By B.B. Pelletier
Today’s post answers a question I received yesterday. To get the best performance from a scope, its adjustment knobs need to be in the optimum range, which is very close to the scope’s optical center.
What is the optical center of a scope?
The optical center refers to the reticle and the field of view. An optically-centered scope shows zero reticle movement against a distant backdrop when the scope tube is rotated in a complete circle. Theoretically, that’s possible to achieve, but I’ve never seen it. The best I’ve seen is a reticle that moves about a quarter inch against a target 20 yards away when the scope tube is rotated in a complete circle
Use a box instead of a gun to optically center your scope
I suppose you could rotate the scope in its rings if the rings’ top caps (the top half of the rings) were removed and the turrets would clear the top of the gun when they came around, but I like another method.
With a small box sitting on a shooting bench, cut two v-shaped slots into the top edges and rest the scope on those two slots. Once the scope is focused on the target (which is 20 yards away), look through the scope and put a dot or mark on the target close to where the crosshairs are. It helps to have two people when you do this – one looking through the scope without moving it and the other downrange to draw the mark.
Rotate the scope tube around a complete circle and watch how the center of the crosshairs moves against your mark. It will probably move several inches at first. Figure out which adjustment knob to move and make your correction to reduce reticle movement against the target. This takes some time.
On the best day, I’ve adjusted a scope in 45 minutes. At worst, it took almost two hours to get it as good as it would get. Remember, it’s almost impossible to remove the last bit of movement out of the reticle. Both the windage and elevation knobs will probably need some adjustment.
DO NOT think you can count the clicks of adjustment between lockup and complete spring relax and go to the center of that number for the optical center. It sounds good, but that way never works. Optical center is more precise than the center of the click adjustment range.
Once the scope is optically centered, put it in an adjustable mount like B-Square’s AA adjustable mount and use the mount’s adjustments to zero the scope and gun. Once you’re zeroed, you’ll still have all the scope’s best range of adjustments remaining.
Is optical centering nesessary?
Absolutely not! You can just mount a scope and use it without going through this drill. But, if your scope runs out of adjustments or if you find yourself shooting to the right of the target at close range and to the left at long range, centering will cure the problem.
39 thoughts on “How to optically center a scope”
Great adivce. I was planning on turning either way and counting the clicks…but I figured there was a better way. Let me ask you another question. I took my scope off the other day and found the scope stop pin had slipped back about half an inch and scratched the top of the gun. Is it common for this to happen? Will the B-Square mount have a longer pin that will keep the scope from slipping back? This also caused the back of the hole on the gun to become “shallowed out.” Should I take any corrective measures there or just hope future mounts don’t slip.
Centering of a scope’s adjustment dials
The elevation and windage adjustments of a scope are easily centered. Place a small mirror against the objective end of the scope. That would be the end farthest from your eye as you look through the scope. Make certain that the mirror is large enough to cover the entire objective. It must also be flat against the objective. With the scope’s power selector ring set at the lowest magnification, look through the eyepiece as you would while aiming at a target. If the scope’s windage and elevation adjustments are off center, you will see two images of the reticle (cross-hair). To reach the center of the adjustment range, simply turn the elevation and windage dials until you see only one image of the reticle.
Do you have to center this way before using adjustable mounts?
To comment #1
Do you have an RWS rifle? What looks like a scope stop hole in their scope base is too small to contain ANY scope stop pins, and scope stop pins always dig a trench in the base.
Some of their bases have a large-headed screw and people sometimes butt the rear scope ring against that, but I’ve heard of cases where the recoil eventually sheared off the top of the screw.
I always reverse my rings, and hang the stop pin in front of the RWS scope base. That means the front ring base is LESS than halfway on the RWS base. If you have a one-piece scope base, it works better.
If you have another type of gun, tell me what it is and I’ll try to offer some advice.
This is for adjustable mounts:
Yes, scope centering is used when you intend using adjustable mounts. Adjustable mounts are really wonderful, once you get them sighted-in, but they can be a real bear to set up.
Maybe I’ll do a post or two on them in the future.
In reference to the scope stop pin, its on the same gun that started this whole optically center scope discussion….the Remington Genesis. Does the B-Square mount have a longer pin?
Okay. I’m sorry, but I forgot that. I answer several questions a day and I sometimes get confused.
The Remington Genesis is a Gamo (I bellieve, from the trigger and the stock styling), and I don’t recall having the same problem with Gamo as I mentioned with RWS.
The B-Square mounts have a stop pin that’s adjustable for depth. The pin screws up and down in its hole via an Allen screw that comes with the mount.
Is your current mount’s stop pin adjustable? Some are just roll pins that have to be drifted with a pin punch.
LIVING IN FLORIDA, I WONDER IF TEMPERATURE DURING A 2 HOUR SHOOT ON AN OUTDOOR 33 FOOT RANGE WOULD CHANGE THE POINT OF IMPACT?
