The peep sight is an aperture, not a peephole

Three systems, not one

The mainstream advice on peep sizing says: size your peep so you see the scope housing as a perfect circle centered inside the peep opening — the "circle-in-circle." Every arrow lands in the same place as long as those circles are concentric.

This advice is coherent. It is also built on a hidden assumption that is frequently false, and it is not the only viable system. There are three distinct aiming configurations a compound archer can use, each with different error properties and different requirements.

System A — Peep larger than the housing. The mainstream approach. You see the housing as a circle floating inside the peep. You center the housing in the peep, then place the pin on the target.

System B — Peep matched to the housing. The peep inner diameter is sized to just fit the housing outer diameter. The two circles are concentric by definition at correct anchor. Any anchor drift shows immediately as the housing clipping the peep edge.

System C — Peep smaller than the housing. The housing outer edge is outside the peep frame entirely and invisible in the sight picture. You see only the peep circle and whatever the scope shows through it — the inner aperture or light ring, the pin, and the target.

The choice between them is not just preference. It determines what your sight picture can and cannot tell you about your anchor.

The hidden assumption in Systems A and B

Both A and B use the housing outer edge as a centering reference. This only works if the pin is centered inside the housing — meaning when the housing is centered in the peep, the pin is also centered relative to the peep, and the entire reference chain is aligned.

In practice, the pin is often not centered in the housing. On a compound bow, windage adjustment moves the entire housing left or right — the pin stays in the same relative position within the housing. Elevation is different: on multi-pin sights, individual pins are adjusted vertically within the housing to set different yardages, and none of them sit at the housing's geometric center. Manufacturing tolerances compound this further. A multi-pin scope dialed for actual yardages will have pins distributed across the housing face, none of them centered.

When the pin is off-center in the housing, centering the housing in the peep produces a sight picture where the housing looks perfect and the pin sits somewhere other than center. The archer now has two references giving different information. Most archers resolve this unconsciously by ignoring one — usually the housing — which means the housing was never actually doing useful work.

System C sidesteps this entirely. The housing is not in the sight picture. There is no conflict to manage.

The error model

In a two-reference sight system, the sight radius L is the distance from the peep to the scope. When the peep shifts laterally by δ due to anchor inconsistency, it introduces angular error θ = δ/L. At target distance D:

miss = δ × D / L

What limits δ depends on which system you are using.

In Systems A and B, δ is bounded by the radial clearance between the peep and the housing — the maximum the housing can drift before the sight picture shows the shift:

δ_max = (d_peep − d_housing) / 2

The complete expression for maximum silent error is therefore:

miss_max = D × (d_peep − d_housing) / (2L)

System B drives the gap toward zero, which drives miss_max toward zero. System A accepts a larger gap — and pays for it.

In System C the housing is absent from the sight picture. If the scope's inner aperture or light ring is visible inside the peep, it becomes the centering reference and the same formula applies — substituting the scope aperture diameter for d_housing. If no inner reference is visible and the archer aims purely by placing the pin on the target, then δ is bounded only by the peep's full radius:

δ_max = d_peep / 2  (no inner reference)

This makes System C without an inner reference geometrically weaker than System B — the peep can drift a full half-diameter before the sight picture reveals it. What System C gains is immunity to conflicting housing signals, not superior drift detection on its own.

Peep-to-eye distance and the apparent aperture effect

There is a second distance that matters, separate from the sight radius L. Call it E — the distance from the archer's eye to the peep at full draw. E does not appear in the miss_max formula, which depends only on the physical gap and L. But E governs how demanding the peep is of eye alignment consistency.

The peep subtends an angle at the eye of:

θ = d_peep / E

As E increases — peep further from the eye — θ decreases. The peep appears smaller. A smaller apparent aperture demands more precise alignment between the eye axis and the peep axis: any head position or anchor inconsistency that shifts the eye off-axis shows up sooner as a partial or shifted image through the peep. The system constrains the archer more tightly before the full drift budget is consumed.

This effect is distinct from the gap formula. Moving the peep further from the eye on a compound bow slightly decreases L (the peep moves toward the scope), which marginally increases the miss_max ceiling. The apparent aperture effect runs the opposite direction: it reduces the probability of drift reaching that ceiling by making smaller anchor errors detectable earlier. In practice, the apparent aperture effect dominates for experienced archers whose anchor variability is already small — the tighter apparent aperture acts as a diagnostic before the physical error budget is reached.

When each system is actually optimal

System B is optimal when two conditions are both true: the pin is verified centered in the housing (measurable — zero it out deliberately when tuning), and the peep is sized to match the housing outer diameter as closely as possible. Under these conditions, δ_max → 0 and the sight picture catches any anchor inconsistency immediately. This is the highest-precision configuration available. It requires deliberate setup.

System C is optimal when pin centering in the housing is unknown or variable — which describes most archers in actual use. The housing is removed as a variable entirely. If the scope's inner aperture or light ring is visible inside the small peep, it provides a centering reference with the same formula as System B using aperture diameter instead of housing outer diameter. If no inner reference is used, the system is simpler but less sensitive to anchor drift.

System A — the large-gap "must see the full circle of housing" convention — is the worst of both worlds when the pin is off-center. It introduces housing drift error from the clearance formula and presents a conflicting reference. Its legitimate use case is low-light environments where light transmission is the binding constraint, not aiming precision.

The diffraction lower bound

The argument above pushes toward smaller peep diameters. Physics imposes a floor. As diameter decreases, diffraction at the aperture edge increases, and the perceived boundary of the peep opening softens. The Rayleigh limit gives an angular resolution of θ = 1.22λ/d. At d = 3/32″ (2.4 mm) and λ = 550 nm, θ ≈ 0.28 mrad — which projects to roughly 0.4″ of blur at 40 yards. A peep meaningfully narrower than 3/32″ introduces diffraction error at the same scale as a top-level compound group, and gains nothing further in anchor detection. Below about 1/16″, diffraction dominates entirely.

The practical window for precision archery: somewhere between 3/32″ and 3/16″, matched to the specific reference being centered — housing outer edge (System B) or scope aperture (System C). Not selected from a size chart. Measured, fitted, and chosen deliberately.

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Published 2026-07-08  ·  Axial Bowstrings