Fixed vs. Adjustable Parallax: Do You Really Need a Side Focus?

You’ve checked your windage. You’ve dialed your elevation. Your breathing is steady, your trigger squeeze is perfect, and the break is clean.

But when you look downrange, there’s no impact on steel. Or worse, you’ve gut-shot that buck you’ve been tracking for two seasons.

You blame the ammo. You blame the barrel. You blame the wind. But the culprit might be, and very often is, a subtle optical error that plagues even experienced shooters: Parallax.

It is one of the most misunderstood concepts in shooting, often ignored or misunderstood by beginners. Today, we’re cutting through the confusion. Here is your deep dive into what parallax is, why it ruins your accuracy, and how to fix it.

Imagine you are driving a car. You look at the speedometer, and the needle points exactly to 60 MPH.

Now, imagine your passenger leans over and looks at the same speedometer. Because of the angle they are sitting at, the needle looks like it’s pointing at 55 MPH.

The needle didn’t move. The background numbers didn’t move. But the angle of the observer’s eye changed the relationship between the two.

This is Parallax.

• The Needle = Your Reticle.
• The Numbers = Your Target.
• The Passenger = Your Eye moving behind the scope.

If your scope is not parallax-free, moving your head slightly left, right, up, or down will make the crosshair appear to "float" or "swim" off the target, even though the rifle hasn’t moved an inch.

Inside your scope, there are lenses that manipulate light.

1. Light from the target enters the objective lens (the front).
2. The scope focuses that light into an image at a specific point inside the tube.
3. Your reticle sits at a specific point inside the tube.

Parallax error occurs when the image of the target and the reticle are not on the same focal plane.

Think of it like a slide projector. If you project an image onto a wall (the target) and then hold your finger up in front of the projector beam (the reticle), your finger and the image are separated by distance. If you move your head, your finger seems to move across the image.

However, if you walk up to the wall and place your finger directly against the projected image, they are on the same plane. No matter where you move your head, your finger stays on the same part of the image.

Your Parallax Knob (Side Focus) moves the target image forward or backward inside the scope until it sits perfectly flush with the reticle.

Understanding the difference between First Focal Plane vs. Second Focal Plane helps explain where your reticle sits in relation to the magnification assembly.

How do you know if you have parallax error? You do the Nod.

1. Secure your rifle on a bag or bipod so it cannot move.
2. Aim at a target.
3. Without touching the rifle, look through the scope and move your head up and down, then left and right.

The Fail: If the crosshairs "swim" or move across the face of the target while your head moves, you have parallax error. If you pull the trigger now, your bullet will go where the crosshair was, not where it looks like it is.

The Pass: If the crosshair stays "glued" to the exact same spot on the target no matter how much you bob your head, you are parallax-free.

Not all scopes allow you to fix this manually. Depending on your mission, you may have a fixed or adjustable system.

Fixed Parallax

Most LPVOs (Low Power Variable Optics), like our Tomahawk II or Arrowhead, and most rimfire scopes feature "Fixed Parallax."

• The factory permanently sets the focal plane to a specific distance (usually 100 yards for centerfire, 50 yards for rimfire).
• The Good: Faster to use. One less knob to worry about in dynamic shooting scenarios.
• The Bad: If you are shooting at 400 yards with a fixed 100-yard scope, you will have some parallax error. You must rely on a consistent "cheek weld" (placing your eye in the exact center of the eyebox every time) to mitigate the error.

Adjustable Parallax (Side Focus)

Precision scopes like the Kentucky Long, Warhorse, and Patriot feature a third turret, usually on the left side (Side Focus), or a rotating ring on the objective bell (Adjustable Objective).

• This allows you to mechanically move the focal plane to match any distance, from 10 yards to Infinity.

Twenty years ago, if you walked into a gun shop and asked for a hunting scope with side focus, the clerk would have looked at you like you had two heads. For decades, the standard 3-9x40mm scope with a fixed 100-yard parallax was the gold standard.

So, do you really need it?

For the Average Hunter: Probably Not.

If you are hunting whitetail in the woods of Pennsylvania or hogs in Texas where your shots are almost always under 300 yards, adjustable parallax is a luxury, not a necessity.

• The Math of the Miss: Let’s say you are shooting at 400 yards with a fixed 100-yard scope and your eye is slightly off-center. The maximum parallax error might be 1-2 inches.
• The Reality: A deer's vital zone is roughly 10 inches in diameter. If you hold dead center, a 2-inch shift isn't going to cause a miss. In the field, your own wobble, the wind, and your heart rate will introduce far more error than parallax ever will.

The "Ethical Range" Factor

Most ethical shots on game occur within distances where parallax error is minimal. The shift in point-of-impact caused by parallax usually only becomes critical when you are trying to hit a target smaller than 2-3 inches at distances past 500 yards.

Why is it Becoming Standard?

You’ll notice more modern hunting scopes coming with side focus. Why the change?

1. Versatility: Shooters today demand more from their rifles. That deer rifle might also be used for banging steel at 800 yards on the weekend. For that, you absolutely need to dial out parallax.
2. Higher Magnification: We used to hunt with 9x scopes. Now, hunters are carrying 16x, 20x, or even 25x optics. Parallax error becomes much more apparent and distracting at high magnification, making the adjustment knob mandatory to get a clear sight picture.

If you are a dedicated hunter shooting medium ranges, don’t stress if your scope has fixed parallax. It’s one less thing to fumble with when a buck steps out. But if you want a "do-it-all" rifle that crosses over into long-range precision, that side focus knob is mandatory.

This is where 90% of shooters get it wrong. They treat the Parallax knob like a camera focus. It is not just a focus knob. A sharp image is a symptom of correct parallax, not the definition of it.

Step 1: Set your Diopter (The Ocular Focus)

• Look at a blank white wall or the blue sky.
• Ignore the target (there shouldn't be one). Look only at the reticle.
• Adjust the ring nearest your eye (the Fast Focus Eyepiece) until the reticle lines are crisp and black.
• Do not touch this again. This tunes the scope to your eye. If your scope doesn't have any sort of marks on it to confirm that the diopter stays in the same place, it's a good idea to make your own witnessing mark with a Sharpie or paint pen.

Step 2: Range the Target

• Know your distance. If the target is 500 yards, turn your Parallax knob to the "500" mark. (Note: These yardage markings are often approximations. Don't trust them blindly).

Step 3: Fine Tune with the "Nod"

• Look through the scope. The image should be relatively clear.
• Bob your head slightly or move it side to side. Does the reticle move?
• If yes, gently turn the side focus knob until the reticle stops moving.
• Bonus: Usually, when the parallax is eliminated, the image will also be at its sharpest focus.

Parallax is essentially an optical illusion. It deceives your eye into thinking you are on target when you aren’t.

• For LPVO/Hunter users: If you're rocking a fixed parallax, your defense is consistency. A solid, repeatable cheek weld ensures your eye is always in the center, negating the error of a fixed-parallax scope.
• For Precision/Long-range users: Your defense is discipline (though don’t neglect a solid cheek weld either). Don't just dial the yardage and shoot. Do the "Nod," lock that image down, and then send it.

Clear your parallax, trust your glass, and shoot straight.