Eye Focusing Exercise
One simple way to speed up your focusing ability is to practice changing focus between your computer screen and a picture hanging on the wall across the room, or a scene outside the window. You can also practice by looking from the dashboard to other cars on the street, or from a book on your desk to a bulletin board on the wall.
To do this exercise on the field, choose an object close by and another that's further away (but in the same or close line of sight). For example, you could stand near the back stop behind home plate. Put yourself in line with one of the foul lines and squat or sit down. Then switch between focusing on the plate and on the 1st base bag (or 3rd base bag if you're lined up with the 3rd base foul line).
Alternately there are strings of beads you can buy (or make) that are about 5-6 feet long. Have round beads on this string about the size of a marble and space them out evenly over the 5-6 feet of string. Have two players of about the same height put their end of the string just below their nose. They will then focus on the first ball, then go down the line to the end and back.