How scopes really work and what not to do when adjusting them

Monday, April 26, 2010

Recently I had a really nice and informative conversation with Mike Kurtz, technical guru, over at Hawke Optics (www.hawkeoptics.com). He told me that well over 90% of their customer service issues have to do with scope owners improperly adjusting their scopes, and I found out that I have been doing some things wrong from time to time. Perhaps you have too, but before we get to that, let do a quick overview of how a scope works.

Take a look at the picture of the scope below. Light comes in through the big end (on the left) and is deflected down by lenses so that it will pass down the main tube of the scope (which in this case is a 30mm tube). As the light reaches the tube, the image is inverted by a lens.

The light then passes into the erector tube, which holds the crosshairs and is roughly in the vicinity of the elevation and windage knobs. The erector tube is fastened to the main body of the scope at the rear, but the front is free to move. When you adjust the elevation and windage knobs, you are moving the erector tube up and down, left and right, until the point of aim corresponds with the point of impact.

(A side note: theoretically, you could zero your scope in a single step at a given distance by clamping your airgun in a vice, firing a single shot, and then adjusting the scope so that the crosshairs intersect at the spot where the pellet hit.)

When the light comes out of the erector tube, it passes through another set of lenses, which flip the image back to right side up and focuses the image for the ocular lens where, you, the shooter, look through the scope.

Now, before we go any further, please notice this one key point: the erector section of the scope is a tube, and it lives within the outer chassis of the scope, which is also a tube. So the erector is a tube within a tube – got that?

The picture below shows an internal view of the scope as it comes from the factory with the outer chassis and the erector tube in perfect alignment, and the elevation turret and the windage turret are adjusted equally.

Okay, now let’s look at some ways folks get into trouble with scope adjustment. The picture below shows a scope that has been adjusted too far left. The erector tube is pinned against the wall of the outer chassis, and, as a result, the elevation adjustment is severely limited because there is no room for the erector tube to travel.

The picture below shows a scope that has been adjusted too far down and right, pinning the erector tube against the erector spring, the windage turret, and the outer chassis. Again, windage adjustment is very limited since there is little room for the erector tube to move.

Finally, below is a scope that has been adjusted too far up and right, and the erector spring has lost contact with the erector tube to support it. When this happens, the reticle free floats, and you will have point of impact issues. Mike tells me this is the most common of scope mounting issues.

So, how do you avoid these problems? First, don’t use up all of the adjustment in any direction with either the elevation or windage turret. Second, if you find yourself using up all of the adjustment, get yourself an adjustable mount. Put the scope back in optical center, then use the adjustments on the mount to get you pretty close to where you want to be zeroed, and use the elevation and windage adjustments on the scope to do the fine tuning.

Til next time, aim true and shoot straight.

- Jock Elliott

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25 Comments

  1. cna training says:

    found your site on del.icio.us today and really liked it.. i bookmarked it and will be back to check it out some more later

  2. Airgun Guy says:

    Great advise! Thanks Jock for yet another helpful article.

  3. TomPier says:

    great post as usual!

  4. n s says:

    nice post. thanks.

  5. RAMON MORAGA says:

    How can I go to zero my scope?

    1. Jock Elliott says:

      Before you sight in or zero the scope on your new air rifle or air pistol, there are a few things you need to do.

      The first is to identify a safe place to shoot. It could be in your basement, your side yard or your back yard, but it needs to be a place where, if your pellets or BBs miss their target, no people, animals, or property will be damaged. This is particularly important for first time shooters who may be more prone to miss.

      Second, you need a good, safe backstop on which you can mount your target. It could be a bale of hay, a commercial pellet trap, or a backstop that you make yourself such as a cardboard box filled with old phone books. If you make your own backstop, test it under safe conditions to make sure that it will stop the projectile as intended. Just because you think that a particular material will stop a pellet doesn’t mean that it will. A friend was amazed and chagrined when he found that his air rifle would easily blow through a sheet of plywood. If you’re shooting BBs, make sure the backstop is designed for them and won’t bounce them back at you.

      Now you’re ready to sight in. Sighting in is simply the process of making sure that, at a given distance (ten yards, for instance), the sights or scope are pointed at the same spot where pellet or BB hits.

      The easiest way is to start at a distance of 10 feet (That’s right, 10 feet, not 10 yards. A tip of the hat to Tom Gaylord, former Editor of the Airgun Letter for this suggestion.) Shoot one shot with the sights centered on the bull’s-eye.

      Look at where the shot hit. Ideally, the point of impact should be no more than 3 inches below the bull’s-eye and centered from side to side. If the shot is too high or too low, or to the right or to the left, consult your airgun or scope manual and adjust the sights accordingly.

      Take another shot from ten feet and see if your adjustments are getting you closer to where you want to be. Make small changes at first until you get a sense for how changes in the sight settings affect the point of impact. The windage adjustment changes where pellets strike from side to side, and the elevation knob or screw adjusts the height. Continue making shots and changes until your pellets or BBs are striking the target 1-3 inches below the bull’s-eye and centered side to side.

      Next, move back to ten yards, and shoot again. Your shot should hit the target a little higher and should remain generally centered left to right. All that remains is to fine-tune the windage and to adjust the elevation so your shots hit the center of the bull’s-eye. That’s it – your air pistol or rifle is now sighted-in for ten yards. If you shoot from a distance other than 10 yards, you’ll notice that your pellets or BBs will strike higher or lower, depending upon the range.

