Barney88PDC from another forum wrote this and I found it very informative. Enjoy!
DISCLAMIER: I am not a optics professional. This is merely my opinion so take it for what it is worth.
What I am trying to do in this post is attempt to help the reader have a better understanding of tactical scopes and show how using the reticle and turret adjustments in conjunction with one another will make you a more capable shooter. I will attempt to start off very basic and get more technical as the article progresses. I personally lean more towards the tactical crowd and I prefer MIL reticles but the very same principles apply to MOA reticles and adjustments. The only thing that will change would be the math formulas for ranging. I am writing this with scopes in mind that have the ability of external adjustments and are variable power.
Now let’s start off simple. Scopes are usually listed in three numbers for variable power scopes such as 3-15x50. The first number is the minimum power of the scope. What you see in the scope will be under 3 times magnification or 3x at the minimum power or zoom setting. The second number is the maximum zoom the scope has. In this example 15x or the target will appear 15 times closer in the scope than to the human eye. And the third number is how large the objective diameter is in millimeters, in this case 50mm. The objective is the lens that would be further towards the muzzle of the gun. The lens closer to the shooter or the lens you look through is called the ocular lens. Now there are several other combinations such as 3-10x40, 5.5-22x50, 5-25x56 ect.
Now what does this all mean and what matters to you is all subjective to what you are trying to accomplish. This article is not intended to help you choose scope A versus B so I am not going to get into this is better than that debate. What I am trying to do is make you realize that being able to hit targets at further distances is dependent upon having a sighting system that you can actually adjust or correct to make hits on targets at further distances. With modern rifles and ammo I can take your *deer* rifle and make kill zone hits with it at 500 yards with YOUR gun and YOUR ammo but probably not with your scope. If you spend hundreds or thousands of dollars a year on hunting or shooting sports, invest some money in a better optic that can be externally adjusted, then learn how to use it and you have more success being able to make longer shots accurately.
Here are some points that I look for when selecting optics.
Magnification
Personally I like variable power scopes with a lot of magnification. I feel if I don’t need the extra power I can dial the zoom back. Don’t get too caught up in magnification because in the hot summers the mirage gets so bad that you cannot dial scopes to extremely high magnifications when shooting 300 yards plus anyway. The magnification amplifies the mirage and the mirage obscures the targets but I still like to have it if I need it. Personally about 15x or so on the top end is my minimum and I have some scopes with 25x ability. It helps out at the range for me to see my bullet holes and not have to go down range to check the target every few shots. If I am hunting or shooting a stage with multiple targets I do not have the magnification dialed very high. The reason being leads me to my next point.
Field of View or FOV
FOV represents how much you are going to see of a target in the scope side to side and up and down. Naturally as you zoom in you are going to magnify the target and make it bigger. But this also decreases your FOV. It is like you are looking through a straw and the more you zoom the worse it gets.
I will try to explain and keep it really basic. Let’s say you have a 4’ long 2* diameter wrapping paper roll and you are looking at an object (say a truck) that is 20’ away. Then you have a 4’ long 24* diameter cement culvert that is 20’ away from the same truck naturally you are going to see more truck looking through the culvert. That is because you have more FOV through the culvert. It is the same thing with scopes. FOV is important for target acquisition. If you are in a competition setting such as a tactical rifle match or even out hunting once you see that target with your naked eyes you want to acquire that same target in your scope quickly. But you want to be able to do it with reasonable magnification on the scope as well.
Elevation Adjustment
How much you are going to need is going to be dependent on how far you are trying to shoot. Also some scopes have more elevation adjustment per turn than others. This is important so you do not get lost as to where you are on the turret if your scope is not equipped with a zero stop feature. A zero stop feature will allow you to set a point to which the elevation knob cannot go past. Usually this is set at or near your rifles zero. I zero my rifles at 100 yards. So if I dial the elevation all the way back until it stops I know 0 is for 100 yards. If your scope does not have this feature and you had to dial say 3 complete turns to make a long shot then after the shot you start cranking back down to your 100 yard zero but get lost and ask yourself was that 2 or 3 turns………well now you are in a bind with no way to tell until that next shot.
