How to use an adjustable objective scope

by B.B. Pelletier

Before plunging into today’s topic, I want to share some news. I reported that John Whiscombe is no longer making air rifles because in 2003 that was announced on the internet. Well, either he had a change of plans or I got hold of some bad information, because one of our readers just got a quote on a new Whiscombe from John. That’s the good news. The bad news is the rifle with 2 barrels (he only does .177 and .22 now) sells for $3,500 in the UK. Because the British Home Office tightened the export laws several years ago, Whiscombe needs a license to export an FAC rifle to a specific U.S. dealer. Whiscombe may have someone to ship to in the U.S. , but even so, by the time the gun gets through U.S. Customs, you’re looking at almost $4,000 for a rifle with 2 barrels. Makes me appreciate my gun all the more!

Skunked at the range!
I tried to get out to test the Crosman Premier hollowpoints for long-range accuracy on Friday, but the wind skunked me. I was shooting groups in the 1.5″ to 2.25″ range (rather than 3/4″ or less) with the very best pellets. So, that test is on hold…which brings me to today’s post.

How to use AO
A reader asked the following question on Friday: So, if my scope has an adjustable objective – do I just dial it to the correct range to correct this problem?

Quick answer – NO!

Here is how it works
This reader had read my posting about parallax, and that made him aware of scopes with parallax correction. The first scopes to offer this feature used the objective bell for the adjustment, so they were called adjustable objective scopes, which was soon shortened to AO. Nowadays, people don’t know what AO means, but by context they guess it has something to do with parallax adjustment.

Why adjust parallax?
You adjust it to get greater aiming precision. Just as you use a scope level when shooting long range, parallax correction helps you sharpen your aiming solution all the more, because you always see the target from the same perspective. Please read the earlier posting about parallax. I explain why it’s such a problem when shooting.

HOW is a parallax-adjustable scope adjusted?
There is just ONE way to do it. Look through the scope at the target and twist the adjustment ring or knob until the target appears as sharp as you can make it. At that point, you’ve removed all the parallax that you possibly can. That doesn’t mean it’s ALL gone, though. Even the best scopes will have some parallax left after adjustment, which is why your cheek placement on the stock is so important. If your eye is always in the same position relative to the scope, there won’t be a parallax problem. The problem comes when your eye isn’t always in the same position. Small variations in placement introduce parallax, even in scopes with parallax adjustment.

Try this NOW!
Want to see what parallax looks like? Close one eye and point your index finger at some distant object. Hold your finger steady on that object and switch eyes. The finger will move! That’s because both your eyes cannot look at the same thing from the same perspective. The distance that they are separated in your head affects the amount of parallax they induce (how far the finger seems to move).

Now, imagine that your finger is the barrel of an airgun and your eyes are the sights. How could you hit anything if the target image keeps moving relative to the sights? You would pick one place to look from (look from one eye, only) and always use that place to line up the sights. Then when you sight in, you will adjust the sights until that picture, seen from just one place, produces hits where you want them. When you move to an air rifle, that means always placing your head at the same place on the stock so your eye will be in the same place, relative to the sights. That eliminates parallax from any scope – even one that is not adjusted for the range you are shooting.

Scopes with parallax adjustment take care of the largest share of parallax, so the shooter can be a little less careful when placing his head on the stock. That makes them faster to use when hunting. But, that’s not their biggest attraction. Shooters tend to use scopes with parallax adjustments as rangefinders. The focusing function works like the coincidence rangefinder of a 35mm SLR camera. When the target is in sharpest focus, the scope is set to the range (distance to the target), in theory.

It doesn’t always work that way
I’ve had scopes focus sharply but read a distance 20 yards different than the real range to the target. Cheap scope? You might think so, but no. The problem is the temperature. Optics are adjusted to the millionths of an inch (and less!). When you focus on a target, you adjust the lenses inside the scope to precise settings. However, as the temperature increases and decreases, the lens holders expand and contract – by thousandths of an inch! That throws off your rangefinding capability a lot! It’s like having a broken clock that always tells the correct time twice each day!

