Scope Parallax Explained: Side Focus, Head Position and Error

If the reticle seems to slide across the target when you move your head, the scope is telling you something. Here is what, and how to fix it.

What parallax actually is

A riflescope forms an image of the target inside the scope, at a point called the focal plane. The reticle sits on a focal plane too. When the target image lands exactly on the reticle's plane, the two behave as if they were painted on the same sheet of glass: move your eye around behind the scope and the reticle stays fixed on the same spot of the target.

When the target image lands slightly in front of or behind the reticle's plane, they behave like two sheets separated by a gap. Move your eye and the closer sheet appears to slide relative to the farther one. That apparent movement is parallax. Because you aim with the reticle, it moves your point of aim even though the rifle never moved.

The target image's position depends on distance. A scope focused for 100 yards puts the image of a 600-yard target in a slightly different place. That is why precision scopes include an adjustment, usually a side focus knob or an adjustable objective ring, to move the image back onto the reticle plane for whatever distance you are shooting.

Two separate focus adjustments

Most precision scopes have two focus controls, and confusing them is the root of most parallax trouble.

The diopter does not fix parallax. It only makes the reticle sharp for your eye. The side focus fixes parallax. But if the diopter is wrong, your eye will strain to accommodate, and you will set the side focus to whatever looks sharpest to a strained eye rather than to the true parallax-free point.

Setting it up in the right order

1. Set the diopter

1. Point the scope (with the rifle unloaded and pointed safely, or with the scope off the rifle) at a plain, bright background such as an overcast sky or a blank wall. Never look at the sun.
2. Set magnification to maximum.
3. Glance through the scope briefly. Your eye adjusts within a second or two, so quick looks give a truer reading than staring.
4. Turn the diopter until the reticle is crisp at first glance. Lock it if your scope has a lock ring.

2. Set the parallax

1. On the range, with the rifle supported on a bipod and rear bag so it does not move, aim at a target at the distance you plan to shoot.
2. Turn the side focus until the target looks sharp.
3. Without touching the rifle, move your head slightly up and down and side to side behind the scope.
4. If the reticle appears to move on the target, adjust the side focus a little and repeat. When the reticle stays put as your head moves, parallax is removed for that distance.

Steady support matters. If the rifle moves while you test, you cannot tell rifle movement from parallax.

How parallax error shows up

Parallax only causes error when your eye is not exactly centered behind the scope. If your head position were identical every shot, residual parallax would shift every shot the same way and you would never see it in the group. In practice, your head position varies, especially in field positions and on barricades, so residual parallax adds dispersion.

The amount of error grows with how far off the focus is and how far off-center your eye is. That is why two habits work together: set parallax properly, and build a consistent head position. Consistent cheek pressure and a stock adjusted to put your eye naturally in line with the scope do more than any knob. Our chassis vs stock article covers adjustable cheek pieces.

Signs parallax might be hurting you

• Groups that open up in awkward positions but stay tight on the bench.
• A shadow or dark ring at the edge of the image, which means your eye is off-center.
• Different shooters getting different zeros from the same rifle.
• Poor results after a large change in target distance without refocusing.

Side focus marks are a starting point

Many side focus knobs carry yardage marks. They are useful for getting close quickly, but treat them as approximate. Temperature changes, individual eyes and manufacturing tolerance all move the true setting. Use the marks to get near, then confirm with the head-movement test. Some competitors replace printed marks with their own tape marks after testing their scope at known distances.

Fixed-parallax scopes and low magnification

Some scopes, especially lower-magnification and many hunting models, have no parallax adjustment. They are set at the factory to be parallax-free at one distance, and the manufacturer usually states it in the manual. At other distances, a small amount of parallax remains. At low magnification, depth of field is large and the error is usually small enough to ignore for the scope's intended use. At high magnification and long distance it matters more, which is why precision scopes nearly always have adjustable parallax.

Buying with parallax in mind

For precision work, look for a side focus that adjusts smoothly, covers the closest distance you shoot, and is easy to reach from your shooting position. Precision scopes in the comparison on this site are covered in Tango-MSR vs XTR Pro vs Tenmile, and the budget build in the Tango-MSR article. If you are starting from scratch, the first precision rifle checklist puts the scope in context with everything else.

You can also browse the riflescope category for other options.

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