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Let’s begin at the top of the digital food chain with the pro D-LSRs. These are the cameras of choice (and necessity) for journalists, sports photographers, travel shooters and an increasing number of wedding photographers. They offer a superb array of features, an extremely rugged body (typically a one-piece magnesium casting) and the highest possible image quality—and, of course, a price to match. Price ranges from a low of about $1,500 to as much as $8,000—BYOL (bring your own lens). But the question isn’t whether you can afford a pro-level camera (perhaps you bought into the Google IPO at the right moment and price isn’t a concern)—it’s whether or not you need it.

One of the two most significant incentives for owing a D-SLR is that they are true WYSIWYG ("what you see is what you get") cameras: what the lens sees is exactly what you see in the viewfinder. This is a crucial issue when it comes to macro photography, for instance, where even a slight bit of parallax (the difference between what the taking lens sees and what the viewfinder sees in non-SLR cameras) can ruin an image. To be fair, even lesser cameras such as advanced zoom and compact models can provide very accurate viewing via their LCD panels—but seeing a clear image on an LCD panel is not always easy in bright sunlight.

Because they accept the manufacturer’s entire catalog of lenses (though some older AF lenses and many manual-focus lenses may not activate all metering features), D-SLR cameras also offer supreme optical flexibility. From fisheyes to super telephotos, you can find a lens to fit virtually any shooting need. Also important, D-SLRs open up the possibility of using numerous lens accessories, such as bellows and extension tubes for extreme close-up work and tele-extenders for increasing the focal length of telephoto lenses.

And Speaking of Lenses

More lens choices means more creative options, but there are other lens-related issues to keep in mind. For one, you’ve got to carry all of those lenses with you—fine if you’re shooting in the backyard, but if you’re toting that bag through airports or taking a long day hike, the pride of ownership can get old quickly. Also, unless you’re willing to spend premium bucks for lenses with large maximum apertures, you’ll have to settle for slower lenses than those you might find on a high-end compact zoom camera.

And because many (not all) D-SLR cameras have sensors that are smaller than a 35mm film frame, they create a magnification factor (also called a "cropping factor") when used with lenses designed for 35mm SLR bodies that typically increase apparent focal lengths by factors of 1.3 to 1.6x. This can be a blessing if you’re a sports or wildlife shooter because your telephoto lenses get longer (they magnify more) with no loss of speed, but it can also be a curse since wide-angle lenses gain focal length making them less wide. While I think that it’s terrific that my 400mm f/5.6 Sigma lens is now a 600mm lens on my Nikon D70s (and at the same lens speed), I’m not as thrilled that my 24mm Nikkor lens (now equal to about 36mm) is no longer the exquisite wide-eyed beauty it once was. Cameras such as the Canon EOS-1Ds that feature full-frame (24 x 36mm) CMOS sensors, of course, have no cropping factor.

Speed & Quality

Another absolute advantage offered by D-SLRs is their speed of use: they have no start-up delay and no detectable shutter lag. A D-SLR is on and ready to shoot from the instant you switch it on. If you’ve ever pointed a compact camera at a romping five year old only to have her scurry off before the camera fires, you can appreciate this luxury. (To be fair though, most newer point-and-shoot or compact cameras have no noticeable lag time.)

Equally important, D-SLR cameras feature both high frame-per-second (fps) rates and large buffers (an area of memory where exposures are held until you give the camera time to process them). The Canon EOS-1D Mark II and the Nikon D2H, for instance, have burst rates of about 8 fps (in JPEG mode, about half that in RAW) and can store 40 frames before you have to pause to transfer images to your memory card. If you shoot sports, this is hard to ignore.

Also, while auto-focus speed and accuracy is blazingly fast in all categories of digital cameras, it is fastest with D-SLRs. Important too, most offer multiple user-selectable AF points so you can tell the camera what part of your subject is most important.

Better Image Quality: Because the sensors in all D-SLR cameras are noticeably larger than those found in any of the advanced compacts—even those that have higher pixel counts--the individual photosites are also larger. This means less image noise even at higher ISO speeds. While advanced zoom cameras typically use ISO 50 or 100 as a default ISO speed, most D-SLRs use ISO 200 with no noticeable noise and noise is still not an issue at speeds up to ISO 400. By comparison, both point-and-shoot and zoom cameras show appreciable noise at ISO 200 and at ISO 400 files are noticeably degraded.

Virtually all D-SLR cameras provide a full range of metering modes, including: complex multi-segment matrix metering and precise one-degree spot metering. In addition, you enjoy absolute control over exposure with shutter-priority, aperture-priority and full-manual metering standard—as well as full TTL integration with sophisticated accessory flash units.

Finally, while all digital cameras can record in JPEG, all pro SLRs also have the ability to record in RAW. The RAW format, which uses minimal or no in-camera processing enables you to alter settings such as exposure, white balance, contrast and sharpening after the fact, but it creates much larger and slower files. Some sensors can record both RAW and JPEG simultaneously enabling you to use the RAW files for their post-production flexibility, while retaining smaller and faster JPEG files for faster browsing.

Are there any downsides to using an SLR?

Yes, a few. As I mentioned above D-SLRs are much heavier: a Nikon D2H body weighs 38 ounces, sans lens, while an advanced-zoom (Sony calls them "fully-featured zoom cameras") like the Sony Cyber-shot DSC-F828 camera weighs just 29 ounces including a 28-200mm lens. Add two or three lenses and a flash to your SLR kit and pretty soon you’re lugging around a medium-sized wet dog. And if you travel, that dog has to go through airport security.

Also, since the camera’s reflex mirror is blocking the sensor when you’re composing, there is no “live” LCD viewing capability on a D-SLR. And while these bodies have superior dust and moisture sealing, getting dust on the sensor is a real problem because each time you take a lens off of the body, you make it vulnerable to whatever tiny flotsam is floating in the atmosphere. I first encountered this while downloading shots in a hotel room near Utah’s Monument Valley and realized I hadn’t the foggiest idea how to remove the dust from the sensor. I knew I’d be facing a lot of extra Photoshop work later on.

Remember though, dust on sensors is easy to have cleaned and also, many manufacturers are finding ingenious solutions. The SONY Alpha, for instance, uses sonic waves to knock dust off of the sensor every time you turn it on.