Adjusting a scope
by B.B. Pelletier
Before I start, just a mention that today marks the first anniversary of this blog! Now, to business.
Scope sights are very common these days, but have you read and understood the instructions that come with them? Many shooters have scopes on their guns that are so far out of whack that it's a wonder they can see through them at all! Let's take a look at the basics of adjusting a scope on an air rifle.
Eye relief is important
To be the most useful, a scope should be positioned so you see as much as possible of the image that exits the eyepiece. This image is called the exit pupil, and you are seeing all of it when the image appears as large as it can be. If you see only a small image surrounded by a lot of scope, you are either too close (unlikely) or too far from the eyepiece, which is common. To fix this, you must loosen the scope in the rings and slide it back (or forward) until your eye sees the best (largest) image. Sometimes, you may also need to move the scope rings, which is why I always use two-piece rings whenever possible. One-piece rings are not as easy to position as two-piece rings.
It's important that your eye is always in the same place to look through the scope. This helps reduce parallax as much as possible and makes sure the scope is positioned correctly when your eye goes to the right place (as naturally as possible) on the stock. Once the scope is situated so you see the biggest image, you can move on the the next step.
Focus the scope
Focusing a scope does not mean adjusting it to see the target clearly. That adjustment is done by the AO (adjustable objective) and comes later. Focusing the scope means adjusting the eyepiece so you see the reticle clearly. To do that, you need to be looking at a plain background so your eye is not distracted from the image of the reticle. You will find that the eyepiece of the scope rotates in both directions, just like the eyepieces on a pair of quality binoculars. Rotate the eyepiece until the reticle appears sharp, then leave the adjustment there for the rest of the time you use the scope. This adjustment is not used in conjunction with the target - it is applying correction for your eyes. This helps most people use scopes without their glasses, because the scope corrects for their vision. There are a few eye problems a scope cannot correct, so you may still need to wear glasses.
The focus ring on a Leapers scope adjusts for your eye, bringing the reticle into sharp focus. Photo borrowed from the Pyramyd Air article All about scopes. - Part 1.
Here's a tip for making this adjustment. If you stare at the reticle for a long time, your brain will focus your eyes to make it sharp. So, look through the scope for only a few seconds at a time while making this adjustment. I use a light-colored wall as my backdrop. I look through the scope and adjust the eyepiece, then look away. When I look back, if the reticle is in sharp focus, I'm done. If not, I make another small adjustment. Some inexpensive scopes may not have the eyepiece adjustment. If that's the case, there is nothing you can do but move on.
Adjusting the parallax adjustment correctly
Tankers used to use a device called a coincidence rangefinder to determine the range to a target. Cameras used to have them, too. You look through the eyepiece and see a double image of the target; by turning a dial, the two images come together in sharp focus. The rangefinder uses mechanical triangulation to determine range based on how much the two mirrors must be moved to bring the images together. Sounds great except for one thing. If you are not extremely careful, it is so easy to be off by a few hundred yards and not know it. But I know a trick that will help you do this easier!
The parallax ring on an AirForce scope adjusts by bringing the target into sharp focus. The distance may then be read off the scale, as shown here. Photo borrowed from the Pyramyd Air article All about scopes. - Part 1.
Adjust from both directions
The trick is to adjust the AO from both directions. Sometimes, you won't see the target get sharp as easily coming from one direction as you will from the other. If you adjust from both near and far, you will get a thin band of possible range within which the target probably lies. You do everything while looking through the scope and adjusting the AO. Only when you believe you have the range determined correctly do you take your eye off the target to dial in the elevation needed for that distance. This is where you will come to appreciate scopes with sidewheel parallax adjustment, like the one you find on the 30mm Leapers scopes. You can REALLY get precise when that big optional sidewheel is installed!
That's all there is to adjusting a scope correctly. After you've done it a few times, it takes only a few minutes, with the reward of a scope that operates perfectly!