What does AO mean? And why should you care?
What does AO mean? (September 2005)
By Tom Gaylord
Pyramyd Air gets more questions from customers about scopes than any other single item, including the airguns, themselves. Most of the questions involve scope terminology, which has been addressed in the article All about scopes. Part 1., but the term AO was not covered very well. And many of the better scopes have it, such as Leapers 4x32 Mini A.O. The Bug Buster. They figured if they were going to advertise them that way, someone should explain what AO is and why it matters to you.
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AO stands for Adjustable Objective. That term means nothing by itself, but since the earliest parallax adjusting scopes put the adjustment feature into the rotating objective bell of the scope, the term AO caught on. Look at the BSA AR 3-13x50mm scope and you will see the range numbers (in yards) engraved on the objective bell.
The parallax ring on this AirForce 4x16 scope has been set to about 12 yards!
Now-why do you care?
Now we know that AO means a scope has parallax adjustment, but what 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 appear to move just a little, in relation to the target. That's going to move the strike of the pellet just a little, too, and that means less accuracy!
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.
But 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.
Parallax correction ALSO means rangefinding!
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. (You may want to read the article Velocity and pellets.)
Some firearms scopes have parallax correction
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.
Sidewheel adjustment knobs
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.
Here you see the third adjustment knob on the left side of the turret. It is for parallax adjustment.
Larger knobs, or sidewheels, make for more precise adjustments
By making the adjustment knob very large, a shooter can place white tape around the rim of the knob and write in the actual distance figures, as actually measured on a range! Because the adjustment wheel is on the side of the rifle instead of on the scope objective bell it can be very large and still clear the rifle. Marks made on the edge of a large wheel are far enough apart that real precision is possible. Look at the optional Leapers Accushot SWAT Side Wheel accessory to see what this looks like. This wheel will mount on any Leapers scope that has a side-adjustable parallax knob, like their 16x50 AccuShot 30mm scope.
Leapers optional sidewheel fits the side-mounted parallax adjustment knob found on many of their scopes.
Ranges are not always exact
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.
Do you REALLY NEED AO?
Well, no, you can get by without it, if need be. In fact, you can get by with open sights, if a scope isn't in your budget. But, if you are contemplating a future scope purchase, I recommend that you consider saving a little longer until you can afford a scope that has AO. It makes your shooting that much better!
In the meantime, until you get a scope with AO, here is what to do. Practice mounting the rifle in the same way every time, so your face contacts the stock at exactly the same place, shot after shot. That way you will reduce whatever parallax error your non-AO scope has, and you will get the best accuracy possible under the circumstances. I owned an FWB 124 with a Tasco scope long before parallax adjustments were heard of and I did all right at the longer ranges because I always held the rifle the same way.