parallaxinfinity

Ocular Lens Focus and Parallax Adjustment

In Blog by Rich6 Comments

There is often some confusion when discussing the topics of ocular lens focus and parallax adjustment. People confuse the terms for each other and they misunderstand what it means to have a parallax free image. We are going to discuss how to properly set the focus of the ocular lens and we are going to discuss what parallax is and how to remove it with your scope. People don’t realize just how big an effect that parallax error can create when shooting and they often misadjust the parallax as they try to correct problems with the focus of the ocular lens. Lets break the topics down and discuss how to get it right!

Ocular Lens Focus

What is the ocular lens, how do you focus it properly, what’s the purpose? The ocular lens is the lens at the rear of your scope that you look through with your eye, hence ocular lens. What people often don’t realize is that it has its own focus adjustment and that you probably aren’t focusing what you might think. The whole point of focusing the ocular lens is to get a crisp and clear image of the reticle. Not the target. Just the reticle. The best way to set the focus on the ocular lens is to look through the scope at a well lit white wall, or in the backyard looking at clear sky. Set the parallax knob to infinity, which is the little figure eight symbol. You change the focus of the reticle by adjusting the diopter.

diopterandlockring

Photo of the ocular lens, and the lock ring for the diopter, which focuses the reticle

 

Now look through the scope and look at the reticle. Is it clear? Or is it fuzzy? If it looks fine, look at something else, a car on the street or what your wife is making for dinner and count to five. Now look through the scope again, does it still look clear or is it fuzzy? Your eyeball will adjust and focus an out of focus image remarkably fast. So you have to sort of snap your eye behind the scope and get a quick read on whether or not the reticle is clear. If it isn’t, and even if it does appear clear, try adjusting the ocular lens focus. Most of the time this is accomplished by grabbing the whole eye piece and screwing it either clockwise or counter clockwise. If you have trouble, look for a locking ring, like a giant lock nut, just in front of the ocular lens. That holds the adjustment of the diopter from accidentally changing.

Bushnell G2 Reticle

Bushnell G2 Reticle

When adjusted properly you should be able to snap your eyeball behind the scope and get a crisp and clear view of the reticle immediately. Little changes can go a long way so go slowly and see what looks best. This is important because your eye will have to work harder when you shoot if its constantly trying to refocus the reticle for you. So make sure you get this set right, then leave it alone. It is not something you should ever have to readjust once you get it set up right the first time!

Parallax Adjustment

You probably hear this one a lot, but do you understand what it means? The absolute best way to describe this that I’ve heard is Frank Galli of Sniper’s Hide and his method. Draw a circle on a piece of paper with a pen. Now hold the pen vertically between you and the paper. Close one eye and line up the pen with the dot, then switch eyes and notice how the relationship between the pen and the dot changes. That is an image with parallax error. To remove parallax error, you want the target and the reticle on the same focal plane. Take the pen and touch the tip to the dot, now do the same drill closing one eye and alternating. That is a parallax free image and it is what we want to replicate with your rifle scope.

parallaxinfinity

Closeup of the parallax knob and what the infinity setting looks like, a small figure 8

 

In order to accomplish the same task with your scope use the parallax knob, sometimes referred to as the focus knob, on the side of the scope. Sometimes there will be a ring in front of the ocular lens that you can twist for parallax adjustment, if you aren’t sure, consult your scope’s manual. I start by focusing the image in the scope so that it is crisp and clear. However, its important that you understand a clear and focused image may still have parallax error present. In order to check, look at the position of the crosshairs of the reticle in relationship to the target. Now very slowly and gently, don’t move the rifle, slide or roll your head from side to side or up and down.

If you have a truly parallax free image the crosshairs will appear to remain in a fixed position. For example if you slide your head slightly to the right, it will look like you are looking at the target from an angle, but the X of the crosshairs will still be on the bulls eye. If there is parallax in the image you will see the X of the crosshairs move laterally away from the bulls eye. If that happens you need to readjust the parallax knob and try again. Understand that breathing and shooting, not to mention running around or moving through a rifle match or battlefield, will force your head to move around a bit on your stock. If you have a parallax free image, this won’t cause you problems. If you have parallax error present in the scope image, you will get shifting point of impact issues with regard to the point of aim.

Wrapping Up Ocular Focus and Parallax

Most of this stuff is pretty easy once you get the hang of it but having a good understanding of what adjusting the diopter on the ocular lens does versus what the parallax knob does will help keep you from confusing the purpose of the knobs. Once you understand what the functions of the different knobs are you can much more easily tweak the adjustment of the scope for maximum performance. Remember you want to adjust the ocular focus with the diopter until the reticle looks crisp against a clear background as soon as your eye lands on it. You only adjust it once, then leave it alone. The parallax knob is how you get the target and reticle on the same focal plane and that may or may not give you a perfectly ‘in focus’ image of the target.

Owner and Proprietor of AccuracyTech, LLC. Rich is a Firearms Enthusiast, Precision Rifle Competitor, and Writer. He is committed to bringing readers quality reviews and articles related to the Precision Shooting Sports. If you have any questions for him, please use the contact form on the site.

Comments

  1. I have a leupold vx-1 4-12×40. Will adjusting the focus (turning the entire eyepiece and securing with lock ring ) affect the poi? Do i need to re-sight in the scope?

    1. Author

      Kinda depends on the scope. The ocular only focuses the reticle so I would say no. However, some scopes have a focus ring that affects parallax juat ahead of the eye piece. So be careful what you adjust.

      Neither ocular focus ring nor parallax will affect POI like moving the turret. However, if parallax error is present and your eyeball is off center it can cause a shift in where the reticle appears to be. Makes sense I hope.

  2. “However, its important that you understand a clear and focused image may still have parallax error present.”

    I have a hard time with that statement. If the target image is focused to the reticle plane and the eye is focused to the reticle, both are in the same plane and focused to the eye. If the target image and reticle are in the same focus plane, by definition there is no parallax.

    If a clear target image show parallax relative to the reticle it would stand to reason that the target image is not focused to the same plane as the reticle. I would return the scope to the manufacturer.

    1. Author

      If only life and manufacturing followed along with definitions. Unfortunately, every pair of eyes is different, every set of scope manufacturing standards is different, variety and variables are part of life.

      Me personally, I focus the image first and then check it for parallax.

  3. Hi Rich,

    I wear spectacles. I shoot without spectacles which means that I have to screw the ocular well out in order to get the reticule in focus.

    Both my shooting mates observed that this extreme adjustment introduces large parallax deviations. When they dial the scope in for their eyes the parallax disappears.

    Your comments.

    Regards

    1. Author

      Everybodys eyes are a little different. As long as the reticle is in focus for whomever is shooting and you use the parallax knob to remove any parallax present…you should all be able to connect with the target.

      It’s adjustable for a reason.

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