That's when I first got tunnel vision
Tunnel vision is when you get so faded that it looks like you’re looking through a tunnel. The sides seems to black out.
It’s used here in a different sense, however. Krizzle’s tunnel vision gives him a sense of clarity. All of the other shit in his life has been put to the side of his vision, and now the only thing he sees are his rhymes.
In other words, his tunnel vision has put his eye on the prize.
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