View Full Version : AO is off...should I be concerned

March 12, 2014, 01:59 PM
I've recently put an old USA Weaver back in service. The scope has an adjustable objective. Windage and elevation is good and responsive,....clarity is acceptable....but the markings for "focus" on the AO are way off. "Focused" for me at 200 shows 75 or so on the AO ring. Other distances are similarly off.

Is this worth sending the scope off to the "Ironsight" people.....I can live with it?

Bart B.
March 12, 2014, 02:06 PM
I would first ensure the scope reticule is in sharp focus as soon as you look through it at a clear blue sky. If not, adjust the scope eyepiece until that happens. Don't look through the scope more than one second else your eye will try to focus on a blurred image. Once instant views in the scope have the reticule sharp and clear, then check the scope focus range versus actual range.

With the scope fixed solid, moving your aiming eye around looking through it should have the reticule fixed in place on the target. If the target moves to the right of the reticule as your eye moves to the right, the scope's focused further than the target is. Move the scope's AO to a closer range. Use a further range if the target moves to the left of the reticule as your eye moves to the right.

Brian Pfleuger
March 12, 2014, 02:20 PM
AO isn't "focus". Bart describes the correct procedure.

You can, in fact, use the AO as a crude range estimator by turning it one way until it's blurry, note the setting, turn it the other way until it's blurry and go to the middle. That's crude though and the AO is not meant to "focus" the image.

Bart B.
March 12, 2014, 02:24 PM
Brian, adjustable focus scope sights are exactly like camera lenses. Their front elements move forward to focus on closer things. Both focus distant images on a plane; sensor in cameras , reticule in scopes. Same optical formulas are used designing both.

SLR Camera eyepieces moves their lens to focus the ground glass image to the eye exactly like a scope eyepiece does focusing the reticule for the aiming eye.

AO = focus.

Brian Pfleuger
March 12, 2014, 03:08 PM
I don't think I've heard of Adjustable Objective meaning "focus". The focus is on the eye piece/Ocular lens while the AO (parallax adjustment) is either on the side or the objective lens.

Bart B.
March 12, 2014, 03:21 PM
AO scopes were called focusing decades before yuppies could not understand that parallax happened when the AO was set to a range different than target range and their aiming eye was off the scopes optical axis. Scope company reps couldn't find a way to easily convey that to customers. So they chose to appease them instead of educating them. Hence the term "parallax adjusting" came into popularity.

Parallax is only corrected physically by moving the aiming eye back on the scopes optical axis where parallax never exists regardless of where the AO is set for distance.

Read "parallax adjustment" in


Both methods change objective lens groups position to focus target image on the reticule. One moves the front elements and the other moves the back ones. Both are in front of the reticule. The eyepiece/ocular lens behind the reticule does nothing but position it's lens so the eye sees the reticule in sharp focus.

March 12, 2014, 03:53 PM
FWIW the opinion of someone who worked with optical systems for 50 years.

As long as it "seems in focus" & the reticule does not exhibit parallax, then you're good to go. It is actually mind-numbingly simple to fix the scale if it really bothers you though. A monkey with a watchmaker's screwdriver could do it.

Notice I say "seems" what can happen is the eye can be distracted by trying to focus the out of position "Aeriel Image" produced by a poorly set AO front end, & try to work out 2 different planes of focus at the same time. While the front end (objective) optics are similar to a camera they aren't identical so its not a great comparison. Camera lenses create a "real" image onto a fixed plane, a telescope (or sight, or monocular) collects this image & passes it on to a second set of optics (the eyepiece group) which project out as a "Virtual image" appearing at infinity so the human eye can focus inside the tube.

The focusing scale is engraved on an outer tube (or ring) which is clamped to the actual objective body holding the lenses. This is done at the factory to simplify construction & assembly & the "clamp" can slip. If you want to fix it yourself try this.

BUY A SET OF WATCHMAKER'S SCREWDRIVERS (always use the right tool for the job!)

Set up the rifle & scope in a fixed location with a clear view of a target at a known distance.

Set the objective for zero parallax by looking through it. This sets up the actual focus tube & optics correctly, only the external scale is now "off".

Back off the 3 (maybe 4) small recessed grub screws till the outer ring ONLY moves.

Turn the scale to the measured (or known) target distance. Don't let the internal tube move, if it does re-tighten & reset the optic.

Tighten the screws evenly till it locks again.

That's it you're done.:cool:

Bart B.
March 12, 2014, 05:02 PM
Wogpotter; good post, been there, done that. Refocused fixed power scopes to focus exactly at 100 (75 in one instance) yards by using an optical spanner wrench (made from an old cheap steel caliper) to remove the lock ring in the objective lens barrel, twist objective lens barrel a bit to eliminate parallax at the target range then put the lock ring back in and tightened it.

