Bow sights are used to aid the archer to align the vertical and
horizontal position of the bow so that the arrow hits the centre
of the target.
The reference (sighting) line runs from the
archer's eye through the sight pin to the target centre. By
moving the sight pin up/down, side to side or backwards and
forwards the orientation of the bow is changed consistently with
respect to this reference line. The main elements of a bow sight
are represented in the attached diagram.
What recurve bow sights do not do is assist in keeping the axis of the bow in the vertical plane (or canted at some angle if required). Compound archers may use 'bubble levels' to help in this respect but their use is not legal for recurve archers. Recurve archers have to use skill to get a 'vertical' bow. It is the combination of the head position (tilt) and the angle a line running from the eye to the anchor point makes with the vertical that defines bow verticality. It think it helps if the eye-anchor point line is itself vertical but the position of the anchor point comes down to the shape of the archer's face as well as personal preference.
The aim of bow sight design is to enable a given sight pin position to be consistently reproduced, not to wander about from shot to shot and to enable controlled movement of the sight pin. Vertical and horizontal scales are usually built into the sight to assist with this as well as having a rigid sight assembly.
In order to hit a specific vertical point at some defined distance then you need to have the correct bow angle to the horizontal. By trial and error you find a suitable sight pin position for that distance so that when you line up your eye, the sight pin and the target you have the correct vertical bow angle. The two adjustments you can make are an up and down movement of the sight pin (on the sight bar) and to move the sight pin towards and away from the eye (vary the length of the side bar). It is the combination of up/down and forward/backwrds sight pin position that gives you the correct bow angle. Because it relies on a combination of two positions there are an infinite number of sight set ups that will give the correct vertical bow angle. They are not all equal as will be discussed later.
In order to define the bow position with respect to the eye/sight/target line you really need a second reference point on the bow, a 'backsight'. In principle you could keep the pin 'on the target' while rotating the bow any way you like with the sight pin being the axis of rotation.Compound archers often use a peep sight mounted in the string to provide a backsight so aiming becomes a mechanical operation. Recurve archers are not technically allowed to use two reference points. The second reference point is a combination of the head position/consistent anchor point and the 'sight picture'. In the sight picture you can subconciously (or maybe conciously) see the target, the pin, the bow limbs and the string. The archer learns to maintain the same sight picture from shot to shot e.g. the vertical position of the string with respect to the sight pin or bow limbs.
The following two diagrams illustrate how moving the sight pin up and down the sight bar or changing the length of the side bar (the eye to pin distance) affects the vertical bow angle.
If the sight pin is moved down the sight bar (red bow) then
the (blue) archer has to rotate his trunk to line up the sight
pin with the eye to target line - the important effect resulting
from this is that the bow angle with the vertical increases.
As the side bar is extended, to maintain the eye-pin-target
alignment, the sight pin has to move downwards. For example with
a reducing bow poundage (higher bow angle for a specific distance)
the pin has to positioned lower and lower down on the sight bar
ultimately causing a pin to arrow clearance problem. Reducing the
length of the side bar projecting in front of the bow locates the
required the pin position further up the sight bar regaining the
required clearance.
Changes in the horizontal angle the bow makes with the direction of the target , the 'windage', is effected by moving the sight pin towards or away from the sight bar and or by changing the length of the side bar. The following two diagrams illustrate these effects:
In the diagram moving the sight pin further away from
the sight bar (black to red) has the effect (LH archer
illustrated) of rotating the bow in the horizontal direction away
from the target direction. (the two target pins illustrated
should actually be in the same place).
In the diagram the 'windage' has been kept the same
while the side bar has been extended (black to red). As the side
bar is extended, the angle between the bow direction and the eye-target
line decreases. I.e. extending the side bar has the effect of
increasing the sensitivity of the windage adjustment position.
The 'no wind' pin windage setting can be affected by how the
archer's anchor point technique effects the eye position. The
tuning of the bow will also effect the basic pin windage setting.
If say the archer anchors on the side of the face and the head is not tilted to compensate then the eye is offset to the line of the bow/arrow flight and the pin has to be moved away from the bow line to compensate. (eye alignment in the above diagram). If the bow is mistuned, e.g. stiff in the diagram, then again the pin windage position has to adjusted to compensate for the fact that the arrow does not travel in the same plane as the bow.
How about the geometry of the sight bar and side bar with
respect to the bow? The sight bar is usually designed to be
mounted so its long axis is parallel to the bow string both in
the plane of the bow and at a right angle to this plane. If the
bow is held so that the arrow is horizontal (give or take nocking
point tillering) then the bow sight is vertical with respect to
the ground. As the bow is raised with increasing target distance
then the the angle the bow sight makes with the vertical
increases. At the same time the line from the archer's eye to the
target stays (on level ground) more or less horizontal. As the
following diagram shows as this angle with the vertical increases
the sensitivity of the pin position (up/down on the sight bar)
changes (L2 is bigger than L1 for the same vertical varation in
position). As you change the bow angle the sight mark gaps
between distances will change. If the sight bar is fitted at an
angle to the side bar then as you move the sight pin up and down
you are at the same time moving it towards and away from the eye.
