Technique
How to Set Parallax on a Riflescope
Quick answer
The eyepiece diopter focuses the reticle and the parallax knob focuses the target image into the reticle plane, two different jobs shooters constantly confuse. Set parallax by moving your head, not by trusting the knob's yardage markings, which are a manufacturing starting point. A 100 yard fixed setting carries about 0.2 inches of error at 300 yards for a tenth inch of eye offset.
Parallax is the apparent slide of the reticle across the target when you move your eye off the scope's optical axis, and it happens because the target's image is not landing exactly in the same plane as the reticle. It is a focus problem, not a manufacturing defect, and every scope without a true parallax adjustment has it at every distance except the one it was set for at the factory.
The knob or ring that fixes it, whether a side focus turret or an adjustable objective collar, does one job only: it moves an internal lens element until the target image lands in the reticle plane. It has nothing to do with your eye, which is exactly why it is so often confused with the diopter, a completely separate adjustment that focuses the reticle itself. Confusing the two is the single most common mistake shooters make with an adjustable optic, and it sends a lot of people chasing a glass quality problem that was never there.
How to set parallax correctly
- Confirm the diopter is already set and locked. Before touching parallax, the eyepiece diopter needs to already be adjusted for your eye on a blank sky and left alone. If the reticle itself is not sharp, you cannot judge whether the target image is landing in the right plane, because you are trying to solve two problems by looking through one blurred picture. See the diopter guide if this has not been done yet.
- Confirm the rifle is unloaded before mounting up to check anything. Open the action and visually confirm the chamber is empty and the magazine is out before shouldering the rifle to work through this process. None of what follows requires a loaded rifle.
- Rest the rifle on a stable support pointed at a real target. Use a front rest such as the Caldwell The Rock Deluxe Adjustable Front Rest at $94.99 so the rifle stays put while you move only your head. A target with fine detail, not a blank wall, makes the slide easier to see.
- Turn the parallax knob to the number printed for your actual distance, as a starting point only. The yardage engraved on a side focus turret or an adjustable objective is a manufacturing convention, set at the factory on a sample unit, and it commonly drifts from scope to scope and with temperature. Use it to get close, never to finish the job.
- Move your head and watch the reticle against the target. With your eye centered, note where the reticle sits on the target. Then move your head up, down, left and right within the eye box, a couple of inches each way, while keeping the target in view.
- If the reticle slides across the target, adjust the knob and repeat. Any visible slide means the target image is not yet in the reticle plane. Turn the parallax adjustment a small amount in either direction, then repeat the head movement check. Work in one direction at a time rather than guessing back and forth.
- Stop when the reticle holds still through the full head movement. The correct setting is the one where moving your eye around the eye box no longer moves the reticle against the target, not the one where the printed number matches your rangefinder reading.
- Note where the knob physically landed for that distance. Write down or mark the knob's actual position for the distances you shoot most, since it will likely differ from the printed number and that difference is worth knowing before you need it under time pressure.
How much error does a mis-set parallax actually cost you?
The error is exact geometry once you know the eye offset and the ratio between your parallax setting and the actual target distance, and it is worth seeing in a table rather than a single number, because it scales with both.
A scope with parallax fixed at 100 yards shooting at 300 yards carries about 0.2 inches of available aiming error for a tenth of an inch of eye offset, and roughly 0.6 inches for three tenths of an inch of offset, and the error is exactly zero at 100 yards regardless of eye position.
Sourced figure
| Target distance | Error at 0.1 in eye offset | Error at 0.3 in eye offset |
|---|---|---|
| 100 yd (the set distance) | 0.0 in | 0.0 in |
| 150 yd | 0.05 in | 0.15 in |
| 200 yd | 0.1 in | 0.3 in |
| 300 yd | 0.2 in | 0.6 in |
| 400 yd | 0.3 in | 0.9 in |
| 500 yd | 0.4 in | 1.2 in |
The formula is error = eye offset x (target distance divided by parallax setting, minus one). Keep your eye centered on the optical axis and the error goes to zero no matter how far off the knob's printed number is, because eye offset, not the knob, is what actually produces the error.
