This is the second installment in our tech article that outlines everything you wanted to know about telescopic sights but were afraid to ask. In part two I'm going to cover target and hunting knobs, what the terms waterproof and fogproof really mean and how to check you scope for both, plus adjustable objective lenses, what they do and why is parallax such a dreaded term. I think that if I can cover all of that we'll have a good chance of finishing up with Part III in the next edition.
Knobs and Covers: What They Do and Why You Need Them - I know that I'm putting you on just a little with that title but I once spoke with a customer that called me up and asked me if it was ok for him to adjust the windage and elevation knobs on his scope because he changed pellets and the new pellet was striking a little low. His request made me think of the college professor that I had years ago who wanted to know if I changed my own oil in my car. I responded yes and asked him if he did the same. He shook his head, quickly answered with an emphatic no and went on to give me his "theory" on automobiles. He told me that he thought that underneath the hood of his car were great, illuminated balls of brilliant, white light that made the car work. And, that if he ever opened the hood, those balls of light would escape and his car would cease to function.
To help dispel that theory, please grab you air gun scope and remove the covers from windage and elevation knobs. They will turn right off and shouldn't be more than finger tight. Remove the top cap first. Underneath you will see what looks like a wide, flat, slotted screw head located in the center of a ring that has graduation lines and an arrow that says "up". This is your elevation adjustment. Next, remove the cap on the side of the scope. Underneath you will see the same thing except the directional arrow will be followed by a capital "R" which stands for right. This is your windage adjustment.
If you take a wide-bladed screwdriver and turn the screws a quarter of a turn you will hear a clicking sound. IF you want, you can continue to turn the screw in either direction and it will not harm a thing. If you tighten the adjustment screw it will stop when it reaches the end of the thread. If you loosen the adjustment screw it will unscrew to a certain point and stop when you reach the end of the scope adjustment. In the last installment we learned that the crosshairs or reticle is affixed to the rearward portion of the internal tube. What you are doing by turning the windage or elevation adjustment screws is moving the internal tube and lenses either up and down or left and right. As you tighten or loosen the screws you are changing the relationship of the internal optics with the external optics and this causes a change or shift in the point of impact because the reticle moved along with the internal optics. So you really can't damage your scope by adjusting the windage or elevation. Just remember never to force an adjustment screw past where you feel it becomes tight or loose.
On a scope intended for air guns, each, single click of adjustment graduations will approximate 1/4" change in the point of impact at 100 yards. If you extrapolate that to a more realistic air gun yardage, say 50 yards, the change to the point of impact for each click would be cut in half to 1/8". Some scopes will have coarser graduations that provide 1/2" of adjustment at 100 yards. The measured amount of the graduations is usually an indication of the type of scope or the intended usage the scope was designed for. Generally speaking, target or long range scopes have finer graduations, hunting scopes have coarser graduations. Most air guns or rimfire scopes have 1/4" graduations. I say that the clicks represent 1/4" but really I should say 1/4 click MOA or Minute Of Angle. One minute of angle subtends a distance of just less than 1" at 100 yards. When you hear someone brag about the accuracy of their rifle as being sub-MOA what they are saying is the the rifle will shoot a group that measures less than one inch center-to-center across at 100 yards. Or, less than 1/2" at 50 yards. So if you refer to a graduation change as a 1/4 MOA per click it's roughly 1/4".
Some of the scopes that we carry have target turrets. These are taller knobs that rise about an inch to an inch and a quarter above the top of the scope body. The top edge of the covers are usually serrated or sculpted somehow to make them easier to grasp and remove. Underneath the covers are taller knobs with the click graduations clearly shown on the sides of the turrets with numbers to help you identify how much adjustment is made. If you are making adjustments at multiple ranges and you require several changes in elevation, the taller turrets can be adjusted without using a screwdriver or any other tools. Some scopes feature both windage and elevation target turrets. The addition of the windage turrets are a real benefit if you are shooting outdoors where windy conditions will require additional amounts of adjustment to keep your shoots on target.
I'm sure that some of you are asking the question "If the windage and elevation adjustment are done by a screw, how do I know if I have enough internal adjustment left to make the changes I need to reach a correct zero?" That can be done by first placing the windage and elevation turrets in the optical center of the scope. This is how that's done. Turn the elevation adjustment in the direction of the arrow until it stops turning. Then turn it in the opposite direction and count the number of clicks as you turn it until it stops again. Divide the number of clicks by two. Example: 200 clicks divided by 2 = 100 clicks. The actual number of clicks will be different for each model of scope. Turn the elevation adjustment back in the direction of the arrow 100 clicks. Do the same for the windage. After doing the windage repeat the procedure again with the elevation. Now the scope is optically centered and you will have the maximum amount of windage and elevation adjustment available to you as you begin to zero your scope.
As I mentioned last time, most scopes made in the U.S.A. and more expensive Japanese scopes share the common feature of fully-scaled elevation and windage knobs. That means, even when the caps are off of the scope, it remains moisture proof. Many scopes made in Germany and Austria do have this feature. The caps include a rubber "O" ring or washer that makes the scope moisture proof only when the turret caps are in place and tightened. As I've also said before, I would not buy a scope that is not fully moisture proof. I don't care how good the optics are. I don't want to mess around with having to worry if the turret caps are tight or loose before I venture out into the great outdoors.
