Opening up your field of view is very important in kumite. All too often a fighter is staring at the opponents eyes, hands, or worse the feet, anticipating the attack. In reality the karateka must open up their vision to be able to see not only the eyes, but also the entire body of the opponent.
The eyes are a good place to focus the gaze as the eyes will often telegraph a movement or an intention before or immediately prior to an attack. The focus and the vision should not be constricted only to the eyes though. Instead, open up the concentration of your vision to gaze at the eyes but at the same time be mentally focused on the slightest movement in any part of the opponent's body. Sometimes the eyes do not telegraph the movement especially with skilled karateka. In this case the first shift of the body or the slightest movement of the hand or feet can be an indication of an incoming attack.
Opening of the vision is something that has to be practiced. Practice this when sparring with an opponent. When the opponent moves do not direct your eyes to the source of the movement but instead maintain your gaze on the eyes with your mental focus in the direction of the movement. This way your eyes are not unfocused off the opponent and then refocused in the new direction. Constant focus is maintained and the eyes continue to be a portal of communication for moves as they are executed.
Create an imaginary aura that surrounds your opponent's body and focus on everything inside this boundary. The boundary is created to be able to exclude distractions and background and focus only on the opponent. Disregard any movements or distractions outside of your imaginary circle.
I refer to the focus of one's eyes as a gaze. It is exactly that. You do not make a concerted effort to focus on only the eyes but in your mind you direct your focus to the entire body with the eyes as a reference point of where to direct the gaze. In this way any movement initiated by the opponent is seen.
Practice over time makes this easier and eventually makes your ability to track movement faster and more efficient.