Speaking of repeatability, when we're shooting prairie dogs, the elevation knob on my Leupold 6.5-20 gets cranked up and down hundreds of times a day. If it doesn't return to a perfect zero each time I'll never make a shot.
The problem with the Nikon and its 1/8 MOA adjustments is that I have to go past zero three times to get out to 800 yards. It's way too easy to mis-count the turns to get back to zero. I try to make it a point to return to zero after each shot but sometimes you forget.
Probably the best article I ever read on setting up a scope came from Catshooter over on one of the Sniper sites. It was repeated again on 6BR.com and I copied it from there. At least read the last part.
How scopes work
By Paul Coburn
© June 15 2008 - Coburn Research Laboratories.
OK... here goes, and it's gonna be a long one.
Part one - how scopes work and dealing with that part of life.
There are several things that go on inside a scope, and in the eyes at the same time. Some of them work against each other.
Some terminology first...
And we'll leave out lenses that are there to correct some optical or color errors, but don't have anything to do with image forming. We'll start at the front of it all, and work back.
1. The "Object"... the "object" (target) that you are looking (shooting) at.
2. The "Objective". The front lens is called the "Objective"... it forms the first image of the "object" we are looking at (that why they call it the Objective). It is the lens that "captures" all the light, that is solely responsible for the image quality of the scope... if the objective is poor, you can't fix the poor image later. This lens is usually made of two different types of glasses (called "elements") sandwiched together, and is called an "Achromat". The Achromat is fully color corrected for blue and green. The red wavelengths are partially corrected, but have what is called "residual color errors". These are very minor.
This is the normal type of objective used in shooting and spotting scopes. In quality, they can vary from bad, through sorta OK, to pretty damn good.
If one of the elements is made of an "ED" glass, or a "Fluorite" (CaF) glass, the two element lens can be very good to outstanding. In some instances, objective lenses are made of three elements, and all three colors (blue, green, and red) are completely corrected. This type of lens is called an "Apochromat", and this is the finest lens that can be bought. The best of these can also have "ED" glass, or Fluorite as one of the elements.
3. The "First image plane". The Objective focuses the light to make an image of the subject, just like a camera lens. This image is upside down, and right/left reversed. This is the first image plane, but NOT the "First image plane" that is talked about when shooters talk about reticles.
4. The "Erector lens"... (if it is a group of lenses, it is called the "Erector cell"). Because the first image is upside down/wrong way around, we (as shooters) can't use it... so we flip it around with a simple optical group called the "erector cell". This cell gives us a new image that is right way around, called the second image plane.
But this cell has another very important job. Moving this cell causes this second image plane to move... so micrometer spindles are put against the cell, to get elevation and windage adjustments.
The total amount of elevation/windage available in the scope (MOA from bottom to top) is determined by how much the spindles can move the cell. The amount of movement "per click" is simply determined by the thread of the screw, and the spacing of the
detents on the spindle.
5. The "Second image plane". This is the second real image plane in the scope, and this is the image plane that shooters call the "First image plane" when talking about reticles. In a fixed or variable power scope with a "First image plane reticle", the reticle would be placed in this image plane.
6. The "Zoom group". In a variable scope with standard (non-magnifying) reticle, the zoom group of optics would follow #5. This group of lenses can change the size of the image plane in #5 and then form a new (third) image plane behind it.
7. The "Third image plane". In variable power scopes, this is the plane that the reticle is placed in. By being here, it allows the image to change sizes, but the reticle to stay the same size. In the context of reticles, this is the image plane that is referred to as the "second image plane".
8. The "Eyepiece". This optical group is like a jewelers' loupe. It is (or should be) a super fine magnifier. It's only job in the whole world, is to focus on the reticle.
Let me repeat that for those that live in Rio Linda...
THE ONLY JOB FOR THE EYEPIECE IS TO FOCUS YOUR EYE ON THE RETICLE!!!!
