The single lens reflex (SLR) is most folks' idea of a serious camera. "SLR" means that the same lens is used for viewing and taking pictures. A mirror in the body directs the light from the lens up into a prism for viewing, then flips up out of the way just before an exposure is made. The standard photojournalist's Nikon from the 1960s or 1970s was an SLR with a roll of 35mm film behind the mirror. When the mirror came up the light passed through to the shutter, which opened to expose one frame of film for perhaps 1/60th of a second. A Canon Digital Rebel or Nikon D80 looks very similar and works in almost the same way. The only difference is that instead of a piece of film behind the shutter there is an electronic sensor.
The mirror and optical viewfinder are what enable a photographer to frame images more quickly and accurately than with a point-and-shoot camera. Regardless of what lens or filters you have attached to the camera you see what the sensor will see. The same can be said for the LCD displays on the back of a $200 point-and-shoot camera but those displays are difficult to interpret in sunlight. The typical digital SLR camera viewfinder offers additional information underneath the image, including all the most important camera settings.
The SLR is much larger and heavier than the point-and-shoot camera. If you are leaving the house to socialize and want a camera to keep in your pocket just in case an interesting photo presents itself, the SLR will seem cumbersome. If you are heading out specifically with a photographic project in mind you will appreciate how the SLR and its controls fit into your hands.
Because digital SLRs are more expensive than point-and-shoot cameras the manufacturers typically put in faster computers and better autofocus systems. This makes the cameras more responsive and you are more likely to catch the "decisive moment" as the baby's face lights up with a smile, the soccer ball leaves the player's foot, or the dog catches the frisbee.
A digital SLR may offer the same number of megapixels, individual image elements, as a high-end point-and-shoot. Not all pixels are created equal, however. Resolution is important if you intend to make large prints but dynamic range, the ability to capture detail within bright highlights and dark shadows, is more critical in many situations. The sensors in digital SLRs are typically much larger than those found in point-and-shoot cameras. The main advantage of a larger sensor is better performance in dim light. If there are 8 megapixels spread out over a sensor that is 4 times larger than the sensor in a point-and-shoot camera that means more photons of light will fall on any given pixel. If during an exposure 50 photons would fall on the small sensor then 200 photons would fall on the big sensor. If there is a small change in the light from one part of the scene to another the sensor in the point and shoot camera is trying to notice a single extra photon; the electronics in the camera with the big sensor have four extra photons that are much easier to detect.
First-time consumers of digital SLR cameras focus on the body. Long-time photographers, however, look at the system. An SLR system includes a body, multiple lenses, flash units, and various connecting cords. For most photographers the investment in lenses will come to dwarf the cost of a body. It is thus important to choose a system whose manufacturer makes the lenses that you need for all of your potential projects and, ideally, whose system is popular enough that you can rent special-purpose lenses for uncommon situations. Each camera system has its own lens mount design and a lens that works on, say, a Nikon camera cannot be attached to a Canon body.
Taken from http://photo.net
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