Eyes and Vision in Pool and Billiards

... various concepts and techniques for pool shot sighting and visual alignment.

Dr. Dave's answers to frequently-asked questions (FAQs),
mostly from the BD CCB and AZB discussion forums

maintained for the book: The Illustrated Principles of Pool and Billiards,
the DVD series: The Video Encyclopedia of Pool Shots (VEPS) and
The Video Encyclopedia of Pool Practice (VEPP), the Billiard University (BU),
and the monthly Billiards Digest "Illustrated Principles" instructional articless



depth perception

Is depth perception important in aiming?

Assuming you have already visualized the required impact line and ghost ball target (see NV 3.1 and NV 3.2), aligning the cue stick with the aiming line direction does not require depth perception. (In fact, the parallax created by binocular vision, even with a dominant eye, can make straight-line sighting difficult.) Now, some people might adjust their aim while in their stance based on their perception of the "angle of the shot," impact line, contact point, and/or ghost ball target. In that case, depth perception might be helpful.

A low stance helps one better align the cue with the desired aiming line, without requiring too much of a shift in eye (or head) motion between the CB contact point the aiming target, while fine tuning and verifying one's aim.

For more info, see dominant eye.

Interesting, the following research study showed no correlation between depth-perception ability (or other vision attributes) and pool-playing performance: "Perceptual-Motor Characteristics of Elite Performers in Aiming Sports" by Abernethy and Neal (1992).

 

from Qtec:

The difference between short and long bridge is that the EYES are further away from the QB when you play with a longer bridge. This means that when you focus on the QB or OB, you will see more of the shaft [ie. a longer line ] and therefore its easier to line up the cue on the line on the shot.


dominant eye

Is one's dominant eye important in aiming?

This is debatable, and it might vary quite a bit from one person to the next. Now, head and eye position relative to the cue can be important in perceiving the desired tip contact point on the cue ball, especially if the cue tip is not close to the cue ball when lining up a shot. For illustrations and more information, see “Aim, Align, Sight - Part II: Visual Alignment” (BD, July, 2011).

3D visual perception, applied to aiming in pool, is a complicated topic that is certainly not fully understood. It is difficult to offer simple eye alignment advice (or a "formula") that will work for all people (e.g., always align your dominant eye over the cue or with the edge of the CB).

The most important advice on this issue is find the head position and sighting technique that helps create the most accurate cue tip contact point and aiming line (for a given individual). See the vision center resource page for drills and information on how to find the "vision center" head position. A person's "vision center" isn't necessarily related to which eye might be dominant or not. Regardless of where you place your head to aim and sight a shot, the most important thing is to be as consistent as possible (e.g., with a purposeful pre-shot routine), so the "sight picture" is always the same for the same type of shot.

Wikipedia has a good summaries and references dealing with dominant eye (including several methods for testing dominance) and related topics here:

ocular dominance
binocular vision

The following research study also showed no correlation between ocular dominance (or other vision attributes) and pool-playing performance: "Perceptual-Motor Characteristics of Elite Performers in Aiming Sports" by Abernethy and Neal (1992).

from RSB_FAQ:

For most people, one eye is much more dominant in seeing alignments than the other. Typically, right-handers are right-eyed, and vice versa. About 5% are "cross-dominant" (e.g., right-handed and left-eyed) and some are "ambi-ocular" (no dominant eye). To aim and sight well, it helps to locate your dominant eye directly over your cue. For cross-dominants, this may call for some adjustments in stance or neck/head angles. For ambi's, the stick will be under some spot between the eyes.

Here's how to test yourself: Hold your thumb up at arm's length, visually blocking some distant object (for example, a clock or a lamp). Don't focus on your thumb; focus on the distant object. You'll see a ghost of your thumb, since your dominant eye will be in line with both your thumb and the distant object, while your non-dominant eye will be seeing past your thumb, directly toward the distant object. With one eye seeing the thumb and the other not, you get a ghost. The ghost is centered on the distant object because your dominant eye is the one that tells you what's lined up with what.

So, when you close your non-dominant eye, the thumb becomes solid instead of ghostly, since the dominant eye is looking directly at the thumb. When you close your dominant eye, the thumb appears to jump to the side because the dominant eye (that was making the thumb line up with the distant object) is not in use.

