I call a smart hitter someone who combines their physical gifts , with knowledge of hitting ,being good at recognizing pitch sequences, hitting in various counts, game preparedness like knowing the pitchers repertoire and tendencies.
Eyesight is very important in recognizing spin and processing that info in fractions of seconds to decide the swing path to make the ball hit the sweet spot and drive it. It's all done so quickly on every pitch and requires quick reflexes. and perhaps a quicker mind. Even then you have to gain experience and deal with your failures most at bats.
Imagine all the different ways hitters hold a bat ,all the different stances and swing mechanisms strides, toe tap. Ttsr can literally make millions of combined changes to how he swings the bat. For example where you hold your hands .It can be anywhere from waist high to above your head. Add that to all the ways to tilt the bat, stride . Then their are proper hitting mechanics.
Finding the right combo of things to do that unlocks you to be better is like finding a needle in a haystack for some and comes more naturally to others.
Personally I'd give prospects every eye test under the sun to test depth perception, color blindness, peripheral vision. Since livh colored eyes are more sensitive to light Id see if sunglasses are ever a for different colored eyes or if their are better sunglasses for artificial light like for night games.
It's said Ted Williams had 20/10 vision. But that's the extent most of us ever hear about vision. Test seems like an extremely important thing to do every year to every player.Slight worsening of vision could lead to bad results for hitters just as improved vision in sunlight and artificial light may improve hitting.
Hitting a baseball well is like sophisticated targetting systems in the military. It all starts with how well you see the moving target.