What does an adjustable objective lense do for a rifle scope?

The AO is useful for higher powered scopes used at varying ranges. It adjusts the parallax for a given range (apparent movement of the crosshairs) so that there is no introduced aiming error due to inconsistent eye alignment. It also provides an fine focus for a particular range.

Most non-AO scopes are pre-set at about 150 yards. The AO scopes are best used for target and varmint hunting. The non-AO scope is better (lighter, more compact, less complicated, less to go wrong), and therefore better suited to big game hunting.

Adjustable Objective scopes do eliminate parallax. But, unless your eye isn't in the center of the scope, you'll rarely need them. Certainly not under 300 yds., even for varmiting.
They don't focus the scope, unless they are of the side focus/parallax style as used on Leupold's LR/Mk.4 scopes, or something similar.
They also reduce twilight performance of the scope.
Essentially, they are for very precise shooting at longer ranges. Or, very short ranges. E

Eremicus you are in error young man when you say the AO does not adjust the focus. The AO does adjust focus at all distances when needed. Without the adjustable AO feature on a scope you would not be able to focus the scope at close range and on any power setting at any range.
It is true that at practical hunting ranges you do not need the AO feature on a scope used for big game hunting. The variable powered scope with the AO feature gathers just as much light at dusk and early morning light as any other scope of equal power. I own both models, with and without the AO feature and the AO feature gives me a more versatile variable powered scope adjustable for any shooting range.
In my view buying a variable powered scope without the AO feature is like buying a new car with no engine under the hood.

On powers of 12X and above I would want a AO. Also the AO feature is desireable for air rifle scopes.

The AO feature does seem to focus the scope.

I have a 4X12 Leupold VX-II made in the late 1980's that gets blurry at 12X. I sent it back and Leupold says there is nothing wrong with it! It's blurry Leupold! The lack of AO may be the problem.

The focus adjustment on my leupold scopes is in the eye piece, not the Adj. Objective. There are scopes that do both, and are so labeled.
The adj. objective scopes have an extra lense which cuts down on light transmition. Much like the difference between the fixed powers, and the variables, the scope with the extra lense(s) don't transmit as much light, therefore their twilight images are less bright.
Some variables do do better than fixed powers-by virtue of their higher magnification-under twilight conditions. However, if one compares comparable scopes, same objective lense sizes, same number and quality of coatings, and at the same magnification, the fixed power has the brighter image under twilight conditions. E

Every AO scope that I've seen does focus the picture in the scope. The eyepiece focuses the crosshairs.

I don't think that there is an extra lens in an AO scope. Both have "objective lenses", but the AO scope moves it in/out, while non-AO is fixed at one particular distance.

Just my $.02,

Bill

Wouldn't an AO scope be good for a .22 Rimfire? Since you can focus the parallax down to 35-50 yards, instead of the 150 that most large scopes are parallax free? That way, you can put a better scope on a rimfire, than the little bitty rimfire scopes? Is that right? Jeff

Actually, I believe Bill M. is correct. Certainly is the case with my Leupold V-III 4.5x14 AO. The ocular lenses moves to focus the crosshairs and the abjustable objective lenses focuses the sight picture. The focus adjustment is very noticable and has a considerable effect on accuracy, even between distances like 100 vs. 150 yards.

As for the comments about rimfire scopes, Leupold and Burris both make rimfire/air rifle scopes with the AO feature that focus as close as a few yards. Even some center fire scopes, such as the Leupold VX-11 4-12 AO or M8 fixed 12X AO will work well on a rimfire as long as the crosshair is fine and the range isn't too close for focus. In fact, that is what I am planning in mounting on a CZ .22 rimfire with a heavy barrel. Should make a great rat zapper.

I have a 4-12 AO Leupold on my ULA rimfire and can really tell a difference in focus at 6 power and above if the adjustable objective is not set pretty close to the intended target's range. I use the ocular to focus the reticle (at the clear sky).

Here's a complete description of how parallax occurs.

U.S. Optics

Eagle Eye,

The 4-12 has the fine duplex. A versatile scope for a rimfire.

JB

I use AO scopes on all my rifles with the exception of my 375 H&H.

Some AO scopes have additional lenses, some do not.

The AO does indeed, focus the optical target while the eyepiece focuses the cross hairs. The eyepiece will also focus the the optical target, but that is not its intended purpose. Most manuals instruct you to sight the eyepiece to the crosshairs using a white wall or blue sky as the optical backdrop. Focusing the eyepiece to the optical target may well result in eyestrain as your cross hairs may not be optimally focused and the eyes will quickly adjust to compensate.

I do a lot of long range playing with THE 22LR and an AO scope is a must for precise placement. A quality scope on one's 22 will really elevate the "fun" factor.

Are AO scopes as rugged as their respective counterparts? Of course not. They have more moving parts, Less efficient sealing, etc. If you are planning on banging your scope around, stay with a fixed power model! AO scopes also are at their best with nice clean lenses. You would be surprised at how much light transmission is reduced with a light dust film on your objective. And a crisp optical target is hard to achieve with a smudge on the big lens.

AO scopes are not for everyone, to be sure. So what? There is a model of quality scope available for each and every one of us to play our own little games.

Ain't it great to be a shooter/hunter?!

~Holmes

The distance from the front to back lenses in a scope determines focus. Hence both the front and rear adjustments will affect focus.

The objective (front) lens forms a real, inverted image at a plane in the scope. If that image is in the same plane as the crosshairs, there is no parallax. Putting the objective lens on a fine-pitch screw allows you to move the plane of the real, inverted image to coincide with the plane of the crosshairs.

It's no more complicated than that.