No doubt you've seen those big, sophisticated-looking cameras that professional photographers and hard-core enthusiasts use. Perhaps you've heard the term "SLR" and know that it refers to those big cameras. But what exactly is an SLR camera, and how is it any different (besides being so bulky) from the little digital cameras many people carry around in their pockets?

This article will introduce you to SLR cameras, specifically digital SLRs. We'll help you understand what these technical marvels can do and why you may—or may not—want to invest in one.

What is an SLR Camera?

SLR stands for "Single Lens Reflex," and it refers to a class of cameras. Other examples of camera classes would be "Point and Shoot" (P/S) cameras, "Prosumer" cameras, integrated cameras (like in cell phones), pinhole cameras, etc. While there are many variations among SLR cameras, they all have a couple of important things that make them part of the SLR class.

First, unlike P/S cameras, SLR cameras have two distinct, detachable parts: a body, and a lens. The body, also called a "back" by photographers, contains the brains and the controls of the camera. If it’s a traditional SLR camera, film’s contained in the body and is exposed behind a mechanical shutter. If it’s a digital SLR (a.k.a., a DSLR), the film’s replaced by a sensor chip which—like the film in a traditional camera—is also exposed behind a mechanical shutter.

Along with the body, the other component of an SLR camera is the lens. The lens can be removed from the camera and swapped with other lenses. More on why you might want to use different lenses later.

The other distinct SLR characteristic relates to how, specifically, the picture is captured. With P/S digital cameras, the shooter lines up the shot by watching a live video image on the LCD. Alternately, some P/S cameras also have a tiny optical viewfinder above the LCD that can be used to line up the shot.

The key limitation to both of these methods is that the shooter isn't seeing exactly what the camera's lens is seeing. In the case of the LCD, the shooter’s seeing a video rendering of what the lens sees. In the case of the optical viewfinder, the shooter is looking through a completely separate mini-lens. If you have a P/S with a viewfinder, try covering the main lens with your hand or a lens cap and you should still be able to see through the optical viewfinder.

So how is an SLR camera different? With an SLR, the photographer actually lines up the shot by looking through the lens. Take a look at the diagram below:

When the photographer is framing the shot, light is directed through the lens. It hits a mirror and is reflected up toward the viewfinder. As a result, the photographer’s looking right down the barrel of the lens and sees the exact shot that will eventually be captured. The term for this phenomenon is "WYSIWYG" (pronounced "WHIZ-EE-WIG") which stands for "What You See Is What You Get". When the photographer presses the shutter button to take the picture, the mirror flips up and out of the way of the sensor (making a mechanical "click" noise), the shutter opens, light strikes the sensor, and the picture is taken.

So we've established that detachable lenses and WYSIWYG viewing are characteristics of an SLR camera. So what? Why are those good things?

Why Might I Want an SLR?

The primary reasons for owning an SLR are quality and flexibility.

From a quality standpoint, the bigger body of an SLR camera holds a larger photo sensor than a digital P/S camera. That larger sensor area leads to cleaner image capture because of decreased interference or "noise" on the sensor. For that reason, an 8 MP P/S camera is not the same quality as an 8 MP DSLR, even though they capture the same number of pixels.

Beyond the size and quality of the sensor itself, the SLR improves potential picture quality by giving the photographer more control over how the image is captured. Everything from depth of field to white balance to flash power to metering mode can be controlled by the photographer. For those interested in learning photography, having the ability to control these parameters is invaluable.