A fastball looks very white and clear, with the seams barely visible. A forkball also has a tumbling effect, but it appears more oblong than a curveball. New York Yankees third baseman Wade Boggs, a .334 career hitter entering this season, has two of the best eyes in baseball. Once your vision goes, everything else falls apart. You don't center the ball any more.
Line drives become popups.
That was enough for me so that I couldn't see the rotation on the forkball any more. He was fitted for contact lenses and everything was back to normal. After his average dipped to .259 in 1992, he has been back over .300 every year since. Boggs says he can read the seams on pitches. I see the rotation on the slider, the tumbling effect on a curveball.