It CANNOT adjust, or compensate for, or do anything else when things look bad in the scope, or when you can't hit the target... and you CANNOT use the eyepiece to try to correct for parallax. That is sheer folly at best, and raw stupidity at worst.
OK... now that you know what the insides are like... let's move on. We'll use the zoom scope for our examples, because if you can understand the zoom scope, then the fixed scope is a walk in the park.
In the scope that is set for infinity range, the object forms an image (upside down, right/left reversed) behind the objective (the first image plane)... the erector cell "sees" that image, and flips it over and makes it right way around in a NEW image plane (the Second image plane). The spindles can move the erector cell to move this image plane for elevation or windage corrections.
The zoom group adjusts the size of this image plane, and makes a NEW image plane (the Third image plane) that is the desired size. There is a reticle placed in this last image plane, and the eyepiece focuses on the reticle AND the image at the same time. When things are good, that's how the scope works!
But... now the booger falls into the soup... IF the third image plane and the reticle are not exactly, (and I mean EX-ACT-LY) in the same place, then your eye cannot see them LOCKED together as one picture.
It sees them as two separate pictures, and the eye will look at each separately, and the eye can also look AROUND one to see the other.
Lenses are measured in metrics (aka Millimeters). Not because the Europeans wanted the metric system 25 years ago, but because optical strings and chains of lenses (like scopes) are really a long string of numbers.
There are constant ratios of "this divided by that's" that give image sizes, "F-ratios", and image locations. It's so damn easy to do the engineering using a 10 based system that the optical guys were using the metric system way back in the 1800's.
The objective has a "Focal length"... this is the distance behind the lens that the first image plane falls when making an image if a subject that is at infinity (or very damn far away).
If the objective has a focal length of 100mm, then the image of that 1000 yd target is 100mm behind the lens. See how easy that is?
But the problem with geometric optics (which is what we are dealing with here), is that they follow the laws of geometry... and optics make triangles like rabbits make babies.
AND... in an optical chain, when you change one thing, one angle, one ANYTHING, everything else in the chain follows along and the changes are BASED on the ratios involved at THAT stage.
If we take that same target, and move it to 100 yds, the image in the scope moves BACKWARDS, going further into the scope. Not by much, but it doesn't take much, because we are dealing with very small distances inside the scope, and very high magnifications.
How far the image moves back, and what its new position is, is predictable by the mathematical ratios of the angles formed by the subject and the first image... OR (for us dummies that lost our slip sticks) by the ratio of the distances to the Target and the focal length, multiplied by the focal length, then ADDED to the focal length.
The target is at 100 yds (91440mm), the focal length of the objective is 100, so the displacement is 1/914 x 100, which means that the first image is now at ~100.1mm.
Hmmm only .1mm, that doesn't seem like much.
Read the following paragraph twice...
In a 1x scope, 0.1mm would mean nothing... but this displacement is repeated throughout the chain, AND if any of the optical groups change the image ratio (aka image size), then the displacement (aka ERROR) is changed in direct proportion to the increase in magnification. So in a 3x scope, it would be .3mm, and in a 10x scope, it would be 1mm, and in a 30 power scope, the image would be 3mm behind the reticle. Now, you should have seen a pattern in this last paragraph.
READ THIS TWICE!!
With the same error in the objective (scope focused at 1000, and target at 100), the parallax INCREASES WITH MAGNIFICATION... got it?
If not, READ IT TWO MORE TIMES!!
OK... now, if we do the same math for closer distances, like 50 yds, and 25 yds we will see that the error gets really big, so that with a target at 50 yards, and the scope set at 35 or 65 yds, the parallax makes the combination un-usable.
When the image of the target, and the reticle, are not in EXACTLY the same plane, and by moving the eye up and down... or side to side, either the target OR the reticle appears to move in relation to the other.
You might see the target move and the reticle stay still, or you might see the target stay still and the reticle move over it... both are exactly the same, and which you see, is only a matter of your OWN perception.