Tube Diameter
You need to know this so you can select the correct size rings to mount the scope to a base or rail. Second and possibly most important is tube size plays a big part in the limiting factor for the range of adjustment the scope will have for both windage and elevation. The old standard used to be 1* tubes. Most manufactures are now using 30mm tubes as the standard and some even larger such as 34mm and 35mm tubes. You have to realize that when you make an adjustment to a turret, either the elevation or windage turret, that the erector tube which holds the reticle in the scope is physically moving inside of the tube. The larger the tube body is then the more the erector tube (internal tube of a scope) can move before it hits the tube body and you run out of adjustment. For most hunters sighting in a scope for 100 or 200 yards with no intention of dialing in corrections for longer shots then the internal range of adjustment is not really a concern. Any scope should zero at those ranges without any concern of running out of adjustment. Where this really comes into play is for long range shooting.
For example say the scope you bought has 100MOA of elevation adjustment. That number is the total range of elevation travel. Most scopes come from the manufacturer with the reticle centered so you would have 50 MOA of up and 50 MOA of down. Let’s assume you put the scope on your gun and you did not have to touch a thing. It was hitting bull’s eyes at 100 yards when you bolted it on the gun. Now you want to shoot at a target that was far enough away it calls for 60 MOA up elevation to compensate for bullet drop from your 100 yard zero. In this example you would *run out of adjustment* because you only have 50 MOA up. One way to compensate for this without buying another scope with more internal adjustment would be to add a base that has MOA built into it. What this does is it tilt or cant the optic on the rifle downward which *adds* elevation travel to the scope. It *takes* the elevation from the downward adjustment and adds it to the upward adjustment. So as in the previous example if you put a 20MOA rail (the common rail put on tactical rifles chambered in .308 Win and the like) you would have to dial 20MOA down on your scope to zero at 100 yards. So now you would have 30MOA of downward travel and 70 MOA of upward travel. So now you would be able to dial that 60MOA needed to hit the target.
Tracking
Tracking is the most important factor when dialing in adjustments for long range shooting . If you have a quality rifle with consistent ammunition then you should get very consistent velocity and accuracy. Using a ballistics calculator or spending time at the range learning your gun / ammo combination will determine how much the bullet will drop as the range to target increases. Once you have determined how much the bullet is going to drop for each range that value can easily be recorded in MIL or MOA of correction to dial on to the scope. Write it down and that is your DOPE for that distance. However, if what you dial on the scope is not what the optic gives you then everything else does not matter. If you dial 10 MILs then correction applied in the optic better be 10MIL’s. Scopes that do not track the same every time are useless for long range shooting.
For instance at the LRA match we had one stage at 1000 yards. A legit KD or Know Distance 1000 yards, not a camp story of I shot that deer at 1000 yards. My DOPE or correction was 10.6 MILs of elevation or to put it another way almost 32 feet of correction (from a 100 yard zero) and I had 1 MIL or 3 feet of wind correction on the scope to account for the left to right wind. When you dial 10.6 it better be 10.6 not close or you WILL MISS. I am sorry but you are not going to do that with a $200 scope.
Light Gathering Ability
There are a few ways to gather more light. The physical limitations are the objective lens diameter and tube diameter. The larger they are the more light they can acquire. But really it comes down to glass and coatings. The better the glass and the coatings are the more light they transmit. The better coatings filter out the *bad* UV light and allows more of the *good* light to pass. This is going to largely depend on how good the glass is and what kind of coatings the manufacture uses on their optics. This is one of the big areas where you get what you pay for. The question you really need to ask yourself is do you need it? That is subjective to each person for the intended use of the optic.
Eye relief
This is simply the distance from the ocular lens to your eyeball. This is a major concern with larger caliber rifles. You want to have a big enough of a gap between your eye and the scope so when the gun recoils you do not injure yourself by the scope coming back and hitting you in the face. There have been some really nasty cuts requiring ER visits and stitches by not having proper eye relief. The end of the ocular tube is usually very sharp and can cut.
