When practicing, I noticed that in forte scale runs I always reach the bottom limit of every key I press, but when playing the same run piano or pianissimo I don't always reach the bottom limit.
So, my question is: How important is to reach the bottom limit of the key when pressed? Should the distance vary depending on dynamic (piano, forte, etc)? Is there an ideal/optimal height the key should end when pressed?
This happens on both MIDI keyboard and acoustic piano.
The let-off on an acoustic piano is adjustable. Your tuner can show you how to do it. It determines how softly you can play.
The answer to your question is: it depends on how the let-off is adjusted.
An interesting thought !On all of my acoustic pianos, so it's not just one, a key can be pressed down less than a quarter of the way to make the note sound.HOWEVER, if that key is pressed in the conventional way, straight downwards, it may or may not make a sound.Pressed all the way gently, no sound. Pressed with more force, sound. Mostly with volume commensurate with the force applied.By using a different technique to depress a key, rather like a claw-like movement, thus giving more impetus to the key and hammer, the key only needs to be depressed by 25% for the note to sound.This movement is to touch a key and slide the finger off the end of the key, quickly.I blocked under the edge of the key so it could not go down more than stated.Range of volume is available. It's not so much the depth that is important, than the velocity to start the hammer moving. Once it starts to move, the extra depth the key goes to has little effect.
Digital pianos usually have a completely different mechanical arrangement, so there is no simple direct comparison.
If you find that sometimes the notes don't sound when not hitting the bottom, then it's important to hit the bottom. If this isn't the case, then it doesn't matter - unless you're interested in playing unfamiliar pianos regularly.
The way to play softly is with a slower action, or with the soft pedal, or both. Different pieces may require different methods of playing softly, but not pressing all the way down is not a recognised technique for playing softly, probably because that's pretty difficult to do deliberately and consistently.
I'm not one to suggest there's a right or wrong way, but when I do it, the keys go all the way down, except by mistake (which occasionally happens with very short notes, regardless of whether trying to play soft or loud).
Playing to the bottom of the keys no matter how loud or soft makes for a solid and consistently even tone that you can control much better than one created by haphazardly pressing notes at various pressure levels.
When playing it's important to use more than just your fingers to produce the tone, and this is done by putting some force behind the moving using your arms and shoulders. By doing this, you'll find it's very easy to play both loudly and softly without working too hard at it.
Keep in mind that this won't happen instantly and like everything else with the piano, you need to practice and use it all the time so the movements become natural.
