In a prior thread the complications in eye dominance were discussed (See
http://forums.azbilliards.com/showthread.php?t=297099 )
Several scientific studies were referenced and discussed. Probably the most important finding is that eye dominance shifts over distance to the target. The size of the image on the retina is the primary cause for this shift. The amount of shift changes for different distances.
While the player may have a preferred head placement the eyes will accommodate as needed.
I developed a method for determining head placement (relative to the cue stick) as referenced on Dr Dave's resource page but have since come up with a method that is better and allows for the complications discussed here.
Have the player place the cue stick on the long rail over the intersection between the table and the rail cloth. Sight down this line to the side pocket and see where the head is relative to the stick. Now have the player sight the stick to the corner pocket and determine head placement. Interestingly the head placement is not always the same for the two different sightings. The "best" head placement is usually relative to sighting the corner pocket,
In my opinion the player should sight the OB as they bend over (far point eye dominance needs) and look at the cue ball after they have bent all the way over.
Other studies have shown that eye dominance often (not always) shifts as one moves across the lateral field (from right to left and from left to right). For this reason the player should not sight a shot until they are behind the cue ball and take a step back. However, because eye dominance shifts with image size the player should not stop looking at the OB until they reach the shooting position.
These recommendations are based on studies of how binocular vision works in humans.
Given the vagaries of eye dominance over distance and lateral gaze I am of the opinion that placement of the cue under the chin (along the mid line of the head) is the "best" placement for the player (unless there is some physiological anomaly). We use the mid line for accommodation throughout life and the body naturally makes its adjustments relative to this line. However, there are many reasons for adjusting this reference point based on the body's needs and the physiological limits of each individual eye.
After reviewing the head placement used by several pro players it also appears that one needs to have the front plane of the face at a right angle to the cue stick. That is, no matter what the location of the head relative to the stick, both eyes should have the same 90 degree angle relative to the stick.
Incidentally, Earl Strickland is one of the few players who often (not always) sight length of table shots with one eye and sights close shots with two eyes. Obviously, there are all sorts of ways to sight a cue stick.