Parallax
Most tactical style scopes have an adjustment for parallax. A common misconception is that the parallax adjustment is a *focusing* knob. While it may indeed bring the image in and out of focus this IS NOT WHAT THIS IS USED FOR! It is to bring the plane of the image inside the scope into the same plane as the retical. What happens is as the distance to the target changes the image moves out of the same plane as the reticle. When this occurs if you move your head side to side, or up and down, without moving the gun, the reticle will move or *floats* on the target. THIS IS BAD and it will cause your shots to be off and your group size to increase. Matter of fact in lower end scopes when the parallax is set properly the target may not be in the best focus.
Think of it like this, if I tell you to stick a target on a wall then I give you a cross on a stand (crosshairs) and tell you to stand 10 feet from the wall but put the cross 5 feet from the wall in perfect alignment with the bull’s-eye of the target. The cross hairs will only cover up the bull’s-eye of the target if your eye is in perfect alignment with the cross (crosshairs). If you move your head side to side or up and down the crosshairs will move on the target because the crosshairs and your eye will no longer be in alignment with the bull’s-eye. However, if I tell you move the cross to the wall (adjust the parallax) so that the cross (crosshairs) is in the same plane as the target (ie against the wall) then you can be standing / sitting / move your head up or down, left or right and no matter what the cross covers up the bull’s-eye of the target. That is parallax!
In my example we moved the retical which is technically opposite of what actually happens (the image moves to the retical not the retical to the image) but the example explains the principle. When the parallax is set properly moving your head will not affect the retical on the target, it will stay stationary. To check to see if you are parallax free; with the gun supported front and rear by sand bags or a combination or a bipod and rear bag; without moving the gun, move your head very slightly up and down or side to side and see if the reticle moves or floats on the target. If it does then the scope is not parallax free. You need to adjust the parallax knob until there is no *floating* of the retical on the target. It takes some practice to be able to move your head slightly without moving the rifle but once you get the hang of it and see parallax it will explain why your zero has moved on you in the past. Meaning *man last time I was at the range it was dead on, now I am high*. You cannot be this steady if you are trying to free hand the rifle or do not have a good rest. To see the effects of parallax for yourself set the parallax to say 100 yards then focus on a target that is 300 yards away and move your head without moving the rifle. The cross hairs will float all over that target without the rifle moving. That is how much error is in your shot if you do everything else perfect! You need to adjust the parallax out of the scope before making the shot to remove this inherent error.
With Premier and S&B optics when the parallax is not set properly at closer ranges (within 300 yards or so) the image of the target is blurry and you will naturally adjust it to bring the image into focus and hence correct for the parallax error. However I have noticed that with NightForce scopes the image remains in focus at almost all parallax settings. A friend and I were trying to sight in his gun with a NightForce optic and we were getting groups over an inch with the groups from string to string shifting point of impact. One group would be high then the next would be right ect. Finally we checked for parallax and the retical was moving about 2" on the target. This has been the cause of MANY missed shots and wandering zeros.
Eye Box
I do not know of a true quantifiable number or measurement of this but the following is what it refers to. When you shoulder your gun assuming everything is properly set up then you should get a clear sight picture without having to try to "find" the target. The image in the scope should be clear from the very edge of one side to the other both up/down, left/right and should not have any blackness or darkening to it. This will depend on having the cheek piece of the stock at the proper level for the height of the rings and the scope being the correct distance from your eye so that you do not have to lean towards the scope or away from it to have a full sight picture.
What "eye box" represents is how much you can move your head left/right or up/down and not lose the image in the scope. Some scopes have a very narrow eye box and just slightly moving your head will result in you losing the image. Now this is not to be confused with the stock and optic not being set up for the shooter. Example, if you go with higher rings then you would need a higher cheek weld. To achieve this you may need to add height to your stock by way of a stock pack or even say a mouse pad or some foam and tape. (It may not be pretty but it works). Or if you have to lean forward a bit to get a clear image then this tells you the scope is too far away from your eye and you need to move the scope back some or shorten the length of pull (distance from the rear pad to the trigger) by cutting the stock or removing spacers that some stocks are equipped with.
Now there are other aspects of scopes that may be a consideration as well but I think I hit the high points.
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