Why the yardage markings on the knob cannot be trusted outright
The number stamped next to a side focus turret comes from the factory testing one sample unit, often under conditions that do not match the temperature or the specific target distance you are shooting at. Manufacturing tolerance in the erector assembly and the parallax mechanism itself means two copies of the same model can land their true parallax-free point several yards apart at the same knob position. Temperature shifts it further, since the internal lens elements move slightly as materials expand and contract.
None of that makes the marking useless. It is a genuinely useful starting point that gets you close before the head movement check does the real work, and on a scope you shoot often, a marked knob you have personally verified against your own head movement check is more valuable than any printed number could ever be on its own.
Why parallax matters more for some shooting than others
A large mismatch between the parallax setting and the actual target distance, combined with a sloppy or inconsistent head position, is what turns parallax into a real source of missed shots. Precision rimfire competition, where targets are small and distances change constantly, is the sport where this shows up most, which is why serious rimfire scopes almost always carry adjustable parallax rather than a fixed setting.
Most big game hunting inside 300 yards is a different story. The available error at a moderate mismatch is small relative to a vital zone, and a shooter with a repeatable cheek weld and a centered eye will out-shoot a shooter with a perfectly dialed parallax knob and an inconsistent head position. Build the position first. The knob is a refinement on top of that, not a substitute for it.
Sources
- Geometric derivation of parallax error from the separation of the image and reticle planes
- Manufacturer published fixed parallax settings by optic class
Frequently asked questions
What is parallax on a rifle scope?
Parallax is the apparent movement of the reticle against the target when you shift your eye position, caused by the target's image not landing exactly in the same optical plane as the reticle. It happens at every distance except the one specific distance the scope is focused for, and it is a normal optical relationship rather than a defect in the glass.
How do I know if my parallax is set correctly?
Move your head around the eye box, up, down, left and right, while watching the reticle against the target. If the reticle stays put against the target through that movement, parallax is set correctly for that distance. If it slides, adjust the knob a small amount and repeat the check rather than trusting the yardage number printed next to it.
Should I trust the yardage numbers printed on my parallax knob?
Only as a starting point. Those numbers come from one sample unit tested at the factory, and manufacturing tolerance plus temperature change routinely shift the true setting by a meaningful amount on an individual scope. Use the number to get close, then confirm and fine tune by watching whether the reticle slides as you move your head.
Can I fix parallax by adjusting the diopter?
No, and confusing the two is the most common mistake here. The diopter focuses the reticle itself for your individual eye and has no effect on the target image. The parallax adjustment, whether a side focus knob or an adjustable objective, focuses the target image into the reticle plane and has no effect on how sharp the reticle looks.
Does parallax affect where the bullet actually goes?
No, it affects where you perceived your aim point to be. The rifle sends the bullet wherever the barrel was pointed at the moment of the shot, and a parallax error means the reticle appeared to be somewhere it was not relative to the target, which produces a miss that looks exactly like an aiming mistake because that is functionally what it is.
Do I need a scope with adjustable parallax?
It depends on your shooting. A fixed parallax hunting scope set around 100 or 150 yards carries only a small, usually acceptable error across normal hunting distances if your head position is consistent. Precision rimfire, small varmints at varying range, and any discipline shooting small targets at unpredictable distances benefit far more from a true side focus or adjustable objective.
Researched, not professional advice. This page is compiled from published manufacturer specifications, published optical and ballistic formulas, and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are shooter convention rather than sourced numbers, and they are labelled that way wherever they appear. Ballistic figures come from a point mass model using the ballistic coefficient the maker publishes, so treat them as a starting point and confirm them on paper at a measured distance. Confirm your firearm is unloaded with the action open before you mount, level, torque or bore sight anything. Verify a zero only on a supervised range or a safe, legal backstop, know what lies beyond your target, and follow the law where you hunt and shoot, including rules on land access and permitted cartridges.