Understanding Parallax and Side Focus on Rifle Scopes

If you have ever shifted your head slightly behind the scope and watched your reticle appear to drift across the target, you have experienced parallax error. It is one of the most misunderstood optical phenomena in rifle shooting, yet it directly affects point-of-impact accuracy — particularly at the ranges UK centrefire and rimfire shooters commonly work at. In this guide we explain exactly what parallax is, why it matters, and how the adjustment mechanisms on modern scopes eliminate it.

Parallax occurs when the image formed by the objective lens does not fall precisely on the same focal plane as the reticle. The result is that any movement of your eye behind the eyepiece causes the reticle to shift relative to the target image. At shorter distances, the effect is more pronounced because the objective must focus the image further back; at longer distances it diminishes. Most budget scopes are factory-set to be parallax-free at a single distance — typically 100 yards for centrefire models and around 50 yards for dedicated airgun scopes — but that only eliminates error at that one range.

Higher-specification scopes solve this with an adjustable parallax mechanism, usually a side-focus turret (sometimes called a third turret) or, on some older designs, an adjustable objective (AO) ring on the front bell. Both do the same job — they let you move the objective's focal point until the target image sits exactly on the reticle plane — but side focus is far more ergonomic, keeping your shooting position undisturbed. Understanding how and when to use this adjustment is essential whether you are shooting HFT at 45 yards with a sub-12 ft/lb springer or engaging steel out to 1,000 yards with a .308.

Buying Advice

When choosing a scope with parallax adjustment, first consider the distances you actually shoot. UK air-rifle disciplines — HFT, field target, and plinking — demand precise parallax correction from roughly 8 to 50 yards, so look for a scope whose side-focus turret bottoms out at 10 yards or less. For rimfire and short-to-mid-range centrefire work (100–300 yards), most quality side-focus scopes cover the range comfortably. Long-range centrefire shooters should ensure the parallax dial reaches at least 500 yards and ideally infinity, and that the markings are legible enough to set quickly in the field.

Pay attention to the focus mechanism's feel. A smooth, well-damped side-focus turret with clear yardage markings makes a real difference when you need to remove parallax quickly between shots at varying distances. Some scopes also double as crude rangefinders: once you learn where targets sit on the parallax dial, you can estimate distance — a useful trick in HFT. That said, never rely on parallax markings as a primary rangefinder; they are approximate at best and vary with temperature and individual eyesight.

Finally, consider whether the scope is first focal plane (FFP) or second focal plane (SFP). Both can have parallax adjustment, but it is worth understanding the interaction: in an FFP scope the reticle scales with magnification, so holdover values remain correct at every power setting — but parallax must still be dialled out separately. In an SFP scope the reticle size stays constant, so subtension-based holdovers are only accurate at one magnification (usually maximum). Neither design is inherently better for parallax correction; the adjustment mechanism works identically in both. Choose FFP if you vary magnification mid-course and need consistent mil or MOA references, or SFP if you prefer a finer reticle at high power and always shoot at the same magnification.