I thought this was a pretty good read about parallax and how to set up a scope properly. Any one like to add anything.
There are several things that go on inside a scope, and in the eyes at the same time.
Some of them workie against each other.
But 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 thing 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 it 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”.
This is the normal type of objective used in shooting and spotting scopes.
In quality, they can vary from badd, 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 friggin’ 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. Moving this cell causes this second image plane to move… so micrometer spindles are put against the cell, to get elevation and windage adjustments.
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 power scope, or in a variable with a “First image plane reticle”, the reticle would be placed in this image plane.
This is where Premier Reticle puts those magical “Gen II” reticles.
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. Is 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, or do anything else when things look bad in the scope, or when you can’t hit the target… and to try to make the eyepiece correct for parallax, is sheer folly at best, and raw stupidity at worst.
If you expect it to do anything else, then stop wasting your time with long-range shooting, cuz you are never gonna make it past mediocre… and take up golf!!
OK… now that you know what the insides are like… lets move on. We’ll use the zoom scope for our examples. cuz if you can understand the zoom, then the fixed is a walk in the park.
In the scope that is set for infinity range, the object forms an image behind the objective (the first image plane)… the erector cell “sees” that image, and flips it over and makes a NEW image plane (the Second image plane). 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 workie!
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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 things, and the eye can look at each separately, and the eye can also look AROUND one to see the other.
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Lenses are normally measured in metrics (aka Millimeters). Not because the Europeans wanted it the metric system 20 years ago, but because optical strings and chains of lenses (like scopes) are really a 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 it.
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 follows along and changes 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, cuz we’re dealing with very small distances inside the scope, and very high magnifications.
How far the image moves back, and what it’s 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, 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. This means that if you have a variable scope, and things look good at 3x, they may NOT look so good at 12x. Set your parallax corrections at high power.
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.
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Parallax is… 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 impossible 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.
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HOW TO SET UP A SCOPE!
This is the only way to do it…
First, 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 a 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!
Second.
Set the scope down on something sold, where it can see something at a long distance… half a mile of 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.
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 (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!
Now… you gots a friend that says to set up a scope a different way???…
… he don’t know doodly-squat about scopes.
The guy at the range said to do it a different way… he don’t know **** either.
Some guy who’s in the Marines says something different… he don’t know crapola!
You got a friend that shoots benchrest and does it different… he doesn’t know **** about optics either.
This is the way, the only way, there is no other way.
… as Rush Limbaugh would say… this is from GOD-da .
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