Scope Adjustment
Proper Scope Adjustment
Ok, I am going to post this document. I would like to give a little info on how
I got it. And the tone used by the author. I copied this and pasted another
site. His tone may be a bit harsh, but this was written after responding to
numerous questions about scope adjustment then having the person asking the
question say well that's not how I was told to do it. Or another guy said to do
it this way. The author is a man named Paul Coburn and his job is evaluating
optics for a living. He is highly regarded in the field and does testing for
some major companies. So no if's and's or but's regardless of if you agree or
disagree with the writing style he is giving the straight answer on the subject.
Here it goes...
I've answered questions about scope parallax
about 300 times, and it's always a long drawn out thing, going several e-mails,
and a few phone calls. It doesn't seem to make any difference how long the guy
has been shooting, this one always keep screwing guys up.
OK... here goes (Whew, this is gonna be a long one).
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 colour 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 colour corrected for blue and green. The red wavelengths
are partially corrected, but have what is called "residual colour errors".
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 friggin' outstanding.
In some instances, objective lenses are made of three elements, and all three
colours (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 reticules.
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 reticules. In a fixed power scope, or in a variable with a "First
image plane reticule", the reticule would be placed in this image plane.
This is where Premier Reticule puts those magical "Gen II" reticules.
6 - The "Zoom group". In a variable scope with standard (non-magnifying)
reticule, 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
reticule is placed in. By being here, it allows the image to change sizes, but
the reticule to stay the same size. In the context of reticules, this is the
image plane that is referred to as the "second image plane"
8 - The "Eyepiece". This optical group is like a jewellers loupe. Is (or should
be) a super fine magnifier. It's only job in the whole world, is to focus on the
reticule.
Let me repeat that for those that live in Rio Linda...
THE ONLY JOB FOR THE EYEPIECE IS TO FOCUS YOUR EYE ON THE RETICULE!!!!
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.
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 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 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 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
reticule placed in this last image plane, and the eyepiece focuses on the
reticule AND the image at the same time.
When things are good, that's how the scope workie!
---
But... now the booger falls into the soup... IF the third image plane and the
reticule 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 Millimetres). Not because the Europeans
wanted 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 the lense.
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
(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 reticule.
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.
---
Parallax is... when the image of the target, and
the reticule, are NOT in exactly the same plane, and by moving the eye up and
down... or side to side, either the target OR the reticule appears to move in
relation to the other.
You might see the target move and the reticule stay still, or you might see the
target stay still and the reticule 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!
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 reticule 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 reticule starts to look better. When you start getting
close, then turn the eyepiece 1/4 turn each time.
Do this until the reticule 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!
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 reticule 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 got 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.
You know some guy who's in the Marines says to use your eyepiece to correct
parallax... he doesn't know about optics either.
You got a friend that shoots benchrest and says something different... he don't
know crapola!
This is the way, the only way, there is no other way.
... as Rushbo would say... this is from GOD-da .
You got questions, just e-mail me.
You wanna "debate it", then go play golf, cuz you're wasting my time!
'lito (gettin' grumpy in my old age!)
I hope this was helpful and did not burn up to much bandwidth. Again Paul
deserves all the credit for the information not me...
Dirty Steve, Out.