Fogproof vs. Waterproof - These terms are often used interchangeably but are not really the same. Waterproof means that the scope will not leak when placed in water, not just subjected to wet, harsh conditions like when its rains outside. Fogproof means that the scope will not fog internally when subjected to changes in temperature and humidity.
Here are two tests that you can perform at home if you want to know if your scope if waterproof and, or fogproof. Only perform these tests if your scope claims to be waterproof and fogproof. Many inexpensive scopes made overseas do not claim to be either and I can almost certainly guarantee that they will fail. To check to see if your favorite scope is waterproof, simply remove it from your air rifle, run some warm water in the sink, remove the turret caps then immerse the scope into the water for a few minutes. If there is a leak, it will show up as a tiny stream of bubbles rising from the surface of the scope. A leak will most often occur around the adjustment housing, magnification ring or at one of the lens seals. Keep it immersed a minute or tow to let the surface bubbles clear and then look for a leak. If you see a steady stream of bubbles rising from the scope then your scope is not waterproof.
If you want to check and see if it is fogproof, remove the scope from the water, dry off the exterior and then place it in your freezer for several minutes. Take the scope out of the freezer and look through the lens. If any moisture condenses on the inside surface of any lens because of the rapid change in temperature it is not fogproof.
I've performed these tests to several of my better-made scopes and only one has ever failed. The manufacturer fixed the scope, free of charge, a lens seal had simply gone bad. I really should tell you that if you are not prepared to deal with the consequences of knowing that your favorite scope is really waterproof and fogproof then do not conduct these tests. Some folks really don't want to know the limits of their equipment and for them these tests may seem extreme.
The Dreaded Demon, Parallax - Maybe I'm being overly factious with that title but I've read many different articles over the years about parallax, what it is, what it does and quite often the writers have demonized the concept to the point of actually making shooters paranoid about it. Parallax certainly isn't a good thing but it won't make you grow old before your time, rot your teeth, steal your car or kill you. Your shooting accuracy may suffer a little but believe me, once you understand what it is and how to correct it, the condition is only temporary.
Parallax or more correctly, parallax error is an optical condition that exists in all optical devices. Parallax error occurs when the target image does not fall on the same optical plane as the reticle. The optical plane of a lens is determined by its focal length. Do you remember back when you were a kid and you tried to start a fire with a magnifying glass by holding it in the sun light and focusing the lens on the dried grass until it started to smoke? The distance from the lens to the grass where the sunlight was most tightly focused was the focal length of that lens. In a scope you have the objective lens at the front and the ocular lens at the rear and the reticle is in between. If the focal length of either of those two lenses is not exactly the same as the distance from their surface to the reticle then you have parallax error. This is because your eye is focusing on the reticle and if the focal length of the lens is incorrect, your eye focuses either ahead of or behind the reticle. So, if you can change the focus of the lenses you can correct the parallax error. In scopes with 4x magnification and lower the effect on accuracy is really somewhat insignificant. At higher magnifications it becomes very significant.
This optical error can be corrected internally during manufacturer so the scope is parallax free at a predetermined range. The effects of parallax error are compounded when the shooter sees the target at ranges either closer or farther away than the zero-parallax range because the target image falls in front of or behind the optical plane of the reticle lens. This can be observed by looking through a non-parallax adjustable scope and moving your eye off center of the reticle. The reticle center will appear to shift on the target.
Basic Construction Revisited - I received an E-mail that contained a news group re-post a few weeks ago from Tim McMurry. The re-post contained information about scopes from Dick Thomas of Premier Reticles. Dick certainly qualifies as a "scope expert" and wanted to point out the difference in aluminum alloys that are used to make scope tube bodies. In the last installment I implied that any scope tube made from aluminum is of adequate quality and I really need to clarify that statement.
Better quality scope tubes are made from 6000 series aluminum. Lesser-quality scope tubes are made from unknown alloys that are considered "dead soft". I can tell you from experience that 6061-T6 aluminum is used to make some very, high-tech bicycle frames that are very lightweight, exceptionally strong and will withstand unbelievable abuse. 6000 series aluminum is generally referred to as "aircraft aluminum alloy" because it is used extensively in aircraft manufacturing. If a scope manufacturer is using 6061-T6 aluminum in its tube construction their brochures will say so. You can count on it because that grade of aluminum costs significantly more than the dead soft alloy.
This interesting and sometimes overlooked facet of scope making comes into play when we begin to factor in the repeated stress of recoil and its effects on the internal lens braces and the attachment method of the internal tube. The dead soft alloy begins to compress and wear. Soon, the lenses work loose and the scope starts to leak or the lenses fall out. Also, the scope will not hold its zero because of the increased tolerances caused by metal fatigue between the windage and elevation adjustment points and the internal tube. All this boils down to one more reason to spend a little more on a scope than you think you need to. Because if you do your homework, you'll find that higher quality scopes will last for many, many years and always deliver the performance you expect.
In the next issue, I will try to conclude with the section on scope mounts designed exclusively for air powered shooting. Rings and mounts designed for conventional firearms just won't do the job because they were never designed to be used at air gun ranges. As equally important as a good scope is an adjustable mount that will put you right on target and take full advantage of your new scope's potential. So stay tuned and we'll try to finish this epic tome in the very next issue.