Trapper Luke
New member
Alright so I know that it stands for Adjustable Objective but what exactly does havin that AO do for a scope and shooter?
Thanks,
Luke
Thanks,
Luke
Can someone explain an AO scope to me??
- Thread starter Trapper Luke
- Start date
odawgp
New member
Without an AO the target and cross hairs will become blurred or go completely away (if fine) as the magnification increases. That blurriness or "out of focus" is called parallax.
With the adjustable objective part of the scope you can "focus" or adjust the "parallax".
Lower power scopes such as 3x9 have parallax adjustment via the eye piece. get above 10-12 power and there is more adjustment needed to keep everything in focus.
You may have seen scopes advertised with a "side focus" knob opposite the horizontal adjustment turret. This does the same as an Adjustable Objective.
With the adjustable objective part of the scope you can "focus" or adjust the "parallax".
Lower power scopes such as 3x9 have parallax adjustment via the eye piece. get above 10-12 power and there is more adjustment needed to keep everything in focus.
You may have seen scopes advertised with a "side focus" knob opposite the horizontal adjustment turret. This does the same as an Adjustable Objective.
Martyn4802
New member
Originally Posted By: Trapper LukeAlright so I know that it stands for Adjustable Objective but what exactly does havin that AO do for a scope and shooter?
Thanks,
Luke
Luke,
The Adjustable Objective on scopes allows for you to focus the scope from it's minimum distance, out to infinity. That way, the parallax will be perfect for any shooting distance you set the AO for, and the cross hairs will also be in focus at that distance. For other distances, you will have parallax, and out of focus cross hairs..
Minimum distance for Leupold's Adjustable Objective scopes is 50 yards, except for two of their scopes that focus down to 25 yards; thses are their EFR scopes.
Non-Adjustable Objective scopes have the parallax pre-set at about 100 yards, or maybe slightly farther, like 125 yards. You must live with slight parallax at distances other than the 100 yards. However, if you keep your eye centered in the ocular lens, you'll have no problem with parallax shooting at any distance.
Parallax occurs when the image plane, and the reticle plane, inside the scope tube, do not coincide.
Thanks,
Luke
Luke,
The Adjustable Objective on scopes allows for you to focus the scope from it's minimum distance, out to infinity. That way, the parallax will be perfect for any shooting distance you set the AO for, and the cross hairs will also be in focus at that distance. For other distances, you will have parallax, and out of focus cross hairs..
Minimum distance for Leupold's Adjustable Objective scopes is 50 yards, except for two of their scopes that focus down to 25 yards; thses are their EFR scopes.
Non-Adjustable Objective scopes have the parallax pre-set at about 100 yards, or maybe slightly farther, like 125 yards. You must live with slight parallax at distances other than the 100 yards. However, if you keep your eye centered in the ocular lens, you'll have no problem with parallax shooting at any distance.
Parallax occurs when the image plane, and the reticle plane, inside the scope tube, do not coincide.
Last edited:
bigwheeler
New member
Oda, the eye piece is the diopter end, it focuses your eye to the reticle not to
the target. The Objective lens focuses the target to the reticle.
the target. The Objective lens focuses the target to the reticle.
Timberbeast7
New member
Originally Posted By: bigwheelerOda, the eye piece is the diopter end, it focuses your eye to the reticle not to
the target. The Objective lens focuses the target to the reticle.
+1
Pretty good boiled down explanation.
the target. The Objective lens focuses the target to the reticle.
+1
Pretty good boiled down explanation.
Evil_Lurker
New member
AO moves the focal point of the image from the objective lens(es) forward and back to make the focus point fall on the focal plane of the reticle (crosshairs). Then, they will move together, no matter if your eye is directly behind them or not.
Quote:The Objective lens focuses the target to the reticle. Exactly.
The ocular (eyepiece) adjustment is used to focus the image on the crosshair plane. Two different things.
Quote:the eye piece is the diopter end, it focuses your eye to the reticle Yep.
Here's a way to understand it:
If you look at a dial, like a clock, and the hands are 1 foot from the numbers, you'll need to be directly in front of it to read an accurate time.
Now, if you make the hands tight to the numbers, you can look at it from (almost) any angle and see an accurate time.
Parallax is shortening/lengthening the distance of the hands.
Stupid explanation, I know, but it's easy to understand and remember.
The ocular adjustment would be putting on glasses so you can read the numbers from across the room.
Quote:The Objective lens focuses the target to the reticle. Exactly.
The ocular (eyepiece) adjustment is used to focus the image on the crosshair plane. Two different things.
Quote:the eye piece is the diopter end, it focuses your eye to the reticle Yep.
Here's a way to understand it:
If you look at a dial, like a clock, and the hands are 1 foot from the numbers, you'll need to be directly in front of it to read an accurate time.
Now, if you make the hands tight to the numbers, you can look at it from (almost) any angle and see an accurate time.
Parallax is shortening/lengthening the distance of the hands.