It is NOT possible to have parallax while moving up and down, but not have it when you are moving side to side.
If you think that is what you have, you have other problems... either you are moving the rifle, or you have eye problems.
How to set Up a Scope eyepiece!
This is the only way to do it...
Screw the eyepiece out (CCW) all the way, until it stops.
If you wear glasses, put them on.
Hold the scope up and look OVER the scope at the sky, and relax your eyes. Then move the scope in front of your eye.
The reticle should look fuzzy
Turn the eyepiece in 1/2 turn, and do the same thing again. You will have to do for a while before the reticle starts to look better. When you start getting close, then turn the eyepiece 1/4 turn each time.
Do this until the reticle is fully sharp and fully BLACK immediately when you look through the scope.
Than back off one turn and do it again to make sure you are in the same place.
Then LOCK the ring on the eyepiece, and leave it alone FOREVER!
Set the scope down on something solid, where it can see something at a long distance... half a mile or longer is good. It can be on the rifle, and rested in sand bags at the range... but pick something at least 1000 yds away... even further if possible. If the scope has an "AO" Adjustable Objective, then set it for infinity, and look at the distant object, and move your head from one side to the other, or up and down if you prefer. If the reticle seems to move, there is parallax. Change the distance setting and try again... if you are very careful, you can move your eye, and adjust the distance at the same time, seeing which direction gets better.
(Something to know - the graduations/calibrations on the AO of a scope are approximations ONLY... they are to get you close. If shooting is critical, then check for parallax by moving your eye and adjusting until there is NO movement of the crosshair/target images).
With front objective adjustments, you can turn them either way without worry... BUT with side adjustment scopes, like the MK4-M3, the M3-LR, or the other LR family of scopes, the adjustment must ALWAYS be made from the infinity end of the dial. Turn the adjustment all the way until it stops (past infinity), and then start turning it in a little at a time, until there is no parallax. If you "overshoot" the proper setting, you can't just turn back a little, you must go back to stop at the end of the dial, and start over again. While "AO"s dials are locked in place, and if the indicated distance doesn't match the real distance, there's nothing you can do about it... the side focus dials are not locked in place. Once you have found the setting for infinity on the side focus models, then (CAREFULLY) loosen the screws, and set the dial so that little sideways infinity symbol is lined up with the hash mark, so it is calibrated. You can also make little marks or put on a paper tape for other ranges instead of using the round dots that don't match any range.
Now you can set it to infinity, but remember that you MUST turn the dial all the way past infinity to the stop, EVERY TIME before going from a close range to a longer range. If you are set for 500 yds, you can go directly to 100 yds, but if you are set for 100 and want to set it to 500, you MUST go all the way back to the stop, and then go to 500 This is because there is a fair amount of backlash or lash (aka SLOP) in this wheel linkage to the focusing cell, so you can set it only from one direction to make sure the slop is always on one side. The other problem with it is, even if you decided that you wanted to calibrate from the other end... the recoil will push the cell back. SO you must ALWAYS set these dials from the infinity end of their scales.
To make it easy to not have to remember...
I always start from the end stop when I change range, no matter which direction I'm going in... it adds about 0.023 seconds!
That's about it on rifle scopes on the internals of scopes.
There are thousands of "opinions" on scopes on the web, but this is the science from one that does optics for a living.
You have a friend that says to set up a scope a different way?
The guy at the next shooting bench at the range said to do it a different way?
You know some guy who's in the Marines says to use your eyepiece to correct parallax?
You got a friend that shoots bench rest and says something different?
Before you take their advice, ask them to explain how a scopes works from the inside out.
This is the way to do it, because this is the way scopes work.
Part two - setting up the scope on the rifle.
This “part two” is in two parts… first, how to put the scope on the rifle so it looks nice and square.
Second is how to make it actually work, all the time, no matter what the range is, with out constantly making adjustments or taking sighters, or dialing in windage every time you change elevation… for those that must hit on the first shot.
