Parallax error.
john.l
Ken Bour wrote recently
I have noticed and written a lot in RSB about the problems I have
experienced with eye sighting and perspective. I too have noticed
that I
get one line of aim when I stand back and look at the shot from a
distance.
If I hold that line, and move into the shot, just as you suggest, when
I
bend down into the crouch, the shot looks ALL WRONG. In my case, it
looks
like I will strike the object ball too far to the left sending it to
the
right of the pocket. The alignment that I get from a distance is
always
correct and, if I trust it, I will not be off by much. On the other
hand,
if I trust what I perceive from the crouch position, I invariably miss
the
object ball to the LEFT of the intended pocket. I have wondered WHY
this
happens: (a) perspective anomalies that everyone experiences, (b) eye
dominance and/or alignment, (c) near- vs. far-sightedness [I wear
contact
lenses], (d) confusion caused by the rail lines, (e) height of
chin/eyes
relative to aim line, (f) lighting affects, . . . I have never
received a
satisfactory explanation and, in fact, until your post, I don't recall
anyone even agreeing that they experience this same effect.
On the subject of "stepping into the shot", Jerry Briesath teaches
this
technique both in his video ("How to Play Pool Right") and at his Pool
School. He advocates getting a perspective on the shot from an
upright
position, lock the chin onto that line of aim, then move the body INTO
the
shot without losing focus on the sight line or moving the chin off the
target line. It is a very useful approach and one that I will pay
even
more attention to, but I still want to understand WHY the shots look
so
differently from the two distinct angles.
--
Ken Bour
Sterling, VA
http://www.erols.com/kbour
Parallax error is a term that is used to explain how, if an
instrument
is read when the eye is not at exactly 90 degrees square to the
surface,an error in reading will occur.
For example, if one reads the time on a wall clock, when standing
slightly to the left of the clock, at exactly 12 o'clock, the reader
might judge the time as slightly past 12o'clock, due to miss alignment
of the eye to clock hand position.
If the eye (dominant) was say 3 inches to the left of the clock centre
and 4 feet away the angle of parallax error would be quite small,
( dependant on how far the hands of the clock are above the
clock face), but if the human face is now moved forward to be
say 6 inches away from the clock face, with the eye still 4 inches
to the left of the clock centre, a completely different error would
result.
Might this be the reason for the apparent change in perspective which
Ken is experiencing ?
Best wishes
JohnL
R. Anne Mayes
>
> Parallax error is a term that is used to explain how, if an
> instrument
> is read when the eye is not at exactly 90 degrees square to the
> surface,an error in reading will occur.
> For example, if one reads the time on a wall clock, when standing
> slightly to the left of the clock, at exactly 12 o'clock, the reader
> might judge the time as slightly past 12o'clock, due to miss alignment
> of the eye to clock hand position.
> If the eye (dominant) was say 3 inches to the left of the clock centre
> and 4 feet away the angle of parallax error would be quite small,
> ( dependant on how far the hands of the clock are above the
> clock face), but if the human face is now moved forward to be
> say 6 inches away from the clock face, with the eye still 4 inches
> to the left of the clock centre, a completely different error would
> result.
> Might this be the reason for the apparent change in perspective which
> Ken is experiencing ?
>
> Best wishes
>
> JohnL
The phenomenon of parallax is the reason many instruments that use a hand
and dial for readings have mirrors on their faces. One lines up the hand
with its reflection in the mirror to insure correct readings from the
dial. In pool, parallax could most certainly be a problem as no such
"mirror" exists to enable one to verify the correctness of aim.
With instruments, the distance from the indicator hand to the dial
numbers/indicators is very small. In pool the distance from the eyes, to
the cueball and then to the object ball is far greater. The line of aim is
generally from the bottom on the cue through the cueball to the point of
contact (or aiming point) for the object ball. One has to "feel" the
correctness of the placement of the cue behind the head while he sees the
aiming point through the cueball. If you think of the cueball as the
"hand" and the aiming point for the object ball as the dial (sic: to be
read), the distance between the two offers ample opportunity for parallax
to be a problem. This phenomenon is probably enhanced by eye dominance.
I would like to see other comments on this subject because I do discuss it
with my students and would appreciate seeing methods for overcoming it.
Anne Mayes
--
Annie O's Billiard Pro Shop - http://www.annieosproshop.com
WBIA Master Instructor - Pocket Billiards - http://www.wbia.org
WPBA Tour Player Last Ranked 23rd - Retired Dec 1996
Ian Richards
Frank Callan discusses this in his book, where he recommends sighting using
the dominant eye, because theoretically, an even sighted approch can cause a
tendency to push the cue down the wrong line. However, most really good
snooker players use an even sighted aproach and it sems to work in practice.
I myself switched from using dominant eye sighting to an even sighted
approach a long time ago. When I read Callan's book I tried reverting to
dominant eye sighting. After a couple of days I reverted back to even
sighting because I could not get as good a feel for the shot using only one
eye.
john.l <jo...@lewis.co.uk> wrote in message
news:38b004dd...@news.dircon.co.uk...
