How to Set Your Scope's Diopter If You Wear Glasses

Decide first what you will wear when you shoot, and set the diopter that way. If you hunt and shoot in your distance glasses, set it with those on. If you wear contacts in the field, set it with contacts in. The setting belongs to one eye and one correction. If you set it bare-eyed at the bench and then hunt in glasses, the reticle will be soft when it counts.

Here is the full procedure. One: mount the scope, or rest it steady, and turn the magnification to the highest power you use. Two: if the scope has a parallax or side focus knob, set it to infinity. Three: point the scope at a plain bright background with nothing to focus on, such as open sky away from the sun or a blank wall. Four: turn the eyepiece all the way out until the reticle is clearly blurred. Five: look away at something distant for a few seconds, then glance through the scope for one or two seconds and judge the reticle. Six: turn the eyepiece in a little, look away, and glance again. Repeat until the reticle is sharp and black the moment your eye lands on it. Seven: tighten the lock ring if your scope has one, and mark the position with a paint pen or a strip of tape.

The short glance matters more than any other step. Your eye works to bring a blurry object into focus, and given three or four seconds it will often manage it. That hides a wrong setting and leaves you with eye strain and a reticle that drifts out of focus when you are tired. A quick look shows you what the scope is doing before your eye starts to compensate. This is also why you use a blank background. With a target in view, your eye locks onto the target and you cannot judge the reticle alone.

Now the difference between the diopter and parallax. The diopter focuses the reticle for your eye. The parallax adjustment, either a side knob or a ring on the objective, focuses the target image so that it falls on the same plane as the reticle. Set the diopter once. Adjust parallax every time the distance changes. To check it, hold the rifle still on a rest and move your head slightly up and down. If the reticle appears to slide across the target, turn the parallax knob until it stays put. The yardage numbers on the knob are a guide and often do not match the true setting.

Many hunting scopes have no parallax adjustment. Their parallax is fixed at the factory, commonly at 100 or 150 yards, and the target can look slightly soft at very close or very long range. That is normal, and the diopter is not the cure. If you turn the eyepiece to sharpen a 25-yard target, you have just defocused the reticle. Leave the eyepiece on its mark and back the magnification down instead.

Distance glasses or readers? Use your distance correction. A scope presents the reticle and the target to your eye as if they were far away, so the lenses that let you read a road sign are the ones that work behind a scope. Reading glasses are made for objects at arm's length and will blur the picture. If you need readers only for close work and see well at distance without glasses, set the diopter with no glasses on and shoot that way, with safety glasses that have no prescription.

Age matters here too. From the mid-forties on, the eye loses much of its ability to change focus. A younger shooter can get away with a diopter that is a little off because the eye quietly corrects it. An older shooter cannot, so the setting needs to be right, and it is worth rechecking each season and after any new prescription.

Bifocals and progressive lenses cause a specific problem. When your head is down on the stock, you look through the upper inside corner of the lens, not the center. With progressives that part of the lens may not hold your full distance correction, and small head movements change the focus. Set the diopter with your head in your real shooting position, not sitting upright. If the reticle still shifts as your head moves, ask your optometrist about single-vision distance glasses for shooting. Take the rifle's stock measurements or a photo of your shooting position so they can see where you look through the lens.

Astigmatism is the case the diopter cannot fix. The eyepiece adds or subtracts plain focusing power, the same in every direction. Astigmatism means your eye focuses differently in different directions, which glasses correct with a cylinder at a set angle. The sign is a reticle where one wire is sharp and the other is soft: you get the vertical wire crisp and the horizontal wire blurs, or the reverse. No diopter position will sharpen both. The answer is to shoot in glasses or toric contacts that correct the astigmatism, then set the diopter with them on. If a red dot looks like a smear or a starburst, the same thing is happening.

A few checks will save you a wasted range trip. If you run out of adjustment before the reticle sharpens, your prescription is outside the scope's diopter range and you need to wear your correction. If the reticle is sharp but the target never is, look at parallax, mirage, or dirty lenses before you blame your eyes. If someone else shoots your rifle, they will need their own setting, which is why you marked yours. For any change in your vision that comes on suddenly, see an eye doctor. A scope adjustment is not the answer to that.