What a sight tape is doing
An arrow leaves the bow along a line that has to be tilted upward if it is going to arrive somewhere other than the ground in front of you, and the further away the target is, the more tilt it needs. A sight pin is a way of measuring that tilt. Sliding the pin down the bar for a longer shot raises the bow, and how far down the bar it has to go for a given distance depends on two things: how much elevation the arrow needs, and how long the lever between your eye and the pin is.
The first part is trajectory. Ignoring drag, an arrow launched at speed v needs an elevation angle where the sine of twice that angle equals gravity times the distance divided by the speed squared. The second part is geometry: for a small angle the pin moves down the bar by roughly the sight radius times the tangent of that angle. Long radius, more movement per yard of distance, finer control and a longer tape. Short radius, everything compressed into less bar.
Why two marks beat a chronograph reading
You can build a tape from a speed figure alone. It is just not a very good one, because the speed number is doing all the work and it carries every error it accumulated on the way to you. Two real marks, well separated, contain the same information in a form that has already been through your bow, your arrows and your release.
The calculator uses them by asking a different question: rather than trusting the chronograph, it searches for the speed at which the model reproduces the gap between your two marks. That fitted speed then generates everything else. It is not the true muzzle speed of the arrow — it is the speed that makes a drag-free model behave like your actual setup over that span, which is the more useful quantity. Take the two marks from distances that are far apart, because two marks close together contain almost no information about the shape of the curve.
Where the far marks go wrong
Errors in the speed figure barely show near the distance you calibrated at, and grow steadily as you move away from it. Suppose you calibrate at twenty yards with a bow that is actually five feet per second faster than you think. At forty yards the model puts the mark a fraction of a millimetre out, which is nothing. At eighty it is out by several times that, and because the marks pack closer together at the far end, a small error in millimetres is a larger error in yards.
| What is wrong | Where it shows | What to do |
|---|---|---|
| Speed a few fps out | Far marks, growing with distance | Calibrate from two widely separated marks rather than a chronograph |
| Sight radius mismeasured | The whole tape stretched or squashed | Measure eye to pin at full draw, not riser to sight |
| No drag in the model | Far marks, always in the same direction | Shoot the longest mark and let it correct the tape |
| One known mark was a bad day | Everything, silently | Use marks you have confirmed more than once |
There is also the flat honesty that this model has no air in it. A real arrow is slowing down the whole way, so it needs slightly more elevation than a drag-free calculation says, and the shortfall grows with distance. That is one more reason the far end of any calculated tape is a starting point to be shot in rather than a result.
Point-on, and the gap underneath it
Barebow archers shooting without a sight use the point of the arrow as the aiming reference. Because the eye sits above the shaft, the line from your eye over the point declines slightly relative to the arrow itself, and at close range the required launch elevation is smaller than that decline. The result is that the point appears below the centre of the target and you hold it there, which is the gap.
As the distance grows, the required elevation increases until it exactly matches the decline of the eye-to-point line. At that distance the point sits on the centre of the target and there is no gap at all. That is the point-on distance, and past it the point appears above the centre and the gaps reverse. Two measurements set it: how far your eye sits above the shaft at anchor, and how far it is from your eye to the tip of the arrow at full draw. Both are awkward to take accurately, so treat the calculated gaps as the shape of the chart rather than the chart itself, and correct them on a target face.
Using it without pretending it is finished
The workflow that works is: build the tape from two confirmed marks, shoot the middle distances to check that the model is behaving, then shoot the longest distance and adjust. If a change to the arrows is coming, make it first — a heavier arrow moves every mark, and the mass and speed involved are the same inputs as on the kinetic energy and momentum calculator. If the arrows themselves are still being chosen, settle the spine band and the weight and FOC before you print anything, because a tape belongs to one set of arrows rather than to a bow.
How far the distances on your tape need to run is set by the ground rather than by the sight, and the layout of that ground is a separate question covered by the range layout calculator.
Questions people ask
How do I make a sight tape without special software?
Take two marks you trust from distances that are far apart, put them in with your sight radius, and let the calculator solve for the speed that reproduces the span between them. That fitted speed then generates the rest of the marks. Two marks work better than a chronograph figure because they already contain your bow, your arrows and your release, whereas a chronograph number contains only what happened in front of the sensors. Then shoot the longest distance and correct, because no calculated tape is finished until the far end has been confirmed on a target.
Why are my long-distance sight marks always off?
Because errors that are invisible up close accumulate with distance, and because the marks themselves crowd together at the far end so a small error in millimetres becomes a large error in yards. The usual culprits are a speed figure that is a few feet per second out, a sight radius measured from the riser rather than from the eye, and the fact that simple trajectory models have no air resistance in them, so they always understate the elevation needed at long range. Calibrating from two widely spaced marks removes most of the first two.
What is the point-on distance in archery?
It is the distance at which the point of the arrow, seen at full draw, sits exactly on the centre of the target. Closer than that and the point appears below the centre, so a barebow archer holds it under and shoots with a gap. Further and the point appears above the centre. Where it falls depends on arrow speed and on the geometry of your anchor, chiefly how far the eye sits above the arrow shaft and how far it is from the eye to the point. A slower traditional setup might be point-on somewhere around thirty yards; a fast one is a great deal further out.
Does sight radius change my marks?
It changes the spacing, not the shot. A longer sight radius means the same change in elevation angle moves the pin further along the bar, so the marks spread out and small aiming corrections become easier to make repeatably. A short radius compresses everything into less tape and makes the same correction a smaller movement. Neither is more accurate in principle; the tape simply has to match the radius it was built for. Measure from your eye at full draw to the pin rather than from the riser to the sight housing, because the eye is where the sighting angle is formed.
Do I need a new tape if I change arrows?
Yes, and it is worth expecting rather than discovering. A tape belongs to a combination of bow, arrows and draw, not to the bow alone. Changing arrow weight changes the speed, which changes the required elevation at every distance, and the change is small at short range and large at long range. Even a change of point weight is enough to shift the far marks. Rebuild from two marks after any change to the arrows, and shoot the longest distance before relying on it.