Use Adjustable Objective Rifle Scopes

Adjustable Objective (AO) rifle scopes are designed to allow the shooter to focus on the target. There are three main advantages: The target is in-focus, being in-focus will eliminate parallax (movement of the point of aim when the shooter shifts the eye position), and the range information on the focus knob can be used for better aiming.

Steps

  1. Be sure that your scope is mounted properly. Sighting-in can have been done preliminarily, but additional sighting-in will be desirable after following these steps:
  2. Know your rifle. There are actually two optical systems in a rifle scope. The objective (front lens) focuses the target image on the reticle (cross-hairs); the eyepiece (rear lens) focuses the eye on the reticle. The eyepiece must first be adjusted before any adjustment of the objective.
  3. Most eyepieces have a lock ring. Unscrew the lock ring a turn or two so that the eyepiece can be screwed in and out.
  4. Adjust the objective for infinity (very far, often a small 8 symbol). This will cause anything you actually look at that is close to be very blurry.
  5. Point the scope toward a blank wall or the sky. At this stage, we are only interested in focusing the eyepiece on the reticle and not on any actual target. Screw the eyepiece in/out until the reticle is in sharp focus. This may take several tries with removing your eye from the eyepiece between tries to give it a rest.
  6. Once the eyepiece has a sharp reticle image, lock the eyepiece adjustment with the lock ring. (This does not have to be more than just snug.)
  7. Adjust the objective. Now that we have a clear image of the reticle, the distant target image can be focused on the reticle. In practice, you will think that you are focusing the image on your eye, but what is actually happening is that the image is being focused on the reticle. Most objectives focus by turning the front lens. Some focus by turning a knob on the side of the scope. In any case, the idea is to get a sharp image of the target. It will be easier to focus if you choose a target area with some contrasting texture such as wood grain - you will see the image "pop" into focus better on patterns than on large areas.
  8. You should now be able to move your eye from side-to-side while the rifle stays still and the cross-hairs of the reticle should not move on the target. This means you have eliminated parallax from your scope system for this target. If the distance to the target changes, you only refocus the front objective (or side focus). You never touch the eyepiece adjustment unless your prescription changes, basically.



Tips

  • Lens shades can help a lot. Many scopes come with an objective shade that screws into the front of the scope. Various slip-on shades are available for the eyepiece.
  • Keeping the lenses clean is important. Never use any paper product on a lens no matter how soft you think it is -- all paper contains silica (think sand). The best cloth is one that opticians sell or give when dispensing eyeglasses; it is a "silk" (same as for men's "silk" ties) that works incredibly well. A LensPen is also an excellent tool -- it comes with a brush to get rid of most dirt and a pad for the smudges.
  • The manual that came with your scope will have this same information but may cover special cases that you will find for a particular scope.

Warnings

  • Use empty chamber indicators.
  • Never assume a gun is unloaded -- check this before working on the gun.
  • Follow all firearm safety rules.
  • Keep the gun pointing in a safe direction when possible.

Related Articles

Sources and Citations

  • Eyeglass-friendly eyepiece shades are not common. One that works well is the iCuff DV designed for video cameras but works well on scopes.

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