When recording acoustic piano, if you are lucky enough to have one, the first thing you need to consider is the room the piano is in.
While someone is playing the piano, walk around the piano, if you can, and listen to how it sounds in different locations around the piano. The technique you use will depend on if you want more of the rooms acoustics in the recording or less of the rooms acoustics in the recording.
The type of microphone you use when recording a piano is normally a condenser microphone, large or small diaphragm. The pattern and type will depend on the sound you want and if you want the rooms acoustics in the recording. The microphone position will also depend of the rooms acoustics. So back to the first thing to consider, the room the piano is in.
Recording Piano technique to include the rooms acoustics.
The farther away the microphone is placed from the piano the more of the room will be recorded. So, to include more of the room in the recording place the microphone up to 6 feet away. The farther away the microphone is from the piano the more of the rooms acoustics will be recorded.
Also, as the sound from the piano tends to travel upward, as you move the mic away from the piano it also needs to be positioned higher.
Recording Piano technique so the rooms acoustics are not recorded.
If you place the microphone about 6 to 12 inches from the hammers of the piano less of the rooms acoustic will be recorded. The position will depend on how many microphones you are using.
Using One microphone
Only use one microphone if you don't have two or more but if you are using one then place the microphone
around middle c on the piano. If recording more of the room then place the microphone where the piano and rooms acoustic sound best. (Remember the first point to consider.)
Using two or more microphones
If you can, use two or more microphones when recording piano as you can record the full range of frequencies of the piano and, it will be easier to get the sound you want when you are mixing the piano recording.
A few suggestions.
Place one microphone above middle c pointing toward the high notes of the piano and place the other microphone below middle c pointing toward the low notes of the piano. This will create a stereo image of the piano, so the recording will sound as if you are sitting at the piano playing it.
You can use the same technique when using three microphones but, place one mic at middle c, one mic at the low notes and one mic at the high notes.
Another technique is to place one microphone at middle c about 6 to 12 inches away and place the other two microphones away from the piano about 2 to 6 feet away. You can get the best of both sounds, the rooms acoustics and the pianos natural sound using this technique and you will produce amazing results.
The last point is the pattern to use when recording acoustic piano. This again will depend on the rooms acoustics and, if you want it in your recording. You can't remove it after it has been recorded.
Cardioid will mainly record just the piano and if you want more of the room recorded try an omni or figure-8.
It is important to try different positions, different patterns and different types of microphones when recording acoustic piano And experiment with one, two or more microphones. There are many techniques you can use when recording a piano, so, experiment and get the best sound you can and, the sound you want.