Most scope companies set their objective lens scale with the lens at infinity on an optical collimator, then set the scale to infinity (or at some shorter distance and set the scale to that one; either way works). Problem is, no single lens nor lens group has the same exact focal length. When several lenses and groups are used, the issue compounds if not corrected with trial and error fitting of each lens group in the scope. There's a tiny difference. It usually doesn't show up unless your scope's focused at 50 yards or feet with decently made scopes. I've checked a few that were focused exactly at 150 yards when the scale was set to infinity.

It's my opinion SLR cameras and rifle scopes have the same optical systems. One focuses the distant object on a ground glass screen, the other in a plane about a reticule. Both have eyepieces to focus the human eye on the image a few inches away.

A scope with side focus moves a middle lens barrel to refocus images from the front lens barrel at the scopes front through erector lenses then back onto the reticule. Both barrel's lens groups plus the erector lens group comprise the objective lens system. Like lens systems are used in cameras.

Magnification/power is equal to the optical effective focal length of all lenses in front of the reticule (the objective lens group) divided by the focal length of the eyepiece lens group. Eyepiece focal lengths are about 2 inches on back focal plane scopes. SLR cameras are the same; lens focal length divided by eyepiece effective focal length equals image magnification.

Rifle scopes are also like military binoculars' right side which has a lined scale in the right half. That scale's on a clear glass plate (like rifle scopes with their reticules on glass plates) that's less than an inch in front of the adjustable eyepiece on that barrel. It's position is fixed and the front objective lens focuses the distant image on the side of the glass the reticule's etched into. Focusing for its user is done by moving the eyepieces in and out by turning the knob between the barrels so the left one's image is sharp and clear, then you turn the right eyepiece to focus the reticule sharply that also has the down range image focused on it from.

March 12, 2014, 09:04 PM
Wow! You guys are so sharp you scare me. Don't mind me much I'm the dullest knife in the drawer. I read every word but just don't understand it all. I suppose it could have some thing with the jargon. I have been stumped with my old 10x Redfield and my 4x Leupold. There clarity espeshely on the Redfield seams to have gone to pot. The Redfield has a AO. I realy don't think I am qualified to do the tool thing and I will be avoiding it. So would you mind going over the tuning again for this monkey? and then for the Leupold fixed 4x without AO PLEASE?

Bart B.
March 12, 2014, 09:34 PM
Longshot, take a scope, loosen the lock ring at the eyepiece, back the eyepiece out 'till it stops.

Look through the scope with it pointed at a blank wall or the clear sky. But look only for 1 second, then look away.

If the reticule was fuzzy, screw in the eyepiece one full turn then do that quick look routine again.

Repeat that until the scope's reticule is now pretty sharp.

Next, turn the eyepiece in only half a turn. Then look in for 1 second.

Repeat that half turn then look until the reticule is clear and sharp the instant you look through the scope.

Tighten the lock ring on the eyepiece; keep it there.

Now look through the scope at some distant target. If it's got an adustable objective, set it at the target's range. If the target's in clear, sharp focus, all's well inside of it.

If not, it's got a problem.

March 13, 2014, 01:25 AM
Man, now I'm fuzzy.

The old Weaver has an adjustable objective lens. I understand and have already focused the rear objective/reticule prior the post. Knew that much.

Have never encountered the scale on the objective bell "off" as far as this one.

Would a set of aging eyes account for the disparity?

Searching for "grub screws" on the bell now.

March 13, 2014, 01:44 AM
Even "Curious George" won't adjust the scale on this bell.....cause it doesn't have any "grub screws" out there. Even looked with a hand lens for them.

The scope will come into focus at practical ranges though by cranking in on the AO till its gets clear. I have lots of room to move the bell/AO forward, but not so much left screwing the bell in.

My other vintage Weaver with an AO bell focuses very well with given distance, but not this one.

Again, could it be my poor close up vision that is effecting the AO setting for me? I suppose I could have somebody else adjust and see what they end up with.

Bart B.
March 13, 2014, 07:15 AM
There's a slight chance that scope's AO is a full turn off.

With the AO in all the way, what's the range it focuses good at?

Wogpotter, you said Camera lenses create a "real" image onto a fixed plane, a telescope (or sight, or monocular) collects this image & passes it on to a second set of optics (the eyepiece group) which project out as a "Virtual image" appearing at infinity so the human eye can focus inside the tube. That's exactly what happens when I mount my Nikon SLR camera lenses on a Nikon Lens Scope Adapter which focuses the len's aerial image (at its focal plane where film or digital sensors are in cameras) so the eye sees it at infinity. Prisms inside erect the inverted aerial image. It's got an adjustment for focus on that aerial image exactly like a rifle scope. Focal length is 10mm so a 200mm camera lens converts to a 20X telescope.


