While it's true that most of the on-board microphones on HD DSLRs provide stereo sound (excepting a few that are monaural), their utility is limited by their fixed position on the camera. They also lack control over incoming audio and have a tendency to pick up the inner workings of the camera and the lens as part of the video's sound.
The best way to avoid these technical shortfalls is to use external microphones. Though a few of the current video-capable DSLRs offer gain control, which boosts or reduces the level of sound coming in—some even include an automatic gain that can override the delicate audio being captured in some situations with superfluous noise—external microphones typically give you much more sophisticated options for working with incoming sound and volume. With an external mic, audio is much cleaner, and it's also more likely to be free of the ambient noise of the camera since audio is coming from an external source designed to pick up sound from a specific area.
Thankfully, there are a variety of solutions that meet the specific needs of HD DSLRs, and here we look at a few of the most basic and common mic configurations. A few words of advice when making a selection: Check to make sure that your camera has a stereo input before purchasing a mic; not all of them do. Also be aware that longer onboard mics can protrude into the angle of view of a lens when using wide-angles, especially with fisheyes lenses.
MIC OPTIONSThe PRO 24-CM camcorder microphone from Audio-Technica includes paired cardioid elements in a basic X-Y configuration, which is angled at around 90º to each other for ideally capturing stereo sound in a two-mic setup. The PRO 24-CM mounts to the hot-shoe of the camera. It's fairly lightweight at 3.9 ounces and includes a removable windscreen. Estimated Street Price: $69.
Perfect for portability, the Azden SMX-10 is built to be compact at a weight of just under 2 ounces and a length of 7 inches. There's a lo-cut switch for reducing ambient wind noise, and the mic is powered with a single AAA battery. It includes a windscreen and hot-shoe mount. Estimated Street Price: $65.
Built to be compact at 6 inches and light-weight at 3 ounces, the Røde VideoMic Pro shotgun mic is an on-camera stereo mic that attaches directly to the hot-shoe or to boompoles. Powered by a 9V battery, the mic features a foam windshield, a two-step high-pass filter for reducing ambient noise, a three-position level control for setting gain and a shock mount for isolating the mic from vibrations that could affect sound. Estimated Street Price: $229.