      A couple of notes: if you back up to 10 yards, and find your shots are going wild, return to 10 feet, check to make sure the fasteners holding your scope or sights haven’t worked loose, and try again. If you are shooting a multi-stroke pneumatic air rifle or pistol, be sure to use the same number of pumping strokes each time.

  6. Steve DeStefano says:

    What’s the procedure to return the scope to optical center? thank you.

    1. Jock Elliott says:

      As far as I know, there is only one procedure that I know of, and it’s tedious: for elevation, count the clicks between all the way up and all the way down, the set the elevation knob to the number of clicks that is half the total. Do likewise with windage.

      1. Jon F. says:

        Cut two V notches on opposite sides of a shoe box (or other suitable box). Put a brick or something heavy in the box to hold it steady and not interfere with the V notches. Lay the offending scope in the notches and focus on a spot on a paper tacked up across the room. (not necessary for the spot to be in perfect focus) While looking through the scope, turn it round in the notches while watching which way the crosshairs move away from the spot. Adjust the turrets until the cross hairs stay on the spot while fully rotating the scope around, the scope crosshair reticle is now centered. You would be surprised how many scopes come from the factory nowhere near centered.

      2. Orin says:

        I use a method similar to Jon F, but here are some modifications I’ve made to the procedure that work really well for me:

        + Use a straight edge (like the edge of the paper or a line drawn on it) instead of a dot.
        + Only adjust one reticle at a time. Rotate the scope back and forth 180 degrees until it is centered. Then adjust the other.
        + Use the highest magnification on variable power scopes. After optical center is found, verify it using different power levels, ensuring POA does not change.

        Procedure:
        Line one reticle up on the straight edge and rotate the scope 180 degrees. Adjust the reticle aprox. half the distance back to the straight edge. Line it up again and repeat. It should only take a few times to get one reticle centered on the straight edge. Now, rotate the scope 90 degrees and repeat with the other reticle using the same straight edge.

        - Orin

      3. Orin says:

        Forgot to mention that counting clicks often works, but sometimes does not. I have a scope that the windage is optically centered at mid-point, but the elevation is optically centered at 15 clicks past the mid-point. Counting is a convenient way to get close, though, without removing the scope.

        The fastest way to count is to turn the turret all the way one direction and then – using that corresponding point on the scale – count the number of complete revolutions plus additional clicks to the other extreme. For instance, 7 full revolutions plus 16 additional clicks max travel on one turret means adjusting it back 3.5 (7/2) revolutions and 8 (16/2) additional clicks will theoretically center it.

        - Orin

  7. Mark says:

    I saw a method on the internet that was different,

    1 shot at one dot.
    2 line up the gun back on the dot and hold it there.
    3 Crank the knobs until the cross hairs move to where the shot hit.

    I haven’t tied this myself yet, but it looked quick and easy.

    Wiz

    1. Jock Elliott says:

      Sounds plausible, but I haven’t tried it.

  8. blake says:

    My gun is still shooting way high. I have used all the adjustment on the scope.what should I do?

    1. Jock Elliott says:

      Blake,

      Contact http://www.airgunsofarizona.com It sounds like you may need an adjustable scope mount.

    2. Maddog70 says:

      Make sure you are using scope mounts of the same height. If your front mount is even slightly lower than your rear mount your scope won’t be able to compensate for it.

  9. Miguel says:

    Jock,

    I thought you might be interested in another way to find the optical center of your scope. Put the front end of your scope on a mirror and you will see two sets of crosshairs, one is the optical center (the reflection) and the other is your scope’s. You then adjust the turrets until they converge. I’m not sure of the actual mechanics or science behind the method, but it works. I suspected that my airgun had barrel droop and this method confirmed it. I would be interested in your thoughts about the validity of the procudure.

    Miguel

    1. Jock Elliott says:

      Miguel,

      I had not heard of that technique, so I can’t comment on it. Sound interesting.

  10. Israel says:

    I just bought a Gamo fusion whisper pro and have set the scope,but just after 20-30 shots it’s off all over the place .How do I know if the scope is defective or broken ?I’ve bought every kind of pellet in my home town big 5…any advice

  11. randy william says:

    this page is helpful:-) I’ll save it for offline reading

  12. Winfield says:

    I was trying to zero my scope and I saw that no mater haw much I trend my vertical and horizontal knobs my cross hairs have not moved any.

    1. Jock Elliott says:

      Winfield,

      Do not worry whether you can perceive the movement of the crosshairs, the important thing is whether the point of impact of the pellet has moved after your adjust the knobs. The whole point is to be able to adjust the crosshairs so that they intersect where the pellet is hitting. That’s zeroing the scope.

  13. Vicente White says:

    I purchased a Hawke 30/30 3x9x40 scope and bore sighted it using a tool as I got close to centering on the bore sighter my clicks became very tight after tinkering back and fourth on paper it seemed like a losing battle but I finally zeroed it in I just worry because it would no allow me to click much further. The elevation knob is tight not the windage? It did not seem to give me that much rotation?

    1. Jock Elliott says:

      Vicente,

      It would be that you need a drooper mount and you are running out of elevation adjustment.

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