The Mid-Side miking techniqe is a very cool miking setup that will give you a way to adjust stereo image width after recording. How does it work? You only need two mics, but they have to be mics with specific pickup patterns.
The first type of mic you’ll need is any mic with a figure 8 pickup pattern. What this means is a microphone that picks up on both sides equally. The silver ribbon mic seen in my posted pics is like this. Other mics with a figure 8 pickup pattern are the Beyerdynamic M130 Double Ribbon Microphone, or the adjustable BEHRINGER C-3.
The other type of microphone you will need is any microphone with a cardioid pickup pattern. This means the microphone is one directional and picks up in a sort of heart-shaped pattern in the area in front of the microphone. Most dynamic micrphones such as the Shure SM-57, or the Sennheiser MD 421 II will work for this. You can even use a condenser mic like the BM-800 I’m using in the 3rd pic above.
Then once you have the correct mics, you need to place them correctly. Whatever you are miking (in my examples it was drums) you need to place the figure 8 mic with one of the sides of the mic that does not pickup, so the figure 8 is horizontally placed in front of the instrument you are recording. Then you take the cardioid mic and point it at whatever you are miking and place it as close as possible to the intersection of the figure 8 pattern without actually touching the other mic. (see my example pics above).
When you open the two channels in your DAW, the first thing you will want to do is copy the figure 8 mic and put the copy on another channel. Then invert the phase of the copied channel of the figure 8 mic. Once this is done, pan the first fig8 hard left and the second fig8 hard right. Then keep the cardioid mic panned center. Keep the two figure 8 mics the same level. If you have the higher the cardioid mic is set in comparison to the fig8 mics, the narrower the stereo image will be. And likewise the higher the fig8 channels are in comparison to the cardioid the wider the stereo image will be. Some DAWs like Qtractor lack the ability to invert phase, but there’s an LADSPA plugin called inverter that you can put on the channel that will do the trick.
I’ve used this technique with great results as drum overheads, and have heard of people using it on guitar too.