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.
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.
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!
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.
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.