Matthew R. Dickinson
This thread brings to mind a question I've been meaning to ask, but wasn't sure
where to start...
The classic test for determining eye dominance goes like this: With both eyes
open, hold up your thumb at arm's length so it covers a distant object. The
close each eye in turn; whichever eye is open when your thumb is still covering
the target is the dominant (looking through the weak eye causes the thumb to
"move" away from the target).
However, When I look at my thumb I see two "ghost" targets in the distance, and
when I look at the target I see two ghost thumbs, neither of which covers the
target. By this test, neither eye seems to be dominant. Does this mean I'm
"ambiocular" and thus predisposed to the even-sighted aiming method, or that I'm
simply crazy?
Ian Richards wrote:
--
Matthew R. Dickinson
Charles A. Long, Inc. 2600 Haddonfield Road Pennsauken, NJ 08110
Voice: 856 665-1665 Fax: 856 665-2024 email: CA...@erols.com
George C. McBane
>The classic test for determining eye dominance goes like this: With both eyes
>open, hold up your thumb at arm's length so it covers a distant object. The
>close each eye in turn; whichever eye is open when your thumb is still covering
>the target is the dominant (looking through the weak eye causes the thumb to
>"move" away from the target).
>
>However, When I look at my thumb I see two "ghost" targets in the distance, and
>when I look at the target I see two ghost thumbs, neither of which covers the
>target. By this test, neither eye seems to be dominant. Does this mean I'm
>"ambiocular" and thus predisposed to the even-sighted aiming method, or that I'm
>simply crazy?
If you're crazy you have me for company.
I see the same thing. When I shoot pool my left eye is over the cue. I'm
right handed.
This "dominant eye" business comes up both in pool and
in trapshooting, and I've never been able to make use of it
in either game. I quit worrying about it some time ago.
-George.
Wyo.Stick
eyes open. Chin in-line with stick. For proof of the
effectiveness of this,,, (and the easiest way too figure out if
your left or right eye dominant) pick up your closest (loaded
but on safty) handgun. Sight on an object accross the
room..With both eyes open.... Then close one eye.. if target
does not shift out of the "sight picture"..that is your dominant
eye. As long as you shoot with both eyes open... then, you are
always sighting with your dom. eye. So whats the problem???
.. guess if you were blind in one-eye....No that wont work
either... if ya only got one eye, its for damn sure dominate!!
Ha.
Paul
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MarkO
close one eye. That will likely be a better test than the arms length
thing.
Mark0 <--And for God's sake don't close both eyes--you're driving!
George C. McBane <mcb...@chemistry.ohio-state.edu> wrote in message
news:38b16a4b....@nntp.service.ohio-state.edu...
john.l
<ca...@erols.com> wrote:
>Good day all,
>
>This thread brings to mind a question I've been meaning to ask, but wasn't sure
>where to start...
>
>The classic test for determining eye dominance goes like this: With both eyes
>open, hold up your thumb at arm's length so it covers a distant object. The
>close each eye in turn; whichever eye is open when your thumb is still covering
>the target is the dominant (looking through the weak eye causes the thumb to
>"move" away from the target).
>
>However, When I look at my thumb I see two "ghost" targets in the distance, and
>when I look at the target I see two ghost thumbs, neither of which covers the
>target. By this test, neither eye seems to be dominant. Does this mean I'm
>"ambiocular" and thus predisposed to the even-sighted aiming method, or that I'm
>simply crazy?
>
There was a discussion on this point some time ago, under the title,
"Shotgun or Rifle" where it was considered how people aimed their
cues along THE LINE OF AIM ( I'm not shouting just emphasizing)
I feel that if one aims a cue, one must use it like a rifle for
precision aiming, but obviously on small tables with big pockets
one could use a genereral ,somewhere near, shotgun type aim.
If you aim with the cue on your chin, equi spaced between
the eyes, in order to focus on the tip, BOTH EYES (just
emphasis) must move inwards, squint, to see the tip, and then
both must move outwardly to see the object ball, and on each
subsequent focusing both eyes have to move in and out etc.
If the cue is aligned under one eye, assume the L eye, when focusing
the L eye does not have to move in and out ( squint ) and the R eye
view would be ignored by the brain. This is what happens with a
rifle. No one could argue that this is the best way to precisely aim
a
gun, or cue, but an infinite number of folk play very effectively
without worrying about how they aim.
When watching snooker on British TV, the camera men do a fantastic
job of getting right in line with the each shot so that the cue is
pointing straight at the camera through the cue ball.
This will seem very conceited but I can usually call if a miss
will occur before a shot is played, because one can see when the
but is not in line with the aim. The guys do not miss often, but it
can be called too many times for my predictions to be lucky.
Has anyone else noticed this?
Best wishes
JohnL