Dear Bastiaan,
The reason I suggest a reference spot on the line about 5 inches (12 cm) out in front of the ball is because of the size and shape of the spotlight of focal vision when looking down at the ball. The back of the eyes where the world registers its light is seen most sharply by a small spot on the back of the retina called the fovea. The "cone" light receptors are concentrated here for sharp daylight color detail vision. The cornea, lens, and shape of the eyeball all cooperate to focus light on the the retina depending upon what object is in the line of sight or gaze and the distance to the surface features of the object. When you look down at the top of a golf ball or the back of the golf ball, standing at address for a putt, your eyes are around 4 to 4.5 feet above the ball. This makes your lens adjust to that distance. Around the ball, visual focus fades rapidly as oibjects are more off to the side of your gaze's fixation point. The concentration of "cone" receptors diminishes like fewer stock traders the farther you get from Wall Street, and the number of peripheral-vision and dark-vision "rod" light receptors increases. Outside about 5-10 inches to the side of the ball, the grass is seen only in the vague periphery. The periphery is really more for seeing things that are MOVING and not really for seeing things (like grass spots) that are still. So keeping the spot close to the ball makes it easier to identify without moving your head or eye gaze to make sure you know where it is.
In more technical terms, visual accuity diminishes rapidly as the object of interest is located farther off the fixation point. The gaze fixation point directs the image to the fovea, where cones are highly densely packed in a small spot on the back center of the retina. This is where vision is "sharpest." If you stare at the bullseye of a dart board, the sharpest vision is of the bullseye. Visual accuity of rings in the periperhy diminishes quickly as the ring is farther out from the bullseye. This chart illustrates the idea with data relating visual accuity to "eccentricity" or angular separation out from the fixation point:
This chart shows diminishing visual accuity under conditions of vision in which the eye is adapted to the ambient light conditions:
Note in the second chart the falloff in accuity between the 10- to 15-degree point. From a typical setup eye height of 4 to 4.5 feet, this angular width of the "spotlight" of reasonably sharp vision extends about six or so balls to the side of the real ball. For more information, the
Webvision site has great information, especially about the
image distance out of the fovea.
Keeping the spot close to the ball helps with "divided attention" like placing two desserts on the table in front of you to choose which one of them to eat. It's fairer. Looking down at the ball and a close-by spot in front of the ball, you can look only at the ball and the spot is still fairly in focus and inside the small "spotlight" of focal vision. You can also look solely at the spot by a simple shift of the eyes from ball to spot on a gaze-shift path that stays along your skull line, and leaves the head orientation undisturbed. And, you can look at both the ball and the spot somewhat simultaneously with "shared attention" and diffuse focus, as if the focus of your line of sight stopped a little above both the ball and spot at the top of a squat triangle. Doing this shifts the brain from "what" the eyes are focusing on to the "where" relationships defined by the line from ball thru spot in relation to your skull line. That's a good thing.
In general, the distance of the spot from the ball does not depend on the length of the putt. While the spot needs to be a minimum distance from the ball in order for the size and shape of the ball not to overwhelm the spot, such that the sense of a "line" from ball to spot is not clearly perceived, once that point is reached, you are not adding much to widen the distance from ball to spot.
The image of the ball seen from 4 feet high has a certain "angular width" as an image occupying the 180-degree range of vision from left to right (about 2 degrees wide). A thumb held at arms length is about the same, so it appears to just cover the ball. The 5-inch distance from center of ball to spot makes a total image width seen from this position of about 6 degrees, or three times the apparent size of the ball. According to the charts above, the 6-degree point is about where visual accuity takes a tumble. So the spot, roughly speaking, is about three balls in front of the real ball. That's about right, as two balls would not quite be enough to give a good sense of the line connecting ball and spot.
It is probably worth mentioning that in miniature golf professional competition, the golfers always pick spots on the carpet in front of the ball, and these spots are not to far off.
Using a spot for help in aiming and alignment really involves three separate uses: seeing where to point the putter face thru the ball, seeing the "line" of the putt to square the body to, and seeing where to roll the ball on a start line. The real deal here is that if you aim the putter face accurately to begin with and setup square to the putter face and the line, you don't really need an aim spot for the purpose of rolling the ball "on line." The putter face itself in its square relationship thru the center of the ball defines the putt line all by itself. A line perpendicular off the putter's face at the sweetspot defines a line thru the ball that includes the backmost dimple on the ball's equator (which is closer to the putter face than any other part of the ball), thru the center of the ball (which is the same as directly below the top dimple on the ball), and the front dimple on the equator opposite the back dimple the same way the South Pole is opposite the North Pole on the globe (and this dimple is closer to the target than any other part of the ball). And since a putter face accurately aimed inherently defines the line of the putt, you ought to setup square to the putter face itself. Especially if you want to check the putter face aim thereafter by turning the head from putter face all the way to the real target, to make sure the face is aimed at the target. But any aim spot on the grass in front of the ball HAS to be on the line thru the ball defined by the putter face relationship to the ball, so why bother with the spot for starting the ball on line? It's not really needed for this purpose. Just putt the line defined by the putter face, which is what you squared up to.
In the case when I am unable to find an intermediate spot to aim the face or setup square with the body to the "line," I just focus on setting the face aimed straight at the real target and then setup the body at address in reference to the putter face itself and make the stroke in reference to the putter face itself.
Cheers!
Geoff Mangum
Putting Theorist and Instructor
Geoff Mangum's PuttingZone
http://puttingzone.com
Golf's most advanced and comprehensive putting instruction.
Over 610,000 visits and growing strong ...
518 Woodlawn Ave
Greensboro NC 27401
336.230.0612 home
336.402.1602 cell