When I peer into my camera's ground glass I am intent on one thing. Sharp focus.
When I check the clock for my 9:45 meeting I focus on the second hand and ignore the hour. I can tell you if I am late or early but not what time it is.
When we narrow our focus in the listening room it is at the expense of the big picture. I might pay particular attention to one area of music like the tonality or imaging and not notice the highs are missing.
This filtered attention helps narrow down specific problems at the expense of the whole.
Some of my most insightful observations of a stereo system came not from narrow focus but by the off-handed listen to the whole without preconceptions.
Ground glass viewing is perfect for razor-sharp focus, though it's often at the expense of the picture.