With target archers they are generally shooting on level ground so the bow angle for a given distance will always be the same. However with field archers and bow hunters (say shooting out of a tree stand) they can often be shooting uphill and downhill. With respect to shooting on level ground when shooting downhill the correct pin position will be higher on the sight bar and vice versa shooting uphill because the arrow trajectory becomes asymmetric. A variant on the bow sight is the pendulum sight where the sight bar remains vertical irrespective of bow angle. This to some degree compensates for the uphill/downhill effect because as say the bow is angled down the pin is effectively raised compared to the pin position on a fixed sight bar.
In the other plane keeping the sight bar aligned with the bow string means that any up/down movement of the pin position will not change the effective windage setting.
If the side bar is not aligned parallel to the plane of the
bow then moving the bar forwards and backwards will have a
similar effect on changing the windage setting as the sight bar
misalignment illustrated above. Moving the side bar will change
the windage setting. In the other plane if the side bar is not
parallel to the arrow then you end up changing the angle of the
sight bar to the vertical as described above.
Whatever bow sight we use we do not want the arrow shaft or fletchings to hit any part of it on the way out of the bow. Most of this is catered for in the design. The bow sight is mounted on the opposite side of the riser to the arrow with sufficient clearance (we hope) that the only possible contact is with the sight pin itself or the pin support bar. As you go to longer distances the sight pin travels down the sight bar until it gets too close to shaft for comfort and the length of the side bar has to be reduced. It's not totally unheard of for an archer hitting low to lower the sight pin and then shoot the following arrow straight through the site pin (with catastrophic results - at least for the sight pin!). If you are seriously underpowered and run out of side bar length reduction then you can usually reverse the side bar in the mount moving the sight bar even closer to the archer's eye (between the bow and the archer) allowing the pin to be located even higher up the sight bar. In this situation in addition to contact with the arrow it's now worth thinking about possible contact between the bow string and the sight bar assembly. A bow string can move a significant amount forward of the bracing height position, maybe particularly for compound bows which have a lot of string wrapped round them wheels.
Can you 'optimise' the use of a bow sight? The answer is yes. Keeping the side bar as long as you can, while maintaining good arrow clearance, increases the distance between the front sight (the pin) and the back sight (your eye/anchor) and improves aiming because of the parallax effect. The length of the side bar also affects the amount of error in the arrow launch direction resulting from bad bow alignment e.g. variation in anchor point.
The diagram illustrates the parallax effect. If
perfectly aligned on the target then both the near and far sight
pins are aligned along the same line. If the alignment is off by
some amount then because the 'radius' of rotation of the far pin
is greater it moves further sideways appearing to track more
across the face of the target. The longer the side bar then the
more sensitive to movement the bow sight becomes - better
feedback to the aiming process. There is a limit to this. If the
side bar is so long that the pin is zooming about all over the
target face this becomes more of a liability than an asset. As a
general guide the side bar should be the length, at a specific
distance, that the pin 'floats' around in the gold.
If when drawing a bow you end up with the anchor point at the wrong point, although the eye-pin-target line may be 'acquired' the bow is at the wrong angle and when shot the arrow will miss the target centre by some amount. How far the actual bow angle is from the correct angle, i.e. by how much the arrow will miss, depends on the length of the bow sight side bar.
In the diagram 'L' is the length of the side bar, 'D' is
the distance from the bow to a perpendicular dropped to the
correct anchor point and 'H' is the length of this perpendicular.
If the bow is drawn correctly (good anchor point) then when the
eye, pin and target are aligned the angle 'A' is defined by the
line from the eye to the pin to the anchor point. If say the
anchor point is too low (by a distance 'DH') then this angle is
increased by an amount 'E'. Any misalignment of the bow can be
regarded as a rotation of the bow with the site pin being the
axis of rotation. The angle 'E' is the error in the bow angle (how
far it is from the correct bow angle).
If we assume (we hope!) that DH is small with respect to the length (L+D) then approximately E = DH/P
As P = Square Root of (H squared + (L+D) squared) and 'H' and 'D' are essentially fixed it can be seen that the value of 'E' for a given value of 'DH' decreases as the length of the side bar 'L' increases. The longer the side bar the more tolerance there will be to anchor point misalignment with respect to where the arrow hits the target.
The attached graphs estimate how far a 'typical' arrow will
hit from the centre, with an incorrect anchor point, at 90 metres
and 20 metres target distance as the length of the side bar
increases. 'D' is assumed at 70 cms, 'H' at 12 cms and DH is
assumed to be 0.3 cms. Bear in mind that there will be a maximum
side bar length after which you will get arrow clearance problems.
The maximum allowable value of 'L' will be greater at 20 metres
than at 90 metres.
For example at 20 metres distance, using a 30 cm side bar the
arrrow will hit about 1.25 cms closer to the target centre than
if a 12 cm side bar was used.
The basic approach to getting the best out of a bow sight is therefore straightforward. Use the longest length side bar you can at any specific distance. This occurs 'naturally' at long distances. At short distances, however, with many bowsights you end up with the pin up near the top of the bow window. If you shoot short distances get a bow sight with the longest side bar that it is practical to use (assuming it maintains adequate rigidity so it doesn't wobble about at full draw).
Last Revision 1 July 2009