They calibrate their own scopes
Field target shooters compensate for this by placing several different scales of distance measurements on their scope’s adjustment mechanism. They verify these scales yard by yard for each temperature range the scale is set to. Sound like fun? This is how exacting some field target competitors get about their equipment.

I could go on, but I hope I’ve explained the situation well enough for everyone to comprehend. When you use a scope with AO, just look through it and focus the scope until the image is the sharpest you can get. Then, you can shoot. You will have reduced parallax by as much as you possibly can.

How does this work if you don’t have perfect eyesight? Please, don’t get me started, because that’s a whole different blog. Write if you want to hear it.

28 thoughts on “How to use an adjustable objective scope

  1. B.B.

    I want to hear about shooting with imperfect eyesight. I am right handed but shoot left handed because I do not have a right eye. But when younger I could get cheek situated on my child hood love, a daisy bb rifle that I won at 10 years old. I have had only one eye for 55 years, so no memory of anything else. I also wear progressive tri-focals, but remove them to shoot with scope. They seem to screw up scope sighting, but have not tried yet to use a peep sight. Plan to put one on a 392 when I order it, soon. I am a pretty decent shot with targets and small animals but probably would not do well in competition. Any suggestions for an old, nearly blind guy who loves to shoot in basement.

    Thanks for all the insight in your blogs. Start every day with coffee and your little tidbits.

    Bill D.


  2. Good post B.B. Keep ‘em coming.

    I would like to know how one’s own eye sight affects paralax adjustment though, I had wondered if my own eyesight screws with my ability to aim. It would be a worth while follow up to this blog. =)

    ~Jensen


  3. B.B.,
    That clears up some of my confusion on why the sharpest focus doesn’t always correspond to the range number on the scope. I use progressive lenses and find it impossible to use a scope while wearing them. But without them I have to load pellets by feel. A serious compromise. And forget about using open sights!!
    BTW, I am working thru your B Square Adjustable scope mount and optical centering advice from Friday but also have been confounded by wind for the moment.
    Thanks again,
    Pestbgone


  4. looks like those ads have really been eating away at this blog, judging by the
    ” Comment moderation has been enabled. All comments must be approved by the blog author.” =( so thats why there was no post earlier.
    so, if the ranges differ with tiny variations in temperature, then wouldn’t that mean every time a competitor shot field target, he would have to re-calibrate his scope? and be meticulous about handling it? a shame those laser range finders aren’t any cheaper than they are. so simple- just click and the range is found!



  5. Interesting topic B.B. I just figured that my eyes were to blame for the difference in focus and range. I had no idea temperature had a role to play. Is there a trick that we can use to find our cheek weld , or is it just practice? I find post like these most interesting. I used to use a Crosman 766 , then a 2100 to shoot black birds out to 40 yards, with iron sights. I don’t know how I did it. After about 100 black birds the stock screws got lose on the 2100.The Crosman 766 wasn’t in very good shape after I got back from Germany 3 years latter. Can you do more posts like this one, scopes,peeps, mounts, mount hight, and the interaction of where things will end up compared to where we will be resting our cheeks? I have a rws 52 , but I’m just using the iron sights to break it in. The front sights are the size of post.At 20 yards I have about 70% chance of hitting a squirrel in head, with Gamo hunter pellets. Thank you for your wealth of knowledge. I hope I can make some converts this year to this interesting hobby ,sport.


  6. dm20,

    Laser rangefinders are specifically prohibited from competition.

    Yes, the competitors do recalibrate their scopes for each competition. The winners do, anyway. Most serious competitors have three temperature ranges on the scope. The temperature can change during a match, as well!

    B.B.




  7. CyberSkin,

    This subject seems to the the number one topic of interest on this blog. Therefore, I will do something about it, as in more posts.

    I will address how to ensure a repeating cheek weld when I cover how your eye can affect parallax.

    B.B.



  8. BB:
    Glad to hear that Mr. Whiscombe is still in business. He gave me a very reasonable quote on a JW80 with one barrel last spring but then I had a stroke. At least now maybe I can get a second chance.
    Ed


  9. Hi B.B.,

    Almost every time you write, “Please, don’t get me started,” I want to read more about whatever is on your mind! It’s kind of like seeing a bowl full of fruit juicy “Jelly Belly” beans labeled, “Only one per customer!”