Stroke into a mirror to see where your dominance spot is, relative to your shaft. It "should" be directly over the shaft. If it's not, but you're not having difficulty aiming or sinking balls, don't worry about it.

from henho:

... there is plenty of research on eye dominance in Medline or Pubmed, a couple of thousand articles to be exact....just one for example: A new interocular suppression technique for measuring sensory eye dominance.
Yang E, Blake R, McDonald JE.Invest Ophthalmol Vis Sci. 2009 Jul 23. PMID: 19628736

Each eye supplies information to the brain differently. The dominant eye is used to fixate on objects. If you look at a point, the fixation of the dominant eye will fall on the center of the point while the non-dominant eye will fall slightly off-center. By comparing the difference between the images supplied by each eye the brain is able calculate depth information in a scene. The degree to which any eye is dominant varies from person to person as well.

In scenarios where estimating depth is important, the functioning of both eyes in tandem is vital. I don't recommend skiing with an eyepatch any more than driving. However, in scenarios where determining a straight line between two points is needed, using the dominant eye confers an advantage over using both eyes. When shooting a gun, you want to align the sight of the gun with a target. If you kept both eyes open while sighting down the barrel of the gun, when you looked at the sight you would see two targets, and when you focused on the target you would see two sights. In this scenario, the depth information your brain is supplying you is not helpful in aligning the target and sight, and it is easier to sight with one eye.

Pool is an interesting scenario in that it requires good depth perception to estimate the line of sight, generally done while standing up, and in addition requires accurately lining up the cue ball with the contact point on the object ball. If you adopt a high stance and hold the cue under both eyes, you have the best depth judgment, which is needed to estimate the speed and direction of the hit. If you adopt a low stance and use your dominant eye, you have the best ability to line up the cueball and object ball. Most people go somewhere in between, where they can line up the balls while still having a general view of the table to guide the speed of their stroke.

This seems to come into play in the nature of various cue games. In three-cushion billiards players generally adopt a more erect stance that provides a good, continuous 3-d view of the table, allowing them to accurately predict the reaction of the balls. In snooker, the accuracy requirement that smaller balls present requires players to adopt a low stance sighting down the cue. In this game moreso than in billiards, players must first estimate what will happen on the shot and determine the line of sight, then remember the layout of the table when they are down on the shot since they don't have a good view of the balls.

from JoeW (in AZB post):

Kahn, AZ and Crawford, JD. (2001). Ocular dominance reverses as a function of horizontal gaze angle. PubMed.
In a reach-grasp task for targets within the binocular visual field, subjects switched between left and right eye dominance depending on horizontal gaze angle. On average, ocular dominance switched at gaze angles of only 15.5 degrees off center.http://www.ncbi.nlm.nih.gov/pubmed/1...?dopt=Abstract

Quartley, J and Firth, AY (2004). Binocular sighting ocular dominance changes with different angles of horizontal gaze. PubMed.
A change in eye dominance occurs when viewing in the contralateral field. Differences may exist in the angle at which this occurs due to the different conditions of the various tests for ocular dominance. http://www.ncbi.nlm.nih.gov/pubmed/1...?dopt=Abstract

The research summarized above indicates that eye dominance can and does change as one moves into a pool shot from across the horizontal field. For some people when they move into the shot making position from the left or from the right their eye dominance shifts as they look at the sighting point.

The obvious solution to this shift in dominance is to walk into the shot from behind. While you may be looking at the shot from the side as you walk to it, you probably should not begin to sight the shot until you are in position and take a step back. This will place you on the shot line, yield a better perspective, and will give your eyes and their dominance preferences time to adjust to the visual field directly in front of you. This type of recommendation has been around for a long time and is now reinforced by the research literature.

Whether you choose to place the cue under the dominant eye or allow the brain to compensate for the dominant eye is another choice. The research literature seems to imply that shift in dominance in the lateral and contra-lateral field is not an invariant process across people: Some people experience more of a shift than others. If you have noted this shift in dominance and or a change in dominance then it is probably more important for you to step back when on the shot line and look directly in front of you prior to sighting.

A more extensive literature review on this topic would probably yield even stronger evidence as we are dealing with a physiological process offset by mental preferences. Pool instructors should seriously review this literature and it probably should be a standard recommendation based on solid research findings. While some people may not have a lateral shift in dominance the recommendation to take a step back allows one to gain a better perspective.

from Billy_Bob:

Here is more reading about the dominant eye...

Ken Tewksbury, Master Instructor (See #6)...
http://www.tableskills.com/article-kt2.php

Robert Byrne: "If you are having trouble pocketing balls, it may be that you aren't bending over far enough and aiming the cue like a rifle with your dominant eye."
http://www.byrne.org/pool/tips/08-1998.html


eye pattern "best practices"

What is "eye pattern," and what are generally recommended "best practices" for eye pattern before and during the stroke?

