The D-SLR Difference
What distinguishes an SLR or D-SLR from all other types of cameras? Introduced in 1959, the single-lens reflex film camera was a groundbreaking change from the design of cameras of the past, as it used a mirror to project an image from the lens into the optical viewfinder for true through-the-lens viewing. To expose the film (or sensors on today's D-SLRs), when you snap the shutter-release button the mirror moves out of the way; it then pops back into position, re-establishing the image in the viewfinder. All of this happens instantaneously.
What this design doesn't allow for is viewing your live image on the camera's LCD. Since a D-SLR gives you through-the-lens viewing, which is achieved with mirrors and not sensors, you can review only your captured images on the LCD; for live viewing you must look through the viewfinder. This year, one camera manufacturer has introduced a D-SLR that does allow live viewing of images on the camera's LCD, but it's too early to know whether all camera manufacturers will adopt this feature.
What differentiates a D-SLR from a 35mm film SLR is that it captures an image on a sensor, a device that converts light into electrical signals. These signals are then converted into data and stored on a memory card. Manufacturers have used a variety of sensors in imaging devices, and we expect that newer types will be introduced as technology improves. The two most common ones are the CCD (or charge-coupled device) and the CMOS (complementary metal-oxide semiconductor).
D-SLR sensors are significantly bigger than those in point-and-shoot digital cameras. This partly explains why D-SLRs have better picture quality and greater dynamic range, whether the sensor is APS-size, Four Thirdssize, or the size of 35mm film. Unless you're getting a D-SLR with a 35mm filmsize sensor, you will have to factor in lens magnification: Since the sensor is smaller than 35mm film, the camera needs to bring it closer to the lens, which magnifies the image your lens sends through the camera. Generally, this will be anywhere from 1.5X to 2X magnification. So your 35mm lens will become a 70mm lens on cameras that have a 2X magnification factor. That can be good news if you like shooting with telephoto lenses, but it also turns your wide-angle lenses into not-so-wide-angle lenses.
The sensor is also what determines the maximum number of megapixels your image files will have. Most entry-level D-SLRs are either 6MP or 8MP, but there has been a steady increase in megapixels, with midrange D-SLRs now delivering images with 10MP to 12MP. This means you'll get images with more data for larger prints, or maybe for more radical cropping in your image-editing program. Keep in mind that a 6MP D-SLR will give you more than enough data for a great 8-by-10 print. If you're interested in fine-art, large-size prints, you might want a 10MP, 12MP, or 16MP D-SLR.