Florida is pretty moderate, to the best of my knowledge. Unless the temperature rose or dropped by 30 degrees, the point of impact should stay the same.
I have seen the temperature drop suddenly enough to affect point of impact in the northern states, but it was a rare occasion.
Also, 33 feet is very close, so the temperature would REALLY have to change to see any movement in the POI.
B Square sent me their catalog that included one with air gun information. It had optically centering a scope. Is the diagram correct on how it gets off centered?
Yes, it is correct. I think the B-Square brochure is a geart explaination of scopes and scope mounting.
Hello, B.B.,,,,,,,This question regards your post about optically centering a scope, on July 5, 2005.
First about your definition of a complete circle….
a.) Do you mean to rotate the scope only one time? or…
b.) Do you mean, as I think, to continue rotating the scope , observing that the reticle actually makes a complete circle around, say the 9-ring, to return to its starting point????
c.) If it is ‘b’ to which you refer, and to which you answered on 7-05-05, please describe specifically, the procedure by which you determine::: left or right, up or down, which one first, how many clicks, how not to lose control and break down emotionally.
I meant B, continue to rotate the scope in a complete circle and watch the intersection of the reticle against a point target 20 yards away.
As for “How many clicks and which knob to do first?” that is your job to discover! Each scope will be off in a different way, so just keep adjusting as needed until you have the reticle moving as little as possible against the point target.
Talking about it is not enlightening. You have to actually do it to understand the process completely. The first scope can take as much at two hours to figure out. After that, you can do the others in lesss than an hour, but not much less.
Good luck, and don’t forget to use adjustable mounts after your scope is centered.
Thank you for your prompt reply
the Ginger Man
I just put a Crosman 4032 scope on my Quest 1000. It looks so beautiful now, but my question is this: I zeroed the scope at 20 yds, as per your suggestion for flattest shooting out to 30 yds. My first 3-shot group printed roughly 4″ left and 2″ high of center. The scope states that 1 click equals 1/4″ @ 100 yds. It took (if I remember correctly, because I cannot now find my data sheet, just the target) 42 clicks to shift horizontal point of impact to center. Does that seem reasonable to you? Seems like a lot to me.
20 yards is 1/5 of 100 yards, so things that move a certain distance at 100 yards only move 1/5 as far at 20 yards. To move 4 inches at 100 yards requires approximately 16 clicks. So at 20 yards it takes 5 times 16 or 80 clicks to move the same distance.
I’d say you got off light!
Whew! So I AM in the ballpark of what’s expected. Thanks for easing my mind. -Joe
I’ve been devouring your archive!
How to optically center a scope
I found this method that worked for me. Did a scope in about 5 minutes.
Basically you look thru scope at a mirror, and align what you see.
Hope this is useful.
Love your blog! except I thought I wanted a pellet pistol, and now I want
a pump up
a single pump pneumatic
a CO2 pistol
maybe a springer.
Haven’t even bought anything and I’m out of control!
Keep up the good work.
To Bud W.
How do you make sure that you are looking straight in the mirror and not at an angle?
If I use adjustable mounts, can I get away without optically centering the scope? I did notice that I shoot to the right at closer than 20 yards..
If you can live with shooting to the right then you don’t have to center the reticle. I don’t do it anymore.
Thanks B.B. I ll probably will try to reduce it…
This post has been great. I used the advise that I saw here, I optically centered my scope, used an adjustable mount (Beeman 5039) on my RWS model 54.
I am shooting consistant groups of 1/2 inch at 20 yards and 3/4 inch at 40 yards.
Being new to airguns this information was very valuable.
I tried to do this with my RWS 300 scope that came on my 460 magnum but i cant seem to get it to work. Looking through the scope when I turn it, the center of the reticle moves a couple of feet, what I can see through the scope moves with it as I turn. If I look down the front side I can see that its like the insides are not centered. I did accidentally dent my scope tube when tightening the caps and not paying attention so I dont know if I broke it or if its defective. Help!
I’m concerned that you say what is seen through the scope moves as the scope is rotated. That’s not supposed to happen. I would get that scope looked at by RWS USA, if you live in the U.S.
Thanks BB. Wasnt sure if you would see this as its such an old blog!
I’ll get in contact with Umarex and see what they say. I just hope I didnt somehow break it myself or void the warranty with the small dent I made with the cap. I have only very recently really gotten into shooting and airguns and your articles have been very helpfull!
This post belongs here, so I have copied it from where I originally posted it.
Fast and easy way to Optically Center a Scope with a Mirror.
I just tested the mirror technique on my 2 spare scopes, a Bushnell Banner 3×9 and an Optronics 4x. It took me less than 3 minutes from removing the caps on the adjustment knobs to centering BOTH scopes! And the Banner took a good 2 full turns to center on one axis and the other scope I needed a penny to turn the knobs with!!
