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.