If you shoot the type of matches where you have sighters, it doesn’t really make any difference how you set up your scope – even if it’s cockeyed, hell, make it up with the sighters… you dial and doodle, and get it on target, then shoot for score.
In Benchrest, the front rests have bubbles to level them, and flat bottom cradles. The stocks are flat on the bottom, and the front rest will even adjust for the width of the stock and hold the rifle in place for you, so you can’t “cant” even if you wanted to. Just shoot the sighters and then you are go to go.
But if you shoot in the field at long range, and you get NO sighters… if you need to hit on the first shot, whether it be PD’s or steel targets in a “practical field match” (a.k.a Snipers match) then you need everything working together in unison – there are no sighters to make up for a sloppy build and/or setup.
Before we get into the optics and mechanics of it, lets give some thought on how aiming and shooting actually work… and what follows, gets bigger as ranges get longer.
When we put the cross hairs on a target, we are pointing the line of sight at the target, and setting an “axis” line. As long as we are using that target from that firing position, that line is the center of everything we do… we put down the rifle for a minute, and we pick it up and reestablish the same axis line – even if it’s moved a little – it is the line that defines what we are doing, and everything we do it centered around that line.
When we adjust the turrets, we reestablish the same axis… so everything we do shooting wise is based on that “fixed” axis.
When we go from a hundred yd zero, and walk to the 1,000 yd range, we establish the SAME AXIS with the scope – but we can’t hit anything, cuz the bullet is hitting 30 to 40 feet low.
When we dial in the elevation turret, what we are doing is LOWERING the back end of the rifle… our visual axis is still the same.
So you must understand that when you adjust the scope, you are moving the butt of the rifle.
Now – if we are shooting at 1,000 yds and hitting ~30 feet low, we want to move the butt of the rifle DOWN, without moving to either side. If our rifle and scope has been properly set up, but we tilt it clockwise by 4 degrees, when we adjust the elevation, we move the butt down and left by 4 degrees and the arc of the bullet flies right – now 4 degrees is not much, and with a level, is less than anyone can detect in a field position, so it is scratched up to a bad wind call or other black magic.
1 - “Cant” Canting means the axial rotation of a firearm around the sighting axis. It causes the bullet to fall to the side of the Point of Aim. If you cant to the right, the bullets will hit right of the POA.
The amount of cant is directly related to the amount of bullet drop from the bore line. At short ranges, canting does not appear, or it’s appearance is so slight, it gets lost in “the noise”.
However, the longer the range, the greater the shooting error for the same amount of cant.
2 - “Crabbing”. Crabbing is the apparent tracking error in a scope, caused by the reticle crosshairs not being true to the spindle faces of the turret shafts in the scope.
Its appearance shows when the POI seems to move to the side as the elevation is tracked from one end to the other. It also appears in the windage turret, but we rarely use the windage turret in large enough excursions for crabbing to show up.
When all is said and done, in the end, it is the spindle faces that MUST BE TRUE to the line of gravity (vertically and horizontally), in order to eliminate crabbing errors.
Setting up the scope on the rifle can be as simple as screw the mounts together and sighting it in.
But in you shoot at ranges that require large changes in elevation then the problem of “cant” comes onto the list of things to worry about.
Canting is (as a generality) when the rifle or the scope is tilted in relation to the line of gravity, and the result is a lateral shooting error that often is blamed on a bad wind judgment.
When the bullet leaves the barrel, it starts dropping straight down from the force of gravity.
Counter to common beliefs, the cross hairs do not have to over the bore, or have any other physical relation to the bore.
It is “nice” to have the scope over the bore, and square to the action and stock, but that is mostly cosmetic… not that it hurts to have everything lined up – it’s just that it is not as critical as thought.
The M1 Garand, “C” and “D” series sniper rifles had the scope mounted to one side by ~3/4” to clear the “En-Bloc” clips, and yet shot very well.