"Eye pattern" refers to how you move your eyes before and during a shot.

Concerning generally recommended "best practices," see stroke technique advice. In particular, see steps 1-4 in the stroke "best practices" document.

Regardless of which eye pattern you choose, it is important to have quiet eyes when aligning your cue with the desired line of aim, and during your final delivery.


finding the center of the cue ball

What's the easiest way to ensure the cue tip is aligned with the center of the cue ball?

First, it is very important that your vision center is aligned with the cue so the tip will look centered when it is centered. It also helps to position the cue tip as close as possible to the CB in your "set" position to minimize any perspective perception problems resulting from not being aligned with your vision center. It also helps to deliberately focus on where the cue tip is relative to the CB center for a deliberate amount of time during the "set" position, with quiet eyes. It can also help some people to key off the resting point of the CB on the cloth. This can help you more accurately visualize the vertical centerline of the CB.

The drills suggested and demonstrated on the MOFUDAT page can also be very useful to diagnose and help correct alignment and stroke flaws. Joe Tucker's Third Eye Stroke Ttrainer can also be helpful to some people.

 

from JoeW:

I have noticed that many players begin the table aim (when they are down on the CB) by placing the tip of the cue on the table. Many players from different countries use this method as part of the aiming process and it seems there may be a reason for it. Perhaps it does help with aiming.

Apparently, when you place the stick on the table oriented through the center of the CB to the contact point, the tip is something like a pointer and you can evaluate the relative distances to either side of the CB from the center of the tip. It is easier to find center and then move relative to this place.

There is a second advantage to this method and this involves the perception of center as seen through the CB to the contact point. With the stick out of the way [there is] an unobstructed view.

There [also] appears to be a third advantage. When the stick is initially placed on the table and the center line is sought, there is a definite tendency to place the stick on this line. The use of FHE and BHE is now relative to this line of travel for the CB to OB and it is easier to determine what needs to be done relative to a line that can be visualized as opposed to an estimated line.


quiet eyes

What does it mean to "quiet eyes" when aiming a shooting a pool shot, and why is it helpful?

"Quiet eyes" refers to focusing on a target with still eyes for a period of time. Studies have shown that top athletes in many sport activities (e.g., golf putting and basketball free-throws) do this. When the eyes are moving and/or are not focused long enough on a well-defined target point, line, or area, performance has been shown to be not as good. Various "quiet eye" resources can be found here:

Keeping your eyes "quiet" both in the set position (while looking at both the CB and at the target, separately), and during the final swing (while looking at the target) is important for consistency and accuracy. Pauses give your eyes time to settle and become quiet. In the set position, you want to alternately verify both the desired tip contact point on the CB (with quiet eyes) and the aiming line and target (with quiet eyes). And before starting your final forward swing (e.g., during the final backswing or backswing pause), you want to have quiet eyes and complete focus on your target point (e.g., the GB center, the OB contact point, a point on the rail, etc.).

Here's a good article dealing with golf putting, but the concepts also apply to pool:

The following research study also looked at potential correlations between vision attributes and pool-playing performance: "Perceptual-Motor Characteristics of Elite Performers in Aiming Sports" by Abernethy and Neal (1992). The study concluded that there is no correlation between various optimetric measures (dominant eye, depth perception, phoria, fixational disparity, etc.) and pool-playing performance.


sighting

What is the "conventional wisdom" and "best practices" concerning how to sight during aiming?

See “Aim, Align, Sight - Part III: Sighting” (BD, August, 2011). It covers this topic fairly well.

Regardless of what method you choose, the most important part is to be as consistent as possible for each type of shot!!!  If you are consistent, your vision and brain will develop to "see" the correct line of aim for every shot.

By "where do you sight," I mean: where do you align your vision center, which is the head alignment that allows you to see a center-ball straight-in shot as straight, with the tip appearing to be at the center of the CB.

An obvious option is to always align your "vision center" with the center of the CB, regardless of the type of shot.  This way, you learn to see all different shots, and how they vary, from the same perspective.

Mike Page's videos on aiming and sighting present excellent background and insights on this topic.

NV B.3 - Mike Page's aiming video (part 1, part 2)

He points out that the only sighting that makes sense for a straight in shot is over the cue through the center of the CB.

He also points out that with a thin cut, the sighting line that makes the most sense is also along the contact-point-to-contact-point line (which will be very close to the edge-to-edge line for a really thin cut).

Then he suggests that maybe you should also sight along the contact-point-to-contact-point line for all shots in between (any cut shot). He also makes a good argument that you should probably never sight along a line that is not parallel to the aiming line (which is along the cue for a center-ball hit).