Stupid explanation, I know, but it's easy to understand and remember.
The ocular adjustment would be putting on glasses so you can read the numbers from across the room.
Last edited:
Jack Roberts
Moderator - Deceased
Since I know the question will come up.
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Martyn4802
New member
Originally Posted By: Jack RobertsSince I know the question will come up.
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Jack, I have a question for you about any difference in depth of field of view between a fixed focus scope with parallax set at 100 yards, and a A/O scope set for 100 yards. Are the depth of field's the same?
TIA,
Martyn
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Jack, I have a question for you about any difference in depth of field of view between a fixed focus scope with parallax set at 100 yards, and a A/O scope set for 100 yards. Are the depth of field's the same?
TIA,
Martyn
Last edited:
sandhillshunter
New member
Originally Posted By: Jack RobertsSince I know the question will come up.
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Yep, that's what we do. We set them at 150 yards and leave'em until needed.
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Yep, that's what we do. We set them at 150 yards and leave'em until needed.
coyotewillie
New member
Years ago, I bought my FIRST AO scope, Leupold 12x that I put on a 220 Swift varmint rifle. Went out to shoot prairie dogs one afternoon. Never could get a clear picture out of it. Boy was I pissed! Headed back home intending to send it back to Leupold. Was in the process of dictating the letter, describing the scope to myself in my head. Got to "Leupold, 12x, adjustable objective........DUH!!! Turned around, went back a couple of miles to the prairie dog town. Tried it again, WOW. That was probably the best scope I've ever had on a rifle, LOL! Of course I was an idiot AGAIN and later sold the rifle and scope. That said, I think the AO type scopes really shine on the higher power, varmint/target type scopes. Greg
Last edited:
Jack Roberts
Moderator - Deceased
Originally Posted By: Martyn4802Originally Posted By: Jack RobertsSince I know the question will come up.
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Jack, I have a question for you about any difference in depth of field of view between a fixed focus scope with parallax set at 100 yards, and a A/O scope set for 100 yards. Are the depth of field's the same?
TIA,
Martyn
Everything else being equal, the depth of field will be the same.
Jack
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Jack, I have a question for you about any difference in depth of field of view between a fixed focus scope with parallax set at 100 yards, and a A/O scope set for 100 yards. Are the depth of field's the same?
TIA,
Martyn
Everything else being equal, the depth of field will be the same.
Jack
Martyn4802
New member
Originally Posted By: Jack RobertsOriginally Posted By: Martyn4802Originally Posted By: Jack RobertsSince I know the question will come up.
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Jack, I have a question for you about any difference in depth of field of view between a fixed focus scope with parallax set at 100 yards, and a A/O scope set for 100 yards. Are the depth of field's the same?
TIA,
Martyn
Everything else being equal, the depth of field will be the same.
Jack
Jack,
Thank you.
Don
No! There is no disadvantage to having an AO.
You can always set it to a fixed distance, leave it there, and it will be just like a non AO scope.
Jack
Jack, I have a question for you about any difference in depth of field of view between a fixed focus scope with parallax set at 100 yards, and a A/O scope set for 100 yards. Are the depth of field's the same?
TIA,
Martyn
Everything else being equal, the depth of field will be the same.
Jack
Jack,
Thank you.
Don
tunered
New member
I just trolled this old bit up from the archives and remember enjoying it, figgered I'd share it around again..........just cuz I liked it and thisaway it'll be handier for the oncoming Wintertime Blues Season.........
Dirty Steve
title: Registered User
status: Offline
Profile
location: San Antonio
join date: Jun 2004
posts: 6
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 it to MS Word off another site. And was sent a copy by the author after I contacted him directly. His tone may be a bit harsh, but this was written after responding o 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 writting style he is giving the straight poop 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 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 [beeep] 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 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 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!
---
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 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 [beeep] 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 [beeep] 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 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.
---
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 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 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!
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.
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 gots 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.
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Dirty Steve
title: Registered User
status: Offline
Profile
location: San Antonio
join date: Jun 2004
posts: 6
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 it to MS Word off another site. And was sent a copy by the author after I contacted him directly. His tone may be a bit harsh, but this was written after responding o 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 writting style he is giving the straight poop 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 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 [beeep] 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 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 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!
---
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 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 [beeep] 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 [beeep] 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 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.
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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 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.
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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 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 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!
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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.
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 gots 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.
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Orneryolfart357
New member
Very interesting read. I read that before, and now that I have a Nikon with Side focus Im wondering if it can be Calibrated? The cap does not look like it can be removed. No screws. Anyone have thoughts on these types of side focus calibration?
Blackhawk43
New member
Thank you for your post Ed, there is a lot of very useful information there. I think we should read that every morning until we understand it well. It reminds me of the saying "Don't let what you know keep you from learning".
Chapped Lips
New member
IMO....this discussion should be anchored permanently at the beginning of this forum....some superb explanations here...optics are FM (friggin magic) to me sumtimes...JMO