What IS important is that the scope’s windage spindle face be vertical to gravity when the rifle he shot in its normal shooting position.
Setting up a scope for a long range rifle is fairly straight forward for the first part, but after that is done, it gets somewhat complicated.
You cannot eliminate cant without a permanently mounted bumble level(s) on the scope tube.
No one is capable of holding a rifle square to gravity without a reference. There are four or five that are made for firearms, and you will need at least one mounted on the scope tube itself.
To mount the scope, set the rifle square and level – you can lay a level across the scope bases to make it easy. Set the scope loosely in the rings, and line up the vertical wire to a plumb line, then snug things up. This is fairly straight forwards and has been described in a hundred places.
It will get you “close enough for government work”, if you are a short or medium range shooter… or if you have the luxury of taking sighters.
Most scopes have a built in error – it is not built in on purpose, it is there to cut costs.
The cross hairs are NOT in the scope straight with the spindle faces of the turret shafts.
If you put a scope in a solid mount like an optical bench (not on a rifle), plumb it to a dropped line, and run the elevation turret from bottom to top, you will see the cross hairs drift away from the plumb line - this is called “crabbing”.
Leupold says they consider 4 degrees of reticle placement error as “satisfactory” and will not repair crosshairs that are within 4 degrees of plumb to the spindle faces. Other high end companies have similar “standards”. Some of the “low end” companies are a lot looser with their standards, and when you complain that there is an error with the crosshair placement, they won’t know what you are talking about.
The only two scopes that I have from Leupold that have true reticles are three MK4-M3 and one M3-LR - all the reticles are true to the spindle faces. They should be true - the scopes have a $1,400 street price and are made for the military.
So what we usually do is set the scope up so the crosshairs are “true”, but the crosshairs are not what counts – it’s that center part that we use to aim.
Suppose you had a dot reticle on a glass plate… I a sense, this is what we are concerned with. Only what happens to the center aiming point.
So we must correct for this ourselves.
The following assumes that your rifle is now WELL zeroed at 100yds - the group is centered on the POA - and you have a bubble level that will attach to the scope tube.)
Once the scope is in the rings (and you are presumably at the range), set up a plumb line on a target that has a large vertical size – equal to the full amount of your total available UP elevation, plus 20 inches. If you scope has 50 Moa of “up” elevation available from the 100yd zero, use a 70” tall piece of paper.
Drop the line and mark a ½” aiming point, 10” from the bottom, on the line.
Then count the total MOA you can get and multiply that number by 1.047 and make another dot that much higher from the aiming point.
Now remove the plumb line (so you don’t shoot it!)
Aim at the lower dot, with the vertical part of the crosshairs intersecting the upper dot, and shoot a group.
Now crank in the MOA that you used to calculate the dot spacing, and AIMING AT THE LOWER TARGET – the same on you used before, fire another group.
In a perfect world, the second group will be smack in the middle of the higher dot.
But – two things – if the click spacing on your scope is not accurate you will see it now - the group will be higher or lower than the second dot. You can use this difference and calculate what the real value of the scope clicks are - ?
If the second group is off to one side of the second dot, we need to correct for crabbing, because your crosshairs drifted off to one side as you cranked in elevation.
To do this, make another dot an equal amount on the other side – if your second group is 2 inches to the left of the top dot, make another dot two inches to the right of the second dot.
Now go back to the bench and set the rifle so the crosshairs are intersection the bottom aiming point, and THE NEW DOT at the top – the one you just drew.
Now with trial and error, set the bubble level so that the bubble is centered when the crosshairs are intersecting the lower aiming point and the new, second dot you drew that is an equal amount on the other side of the top dot.
What we have done is canceled the crosshair offset by making the bubble centered when the spindle is vertical because we have set it up to cancel the error.
Do the above shooting test again, and for the second group, keep the bubble centers and the second group will be in plumb with the first – now the scope can make longrange elevation adjustments without cranking in windage.