Originally Posted by philc
ERR whats paralax error
Many scopes have what people refer to as a ring on the objective lens (the front bit!) or have a side-wheel to focus the scope. It's the thing marked with yards or metres generally going up to infinity and down to 10 or 20 yards/metres.
Strictly speaking that's actually parallax adjustment, to put it simply (and somewhat inaccurately but for the purposes of this post it doesn't really matter) it adjusts the focus of the scope.
Now imagine you have such a scope and you're aiming your crosshairs at a dot on a paper target, 25 yards away. If the scope is 'focused' (or parallaxed) to 25 yards, no matter how much you move your eye position behind the scope the cross hairs will stay put (assuming you're holding the gun still).
If however, your scope was focused to, for example, 10 yards, as you moved your eye about, you'd see the crosshair move too. So you'll think you're aiming at one point but in fact the barrel of your rifle is actually pointed, say, half an inch right. In terms of shooting that's what we call parallax error (or rather the consequence of parallax error).
Some scopes have a fixed parallax (no side-wheel or ring on the objective lens). The Bushnell Elite 3200 is an example. The 3200s tend to be parallaxed, I believe to, 100 yards; these can be reparallaxed to other distances but each model is different and some simply aren't adjustable by the average bod without the danger of damage.
Now if you're shooting a discipline that allows you to change your parallax setting (FT and SFT spring to mind) this may not be much of a problem.
In HFT however you can't change the parallax setting of your scope after you've taken your first shot. Most targets are going to be at distances which aren't the same as the parallax setting you've set your scope to and thus where parallax error is a potential problem. The key to avoiding this problem is to ensure your eye is positioned in the correct place behind the scope.
parallax error (the most you'll be out) generally gets worse the bigger your objective lens is. So a 32mm objective lens should
show less maximum parallax error than a 50mm objective lens. Obviously the less potential parallax error, the less trouble you can get yourself into.
Don't forget too, that a bigger objective lens doesn't necessarily mean more light gathering either. I've looked through some scopes with 50mm that were murky as the bottom of a well and looking through Ryan's Leupold, which I think is 36mm objective (although I might be wrong), is like looking through a magic doorway into a brighter, clearer, happier world!