Another alternative is to always align the inside eye of the shot with the inside edges of the CB and ghost ball, so you can more clearly see the ball-hit fraction (i.e., ball "overlap"). In this case, with a thin hit, you will be aligning nearly edge to edge (ETE). Some people recommend sighting exactly ETE with thin hits. An alternative here is to temporarily shift your head and close one eye to get a look at the ETE line, but then re-center your "vision center" over the cue during your final set and stroke.

Again, regardless of how you align and sight, the important thing is to align and sight the exact same way for each type of shot.  That way your brain is seeing the same picture for the same type of shot every time.

It is also very important to find your vision center and make sure you align it perfectly with the cue for straight in shots.

It is also very important to make sure the cue tip is aligned with the center of the CB when you don't intend to apply english (see finding the center of the cue ball).

from Patrick Johnson:

The things you want to align for aiming (CB/OB contact points, CB/OB fractions, etc.) are rarely on the same line as your stick - they're separate lines. So when you center your vision directly over one line, the other line is necessarily off to the side a little and therefore harder to line up precisely - i.e., if you center your vision over your stick it's more difficult to be sure you're lining the contact points up precisely and if you center your vision over the contact points line it's more difficult to be sure you're lining your stick up precisely. It's a tradeoff.

So I don't think it's a given that one way is always better than the other. I'm not even sure it's necessary for a player to do it the same way every time - maybe some shots lend themselves better to one and some to the other. For instance, thin cuts and shots with lots of sidespin might lend themselves more to sighting along the contact points line, while thicker shots with less spin might be best sighted along the stick.

What's best might also change with the player - some might see the alignment better by favoring the stick while others might favor the contact points line, like how different players have different centers of vision because of eye dominance.

 

Is aiming a pool shot the same as aiming a rifle?

from Patrick Johnson:

Aiming a rifle and aiming a cue stick are similar, but they're not the same.

There's only one place for anybody's eye (singular) when aiming a rifle, but that doesn't mean there's only one place for everybody's eyes (plural) when aiming a cue stick or aligning balls in pool. The difference is that when aiming a rifle you're using only one eye and you can get your eye directly behind and in line with the rifle sights and the target, but when aiming a stick or balls you use both eyes and your eyes are always above the line the stick and balls are on.

You use monocular vision to aim a rifle, but binocular vision to aim in pool, and the two are very different. With binocular vision from above it isn't necessarily best to have one eye directly above the things you're trying to align - sometimes, or for some people, it's best to have the things you're trying to align somewhere between the eyes because that's where your brain assembles the twin images from your binocular vision to present the truest single image (mostly due to which eye is dominant and how strongly dominant it is).

vision center

How can I determine the head position and eye alignment necessary for me to have the best aiming accuracy and consistency?

Aim, Align, Sight - Part II: Visual Alignment” (BD, July, 2011) covers this topic fairly well, as does the following video from Disc I of the Video Encyclopedia of Pool Practice (VEPP):

 

Your vision center is the head and eye alignment, relative to the cue, that allows you to see a center-ball, straight-in shot as straight, with the tip appearing to be at the center of the CB. For some people, this might be with the cue under their dominant eye (if they have one). For others, it might be with the cue under their nose, or somewhere else between (or even outside of) their eyes. To be accurate and consistent with both straight-in and cut shots, you should always position your "vision center" over the desired aiming line for the shot. See shot sighting for more information (and other approaches).

In general, it is best to have your head as square to the shot as possible, and with the eyes as level as possible, in the "vision center" position. For more information, see Diagram 4 and the pertinent discussion in “Aim, Align, Sight - Part II: Visual Alignment” (BD, July, 2011).

A useful technique to find your "vision center" is to set up a straight-in shot (e.g., with the CB and OB along the diagonal of the table, with the OB close to a corner pocket and the CB close to the middle of the table) with a cue shaft carefully laid down on the table centered in front of the CB and perfectly aligned with the straight line of the shot. Then position your head over the shaft close to the place and orientation your head would be while down in your stance. Then move your head left and right, without tilting or turning, until the tip position looks centered and the shaft and shot alignment look perfectly straight. This head position is your "vision center." Here's a good demonstration of this approach, using mechanical bridges (snooker "rests") to support the cue:

A good way to test your "vision center" is to use the MOFUDAT and related drills. This will test if you are aligning with the center of the CB and if you are able to consistently send the CB along a straight line. Another useful drill is the cut shot drill (F6) in Exam I of the Billiard University (BU) playing-ability tests. If you tend to miss shots in one direction more than the other, you might want to recheck your vision center position.