March 13, 2014, 07:52 AM
That was my point about scopes Vs Cameras. You have to attach the other 1/2 of the system which is why they aren't identical, just similar.

Bart B.
March 13, 2014, 08:07 AM
Wogpotter, now I understand. Rifle scope optics in front of their eyepiece and erector lenses are the same as camera lenses. Now we're on the same page. Thanks for clarification.

March 13, 2014, 10:02 AM
Bart, Good communications! My good old Redfield is in tune and ready for action. My 4x Leupold is good but I see now that I could use a AO. A 8xAO might be what that 77/22 K Hornet needs for those bushy tails...
Thanks a lot for your help.:D

Brian Pfleuger
March 13, 2014, 10:38 AM
AO scopes were called focusing decades before yuppies could not understand that parallax happened when the AO was set to a range different than target range and their aiming eye was off the scopes optical axis. Scope company reps couldn't find a way to easily convey that to customers. So they chose to appease them instead of educating them. Hence the term "parallax adjusting" came into popularity.

Parallax is only corrected physically by moving the aiming eye back on the scopes optical axis where parallax never exists regardless of where the AO is set for distance.

Read "parallax adjustment" in

It amazes me how we like to complicate simple things.

Regardless of what may have been "decades ago", in the modern world no major manufacturer (that I've ever seen) refers to the Adjustable Objective as "focus" of any kind. This includes NightForce, Leupold and Sightron, just to name a few.

This is a very simple issue. First the reticle must be focused. This is done with what is often called (NightForce, for example) the "Fast Focus" ring if it doesn't have a lock, located on the Ocular lens portion (closest to your eye). This should be done with the Adjustable Objective/Parallax (not called the focus according to NightForce) set to infinity and the magnification set at max.

Once that's done, the parallax adjustment can be made by sighting at an object of known distance, setting the AO (parallax) to the indicated distance and moving the shooters head side to side while looking to see if the reticle moves in relation to the target object. If it does, the parallax is not correct.

If this is the case, the AO (parallax) should be adjusted without regard to indicated range until the shooters head can move without the reticle moving in relation to the target object.

There's typically a tiny set screw which holds the adjustment ring. The screw is loosened and the ring is turned so the indicated yardage is true to the actual yardage.

If the distance indicators on the ring are correct, the parallax should now be correct at all distances. If they're not, the shooter will have to adjust to different yardages and make their own marks.

March 13, 2014, 11:27 AM
That's true.

You can actually set the parallax visually in a target situation, ignoring the ring completely, assumes you have the range of movement in the ring. Unfortunately if the scale is "off" because it slipped or something, then you are forced to do it visually with the old "bobbing the head & watching the reticule move" trick.

Anyone wanting a "no tools" scale fix can simply set the ranges visually then using white tape & a fine point marker just make your own scale & cover it with clear tape for field use where you just want to dial in a distance.

The tip about focusing the eyepiece with white paper is a good one because it puts the image generated at the front of the scope & the physical reticule in the same plane of focus. Then you can dial in the parallax adjustment with the target & reticule clear & have way less eyestrain.

Bart B.
March 13, 2014, 11:32 AM
Brian, I agree with your comments on what scope maker now call that front objective lens that they all originally called what focused the scope for target range.

That's why they have range markings (yards, feet meters, etc.) on them they're set at to focus at instead of some number for a given amount of parallax. When the AO's set at some distance mark equalling that of the target, the scope focuses the target image on the reticule. Objects at closer or further ranges that the target's range will focus behind or in front of the reticule.

Jim Watson
March 13, 2014, 01:26 PM
That is unusually far off, but I think I have seen more AO scopes that did not match the parallax free distance with the scale on the bell than ones that agreed. Even so, I prefer an AO to a side knob. My Leupold "side focus" is very tedious to get parallax out on.

March 13, 2014, 07:43 PM
My 1978s old Redfield 12x with the OA is in tune thanks to this Forum. It's been at least 15 years since I have looked at scopes of that type. Correct me if I'm wrong but aren't the side focus scopes a new technology. Guys like my self still some times call the AOs focus. Do to the lack of keeping up with speed of it all.

Bart B.
March 13, 2014, 09:01 PM
Side focus scopes use an extra lens group between the objective lens and the erector/zoom lens group. Instead of its objective lens moving back and forth to focus the target image like AO scopes do, that extra lens group refocuses the target image from the objective lens through the erector/zoom group so it'll focus on the reticule.

It's all about bending light rays. Optical design software makes all sorts of imaging focus stuff much easier these days. It'll also help design the adjustment mechanics to move one end of the inner tube a given angular amount for click values desired. Some scope's adjustment click will move that tube 0001" or less.

March 14, 2014, 01:38 AM
I think I've got it! Bart B's comment about a "full turn off" got me thinking as well as the "all the way in" bit.

