SD/SDHC storage and a new higher capacity battery. All in all a fairly impressive range of usage oriented improvements and refinements.
Aperture:
Aperture is a hole where light can pass through. The aperture of a photographic lens is a hole that can be adjusted to different sizes. This lets the photographer control the amount of light reaching the image sensor. The aperture is changed along with the shutter speed to control the amount of light that enters the camera. This amount of light is called the exposure.
F-number:
The size of the aperture is described as an "f-number." This is a number that describes the size of the hole proportional to the focal length of the lens. A low number (such as f1.4 or f2.8) means the aperture is wide open, and lets in a lot of light. A high number (such as f8 or f11) means that the aperture is small, and does not let in as much light.
Shutter Speed:
A fast shutter speed will not let much light in causing the picture to be dark, so the photographer will use a larger aperture to correct for this. A slow shutter speed will need a smaller aperture to stop the picture coming out bright. In addition to its effect on exposure, the shutter speed changes the way movement appears in photographs. Very short shutter speeds can be used to freeze fast-moving subjects, for example at sporting events or light pictures. Very long shutter speeds are used to intentionally blur a moving subject for artistic effect.
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