![]() ![]() ![]() Sounds good, but there’s a fly in the ointment, which is that it is not straightforward to get the light to converge exactly at the plane of the reticle before looking at that, a quick look at the means of focusing the scope. When this is achieved, the light from that single point or any single point on the target, for that matter, will always appear in exactly the same place relative to the reticle, and parallax error simply cannot occur, no matter how far from the axis of the scope you place your eye. The light from a single point on the target bounces off in many directions, some of it entering the entire surface of the objective (front) lens of a telescopic sight refracting (passing through) each air/glass boundary causes the light to bend, and the objective when focusing a scope for parallax purposes is to get all of the light from that single point on the face of the target to converge at a single point, at the plane of the reticle. There are ways in which the POI error caused by image shift can be reduced, or even negated, but before looking at them, we’ll consider what exactly causes the image shift that can lead to parallax error.Īll the light we see when viewing a target in daylight emanates from the sun, and has reflected from the surface of the target. That means the pellet can land a maximum of 6mm – in any direction according to whether your eye is left, right, high or low – away from the intended POI, and it’s important to realise that this figure is the absolute maximum, to achieve which, your eye must be a long way from the centre of the scope, much further than you’ll normally achieve unless you make a special effort to look through the edge of the image.Įxperiments suggest that unless you do make a deliberate effort to move your eye as far from the scope axis as possible, parallax POI shift, in practice, is likely to be nearer a quarter of the absolute maximum, or 1.5mm, which isn’t a lot unless you’re a paper-target shooter. The distance by which the image of the intended POI can move away from the reticle aim point can be calculated using a very simple equation, which is the radius of the objective lens multiplied by the difference between the target distance and the scope focus (parallax) range, and divided by twice the focus range.Ī 40mm objective lens scope parallax focused at 25 yards and with a target at 40 yards has a maximum potential shift of 20 x 15/50, which works out at 6mm. Parallax error is thus a potential cause of inaccuracy. Put simply, the position of your intended pellet point of impact (POI) moves away from the reticle aim point if your eye moves away from being directly behind the axis of the scope. The term ‘parallax error’ is used to describe movement of the viewed image through a scope relative to the reticle, caused by a change in the position of the user’s eye relative to the axis of the scope. What is parallax error and why does it matter? Jim Tyler explains what causes parallax error in your scope, why it matters, and how to avoid it! ![]()
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