There is an autofocus mode on most cameras called AI SERVO that is for moving subjects. Once you “lock on” something that is moving, it keeps refocusing as you track with its movement. So, the subject sets the pace for all the other elements in the shot and all those other elements become blurred in the background.
So it is with music, when one instrument is being featured as a solo instrument. As it moves through the composition, all the other players fade into the background and play more sustained notes to support the soloist. It’s almost as if they become “blurred” to allow the “focus” to become sharper on the soloist.
Another similarity between music and photography occurs when you consider lighting. Your eye is always drawn to the lighter portion of a photograph first, which is why you highlight the subject and reduce the light and/or vibrance on the background. Sometimes this is accomplished by putting a vignette around the outside.
With music, the supporting instruments always play at a lower volume and adjust according to the volume and energy of the soloist.
The “COOKED” version below is a good example of highlighting the subject and minimizing the background. You hardly notice the old abandoned hotel in the background and that it’s off the horizontal plane.