Bruce Potts gives a useful guide
What is parallax and how can it affect your rifle shooting? Parallax is caused by light bending as it passes from one medium to another. It’s why a fish or a stone under water will appear closer to you than it really is.
In rifle shooting, the light transmitted through a telescopic sight will bend as it passes through the various lenses unless (a) they are perfectly ground and (b) your eye is dead centre in the optic lens.
However error in parallax can and does occur. This is when the target image does not fall in the same optical plane as the reticle. It affects all scopes, but many have a parallax adjuster that focuses the scope at one range and aids accuracy. With high magnification scopes parallax error is particularly acute. (Read our review of thermal imaging scopes.)
Checking for parallax
To check if your rifle scope has parallax, fix the rifle in some sort of clamp. If the crosshairs move on the target when you move your head up and down or side to side, then you have parallax at the range of that particular target.
Most ‘scopes are adjusted to be parallax free at about 100 yards (or metres) but some have parallax adjustment at the object glass (the end furthest from the eye). The figures on the ring may, or may not, be accurate and you won’t know without doing the above test at various known distances.
Does parallax matter?
If you always make sure your eye is dead centre (you have a perfectly symmetrical dark ring around the crosshairs) then parallax doesn’t matter.
Do I need a scope with parallax?
Should you get a scope with parallax adjustment?
Parallax in a scope is quite complex but makes a big difference to the performance of a scope and your shooting.
Parallax error is where the reticle seems to shift its point of aim on a target as you move your eye left or right while looking through the riflescope. This is why shooting technique and scope-to-eye alignment must be exactly the same for each shot. Most scopes designed for full-bore rifles are parallax free at 100 yards thus even some eye movement results in minimal parallax error. Rimfire scopes used at shorter ranges have a parallax set for 50 yards.
To allow for more flexibility some scopes, especially higher magnification and variable scopes, have an adjustable parallax ring either on the objective bell or side-mounted on the adjustment bowl. This can be used to eliminate parallax at any range while maintaining focus at closer ranges, even on the highest magnification. So yes, a parallax ring is useful, but if you hunt at short ranges with a lower-powered scope and position your eye the same for each shot, you will be fine.
More on parallax and scopes
Scopes are either set for one parallax-free setting or come with parallax adjustment.
Look through a scope from a differing angle each time and the chances are your shots will shoot differently.
Reader question on parallax
Q: What is parallax adjustment on a scope and why is it important? Is it only a focus knob?
A: There are two types of scopes. The first type is usually a cheaper, more basic model that will have a fixed parallax, typically set around 100 yards or 150 yards. The other type has an adjustable parallax, which is found in most top-end scopes, particularly those designed for long-range shooting.
Parallax, as most will know, focuses the image in the scope, enabling you to see it clearly, but it also has another function. It sets the lenses within the scope to be on the same plane. Why is this an important feature? Well, if the scope is slightly out of alignment, and so not in focus, you will find that if you move your head on the stock slightly to the left or right, it will appear to move the crosshairs off the target. This, of course, could result in a miss that could be more pronounced at longer ranges. By adjusting your scope’s parallax, you bring your target into sharp focus and at the same time minimise ‘parallax error’. You must also be sure to focus the eyepiece correctly when first setting up your scope to ensure correct alignment. To do this, point the scope towards the sky and adjust the eyepiece so as to bring the crosshair into focus.