    I’ve read somewhere about one ‘trick,’ to help established repeatable eye position with a scope. It involves adjusting the eye relief when the scope is mounted, so that a thin blackish ring of darkness remains visible around the outer edge of the eyepiece. I haven’t tried this, so your comments when you write a follow-up blog will be appreciated. Does it work? If there are any downsides of the ‘trick’ what are they?

    Cheers,
    GH


  10. B.B., glad to see that the blog problem was just software and not something that happened to you!

    I found this article very interesting! Had to read it several times to soak in.

    .22 multi-shot


  11. GH,

    Thanks for your kind words.

    About the black ring, I have to say that’s a new one for me! I do know that some scopes go black almost instantly when the head moves. I’ve seen an 8-32 Burris that was most particular that way, and a couple of Leupold 6-24s that were pretty close to the edge, as well.

    The downside is that it’s difficult to see the target unless your head is in the right spot. That can be aggravating in a match. I think I wouldn’t like it if I were hunting, but for making the perfect shot, I guess it’s the best way to go.

    B.B.


  12. I use the black ring trick.
    I put a Leapers 8-32 on my TX200 but only get a chance to shoot at distance about every two months, so I shoot in my garage at 20 ft. I have to wear reading glasses to bring it into focus and paralex is nuts, but it is better than nothing. I move my head back until there is a black ring on the edge and try to make it the same thickness all the way around. I can’t focuse on the ring and target at the same time so it takes a while, but it does help.
    MCA



  13. This blog was touching on that Red Dot sight topic too, where parallax seems to be the issue. I’d certainly determined that it was necessary to keep my head in the same place for each shot, but it made me wonder why use a non magnified Red Dot instead of open or peep sights when the latter don’t suffer from the parallax problem (or do they?). The Red Dot though has the attraction with aging eyes of allowing the shooter to just focus at infinity.

    Gazza


  14. Thanks B.B for your reply. I’m smiling from ear to ear. Its a shame we can’t have a TV show with you being the host. Most people think air guns are kids toys and we could bring up some issues that air gun manufactures could not ignore.



  15. MCA & B.B.,

    Yeah! MCA has got it. He described what I read about how the ‘trick’ is supposed to work.

    I haven’t tried it either. I’ll see if I can run down where I read about that trick.

    Cheers,
    GH


  16. To get perfect focusing, one can use a mask in front of the objective lens. The mask can be a cardboard disk with two holes — typically the holes are about 1/3 of the lens diameter and postitioned equal distances from the center of the disk. (This is not critical — almost any mask with two openings will work fine.)

    When this mask is in front of the objective, you will see two images when focusing and not in-focus. When you are in-focus, the two images will merge into one. This is easier to see for high-contrast targets and the typical FT lane is not a good high-contrast place.

    This is essentially the same as the old range-finder camera system. It has been used by astronomers for a long time.

    Best,

    Joe



  17. Hi BB,

    For my Daystate X2 HFT gun, I use a scope enhancer to ensure my eye is always in the same place each time. The one I use is a stiff rubber tube with an eliptical cup at the end to fit round eye socket. It’s taped to the scope at the spot which gives me perfect eye relief. It works very well.

    ATB,

    Tenuc


  18. hi bb, this is what i’ve been looking for. I have an AO scope bc i read your parallax problem blog. anyway, im near sighted and needs glasses to see distance, so how do i use AO for range finding? glasses on or off? and sometimes my reticule seem fuzzy until i wait awhile and it becomes clear bc my eyes adjust….did you make a new post for this? thnx


  19. Nearsighted,

    First adjust the eyebell until the reticle is clear. Then it’s your choice, glasses on or off. I wear bifocals and I like to shoot glasses off. Safety glasses on at all times, though.

    B.B.


  20. Thnx, BB, one more thing. Since i (and many others) have nearsighted vision, doesn’t that mean we HAVE to use our glasses when using objective lense to find distance? b/c the objective lens HAS to do extra focusing to correct OUR bad vision first, then the range aft?



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