When shooting at extended distances with a riflescope, parallax is a commonly misunderstood optical problem that can affect your ability to shoot accurately. To better understand what parallax is, and how shooters can correct it, I turned to Reg Wales, a technical field advisor at Vortex Canada. He provided me with a copy of Vortex’s Operational Tactical Optics Program, a training course on extreme-range shooting for law enforcement and military personnel.
“Parallax is essentially an optical illusion,” explains the course material. “It presents itself as the apparent movement of the reticle in relation to the target when your eye moves off centre of the sight picture (exit pupil) or, in more extreme cases, it can appear as an out-of-focus image.”