Another good drill to help you find and test your "vision center," per the first video above, is to set up a long straight-in shot into a corner pocket, marking the CB and OB positions with self-adhesive hole reinforcements (AKA, a "little white donuts"). Then hit stop shots. If the CB has no sidespin after hitting the OB, and if the OB goes into the center of the pocket consistently, then you have your vision center properly aligned (and you have a good stroke). If not, then shifting your head will probably help. If the CB consistently goes to the right of target, causing the OB to go left of the pocket, your left eye is probably dominating the perception of the line of the cue. This causes you to position the cue a little to the left of center and to pivot the cue a little to the right of the desired line of aim. With your eye alignment to the left of center, you perceive the shifted and pivoted cue position as centered and straight, but it is not. If this is the case, try shifting your head to the right a little, and then try another set of stop shots. If you are consistently missing the CB target to the left instead, with the OB missing the pocket to the right, try shifting your head to the left. After you find the head alignment that results in the best accuracy (i.e., no CB spin, and OB in the center of the pocket), you will have found your vision center. When you are done, try to remember how the stance feels and/or where your nose or chin is relative to the cue (e.g., by lowering your chin to touch the cue and/or by glancing down at the cue with your eyes, keeping your head still).

When playing, you can verify that your vision center is aligned properly by touching your chin to cue if your stance is low enough to do this. Otherwise, when down in your stance, just glance down with your eyes (with the head still) to see where the cue is relative to your eyes and nose. Obviously, you want the cue in the same place as it was when you determined your "vision center" with the drills above. Regardless of what you use as your "vision center" position, the most important factor is being as consistent as possible so your brain can learn to judge the full range of shots from the same perspective.

For more information, see "VEPP – Part I: Introduction and Fundamentals," (BD, April, 2012).

Here's useful 2-page visual summary of "vision center" concepts and drills.

from Neil:

Take an extra shaft and lay it on the table pointing away from you. Get down in your stance with your cue with your bridge hand at the end of the spare shaft. Place your cue over the shaft. When you see equal parts of the shaft on each side of your cue, you then know you are seeing a straight line. That is where your head should be.

from JoeW:

Photo 1 shows the table with a line drawn from the head to the foot of the table on the center diamond. I used a dressmaker’s marking pencil (cost $1.00 at Walmart) to draw the line. I left the pencil on the table for this photo.

For a ruler I used an 8’ piece of metal wall board corner molding ($2.00 at Home Depot). Note that the line goes from one end of the table to the other.

Photo 1
vision center experiment - part 1

The balls are set with the base of the One ball on the line (see Photo 2) with the center of the numeral One centered on the line. If you look over the top of the One ball you can see the other side of the ball and that the number One lines up with the white line. Go to the other end of the table and set the Three ball in the same way.

The balls are now lined up perfectly. If you hit dead center on the One, stop on the Three ball the three ball should bank off the end of the table and come back and hit the One ball – a really tough shot.

Photo 2
vision center experiment - part 2

Ok so how should you aim? Well first lets get the bridge hand out of the way. To do this I reversed the table bridge and set it where the stick will line up perfectly with the center of the One ball. See Photo 3. I am holding the table bridge with my left hand and site with my eyes and the right hand. Here is the interesting part of this technique.

As I look down the shaft and line up from the One to the Three I get what I think is a good site picture. Notice where your nose is relative to the shaft of the cue stick and this is your site picture.

Now raise up a little bit and you can see over the Three ball to the head rail and you can see the white line. If everything is lined up then you are straight for your site picture. If not, something has to change.

Shooting the shot by stopping on the Three ball and watching the return from the rail will tell you if you site picture is the least bit off. This is a difficult shot and you have to be lined up perfectly. All I can tell you is --- things will change.

Please try it. By the way, you will learn if you shoot most accurately with one eye or with two eyes.

Photo 3
vision center experiment - part 3


which ball to look at last

Which ball should I focus on during the final stroke?

Best results can vary from one person to another, but it is generally recommended to focus on the object ball (or the center of the ghost ball, desired contact point, ball-hit fraction, or a point on the cloth or cushion) instead of the cue ball during the final stroke. Possible explanations or reasons include:

For shots where the cue-tip contact point is especially critical (e.g., jump shot, masse shot, shot with extreme english, when elevated over a ball, when the CB is frozen to a rail), focusing on the CB tip-contact point (instead of the OB target) during the final forward swing can be more effective for many people.


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