I ran the AO all the way in. I then measured off 50 feet (the lowest setting on the bell) and started cranking out 'till an object at 50 feet came into focus. It awhile , took maybe a full revolution out or more,....I wasn't counting.

Lo and Behold! The scale was very near 50 feet. I then set the scale on 100 yds and scoped my 100 rd target across the field. Clear as a bell,,,,I think. Did same drill with scope on another power setting and same results. There seems enough adjustment left to crank "in" the remaining settings at 200, 500 and 1000 and I'm betting those distances will be close on the scale as well.

I will double check tomorrow in better light....but I believe its good. My take is that the AO bell was cranked in/ out WAY to far one way or the other......(obviously its a used scope, and I've never had it on a rifle before) and was giving me goofy readings on the scale as a result. Zeroing the thing all the way in and coming out to a known close distance seems to have provided a solution.

Thanks all.

March 14, 2014, 07:22 AM
There is usually a "stop" inside to prevent over rotation, maybe it was dinged & no longer stops the bell rotating too far?

Bart B.
March 14, 2014, 10:13 AM
You can use a pair of binoculars to look through your rifle scope from different angles (up close and personal) to see greater apparent movement of the reticule to the target.

Anyway, after getting your scope well focused on a target at, say 50 yards, and it's parallax-free, mask the scope's reference mark with something then put a new mark directly opposite the "50" on the scope's AO barrel. It should be well focused and parallax free at any further ranges when that range number is at that new mark.

Found this web site page that might help folks understand parallax and focusing:


Note that the diagram's showing light from the target bends the same angle going through the outside edges of the objective lens regardless of the range to the target. If the target's close, the lens has to be moved forward, closer to it, to get those light rays focused on the reticule. If the target's further away, then the lens has to be moved back. The target image focus point moves back in the scope as the objective lens gets moved towards the target as it gets closer to the scope. The reverse happens if the target is further away. The target and its image move in the same direction and opposite of the direction the AO, be it either a barrel that turns on the front of the scope or a knob on the side of the adjustment turret.

March 16, 2014, 02:40 AM
Well, I was wrong. The focus at 100 yds and out varies with the power setting. What is in focus at 100 at 6x, is not in focus at 9x and requires a change in the bell. When it does clear up, at 9x/100 +, the scale is not anywhere near the correct yardage. Pretty much back where I started. I have not experimented with groups and POI at various power settings, all my zeroing and groups were at 9x, which worked fine. I'm suspect now that the rifle will shoot to different points depending on the power setting.

I think the old scope may go back in the cabinet.

Bart B.
March 16, 2014, 08:09 AM
If the scope's eyepiece is not set right to make the reticule appear sharp and clear, it's possible that a target may appear a bit fuzzy if the AO is set at the target's range.

I suggest you set the scope's AO to a close range value, then look at a target that's exactly that range away. Then adjust the scope's eyepiece until both the target and reticule appear sharp. If this can't be done, then I suspect the scope's got some mechanical issues.

Bart B.
March 16, 2014, 08:40 AM
For those interested in one company's correct use and purpose of setting that side knob on their rifle scopes, check out the following:


March 16, 2014, 09:30 AM
Ideally yo should set the AO at the highest setting & it should then be good for all lower settings at the same distance.
If you set it at lower power you might just be seeing the increase in magnification making it go "soft".
Its beginning to sound like there is something other than the scale being off happening here.
Try this & see what happens.

Rest the rifle so it can stay in place without holding.
Set the highest power for magnification.
Put a plain sheet of white paper in front of the front lens to reflect light into the scope. (usually a 45 degree angle works best.)
Briefly look into the scope & see if the reticule is immediately sharp.
If it isn't adjust the eyepiece focus until it looks sharp & then look away for a few seconds.
Go back to a quick look.
Repeat the quick look~adjust~look away~look back until it snaps into clear sharp focus when you first look in the eyepiece.
Remove the paper reflector.
Set a target up at a known distance. Visually focus the front eyepiece in the same look in~look away technique as you used before for the eyepiece.
Now slightly bob your eye up & down (or left right, either works) till the reticule doesn't "track" (move) in relation to the target.
Reduce the power setting.
do the eye bob again.

If the parallax changes as the power is reduced then there is something wrong in addition to the AO scale.:(

March 16, 2014, 01:01 PM
I've never been concerned about how accurate AO markings may or may not be. I just turn the adjustment until I see the best image. And it matters not to me if the reading on the dial is close to the target distance.

March 23, 2014, 01:01 AM
I put the 6x Leupold back on the rifle. The old AO Weaver seemed OK, but there were so many peculiarities in the set up that I bailed on the whole idea.

Dang scope was likely to big on the rifle (size/weight) anyhow. The click adjustments were great, very responsive and the bullet holes moved around on target very gratifyingly, but the in focus/out of focus, what if...... just got to me.