Ranges are not always exact: June 30, 2008

Temperature affects optics by making them expand or contract. Since optical tolerances are measured in units smaller than one-millionth of an inch, any change in dimensions will affect how the optic works. In the case of parallax correction, the scale of yards will change with the temperature. So the number 20 on your AO scale could really represent 19 yards today and 23 yards next week, if the temperature fluctuates that much. Field target competitors compensate for this change by having three different color-coded range scales on their adjustment wheels, but you don’t need to. Unless you are concerned by your pellet striking one-tenth of an inch away from the aim point at 30 yards, the scale on your parallax ring or wheel is good enough for all shooting. Just understand that the measured distance will not always be the exact distance for this reason. Now you know what AO stands for. Even though many scopes with parallax correction are now using side adjustment knobs, the marketplace still uses the term AO to mean all scopes with parallax correction. Dirt E. Harry

Sidewheel adjustment knobs: June 29, 2008

The sniper scope builders also borrowed something else from airgun scopes, but as yet they have not developed it fully. That feature is a side adjustment knob for parallax. The location makes adjusting parallax easier because the shooter doesn’t have to reach all the out to the end of what is usually a very large scope. The adjustment knob is on the left side of the turret, opposite the windage adjustment knob. Airgun scopes had this feature back in the early 1990s (which is why the term AO isn’t always correct – but who cares?), and ten years ago, shooters began experimenting with the size of the adjustment knob. Dirt E. Harry

Some firearms scopes have parallax correction: June 28, 2008

Not all firearms shooters are cavalier about bullet placement. Tactical snipers, for instance, have to place the bullet exactly or they may not realize their goal. When a suicidal terrorist has his finger on the trigger of a bomb, it matters what happens to that finger after the bullet strikes him, and tactical snipers train for such missions. So optics companies like Leupold borrowed the concept of precision parallax correction from airgun scopes and adapted them to sniper scopes. Dirt E. Harry

Parallax Correction= Rangefinding! June 25, 2008

As it turns out, when a scope corrects for parallax at a given distance, it also focuses the target image as sharp as possible. You don’t notice this at 150 yards, because the image is so small that the details are too difficult to see, but at 30 yards they stand out vividly! So before long airgunners found out they could turn the parallax adjustment ring until the target image was sharp, then read the distance to the target on the parallax ring. Presto!-Instant rangefinding! Distance information is very useful to an airgunners whose pellet may drop five inches out at 50 yards. Many shooters think that’s what the AO feature is there for; they aren’t even aware of the parallax problem. Dirt E. Harry

Striving for Accuracy: June 24, 2008

Airgunners strive for accuracy that other shooters never dream of! We want to hit within half a pellet’s diameter at 35 yards! Firearms shooters don’t usually shoot that close, and they don’t think in terms of hitting within 0.09-inches of their aim point! The effects of parallax are greatest at very close range, so by the 1990s, many better airgun scope makers were putting parallax adjustments on their scopes. The shooter then looked through the scope and turned the objective until the target appeared as sharp as possible. When that happened, the parallax error has been reduced as much as possible for that particular target. Dirt E. Harry

Scopes and Firearms: June 23, 2008

Scopes for firearms usually have their parallax correction fixed at some distance like 100 or 150 yards. At whatever range has been set by the factory, there should be as little parallax as it is possible to have (in an optical sight you will never get rid of parallax entirely). And let’s be honest, a deer hunter doesn’t care if his bullet lands two inches to the right of his aim point – as long as it does the job he intended. Dirt E. Harry

So why care? June 22, 2008

Now that we know that AO means a scope has parallax adjustment, of what benefit is that? Well, the reticle in a scope is closer to your eye than the target. So, if you don’t place your sighting eye at the same place each time you look through the scope, the reticle will move just a little in relation to the target. Dirt E. Harry

The Adjustment Feature: June 19, 2008

The term AO means nothing by itself, but since the earliest parallax adjusting scopes put the adjustment feature into the rotating bell of the scope, the term AO caught on. For example, Look at the Leapers 8-32×56 Full Size AO Mil-Dot King of SWAT and you will see the range numbers (in yards) engraved on the objective bell. Dirt E. Harry

What does “AO” mean? June 18, 2008

I ran across this article by Tom Gaylord who is probably the authority on air guns. Tom has forgotten more about air guns than most of us will ever know. The article is about scopes and what the term “AO” means. AO stands for “Adjustable Objective.” Stay tuned. Dirt E. Harry

To sum it up: June 16, 2008

I don’t know about you, but this article by Tom Gaylord has been most informative. Thanks again, Tom for your dedication to our passion and to our sport. Dirt E. Harry