I used a 4″ mirrored candle coaster that had fairly thick glass to do this.
I found by experimenting that up to a point, the thicker the glass is, the better, you not only get more light in to see what you are doing, but the farther out you are from the mirror surface, the more it amplifies the deviation.
I also found that you need to hold firmly to the end of the scope on the glass because any little movement causes the image to shake.
With that said, you will need to also remove any rubber pads that might be on the bottom of the coaster. Best thing is, you can get these things at any Target, Walmart, Goodwill, Hallmark, or candle store…. etc., and you will not have to cut any glass or worry about sharp edges! I also found that clear plastic gives a false image, so use glass.
As another blog member mentioned elsewhere about this technique, it does depend on the Bell of the scope being “square” with the rest of the scope. Still, even with a crooked one it gets you very close…. very fast! And if the scope is that bent up…. it probably isn’t going to work well anyhow.
I have not tried to center a scope and will be doing my first one soon.
But I am lost on how it helps once you are finished, and have it centered.
You put it back on the gun, and then say you have to adjust the scope for windage, or from 20 yds to 75 yds.
Doesnt this adjusting change everything you just did with adjusting for centering. ??
Here is what optically centering does for you. It keeps all your shots on the vertical reticle as the range changes. A not-center scope can shoot a little to the right close in and to the left far away. We may only be talking less than one inch at airgun ranges below 50 yards, but to those who wish to hit an aspirin on the first shot, it matters. To those who shoot at tin cans, it makes no difference at all.
Deer hunters will never know they did it.
Once you remount the scope you NEVER TOUCH the windage again. All scope adjustment for windage is made with an adjustable scope mount, and only one time. Then you lock it down and just adjust the elevation. If you adjusted the windage, everything you have done goes out the window.
A 75-yard shot with an airgun is like an 800-yard shot with a firearm. I’ve only done that distance a few times in my life.
Ask yourself why you are centering the scope. If it’s for shooting field target, then that’s a good reason. If it’s just because you want the best for yourself, well that’s like blueprinting a SmartCar engine. Not worth the effort.
JBA, Bud W and BB,
I tried to center my scope (Bushnell Banner 4-12×40) with a mirror on my medicine cabinet, without a success. I held the scope’s object lens firmly and squarely on the mirror and then looked into the scope. I could see one set of cross hairs so there was nothing to align. I rotated the windage and elevation adjustments up to two full turns to see if I could see two sets of cross hairs. I adjusted the paralex to the minimum and refocused the eye piece just in case. Again I still saw one set of cross hairs. What did I do wrong?
I know what I didn’t do. That was to dial the magnification to the minimum. Now I see two reticles and can get them close. I can’t align the two reticles perfectly because they change by about 1/4″ when I rotate the scope about its long axis. Anyway, a very quick method.
75yds. isnt all that uncommon i own a career 707 in .25 cal.100yd measured groups of 1 inch or less with korean pellets35 grain dome its a lever action model with power wheel set to max.ialso own a 9mm career fire 201 single shotthat will shoot 1.5 inch groups at 75yds. measured using 90 grain xtp pistol bullets ment for reloading.380
I built a fixture to do just this. Then I came on here looking for some sort of device that would "plug" into the obj or eye piece and allow you to align the reticle.
You can view the video on youtube under precisionlrhunter
I found another way to optically center a scope. I place the scope in a box with the v's cut as described earlier. The box is securely held in a vice. I point the scope at a piece of graph paper that can be made online with several free programs. Be sure the lines are bold enough to be seen at about 6 yards. Rotate the scope until the crosshairs are at the very bottom of the circle made by thr crosshairs. Make a small mark on the graph paper where the crosshairs are located. Repeat this process with the top of the circle and the left and right spots. You should have marks resembling a square rotated 90 degrees (like a diamond shape)Find the center of the four marks by drawing lines thru the top and bottom marks and the left and right marks. Rotate the scope until it is upright and while looking thru it move the adjustment knobs until the crosshairs intersect the center mark. If you do this very carefully the scope will now be close to perfect center.
That sounds like a great innovation on the adjust-spin-adjust-spin-adjust method that I've been using. Is there anything at all special about the graph paper you mention, or is it regular old graph paper? I've long since used up all of my graph/engineering paper stash, so where would I go to find the free programs you mentioned?
Bobby, I got the graph paper printer at: http://download.cnet.com/Graph-Paper-Printer/3000-2064_4-10037453.html
If you play around with the prefrences you can change the width of the graph lines to make them more visable.
If you change the variant to Polar coordinates, you can make some nice targets.
After a scope is optically centered how do you sight it in without disturbing the centering?
It’s sighted in with the mounts, mostly. But people don’t optically center scopes that much anymore. It was popular for about 10-15 years back in the ’90s but not so much these days.