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Old February 24, 2013, 08:21 AM   #26
Bart B.
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Fire Moose, if your contacts and eye glasses have the same prescription, once the scope's eyepiece is set to have the reticule sharp, either set of corrective lenses will work just fine.
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Old February 24, 2013, 05:38 PM   #27
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Re: Parallax Knobs; A Misnomer???"

Thanks guys. Was wanting to take it out this weekend but too much wind for load tests. Ill have to wait it out some more.
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Old February 25, 2013, 10:28 PM   #28
AR15barrels
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Quote:
As long as the aiming eye's kept on the optical axis of the scope, there will never be any parallax of the reticule on the target's aiming point regardless of where the objective (front) lens group's focused.
The issue is that most people are really poor at keeping their eye at the center of the optical axis of the scope.
Also, what do you consider the optical axis?
Are you talking about the external glass, or the erector?

These two are rarely aligned unless you have a scope that adjusts externally like they did 100 years ago.

When teaching new precision rifle shooters, I find that they make their greatest leaps in precision when they fully understand how to adjust their parralax to remove all parallax error.
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Old February 25, 2013, 10:33 PM   #29
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Focus and parallax adjustment is not the same thing. Your sight picture could be 100% in focus while having parallax or vice versa.
I had a nightforce scope that had the focus and parallax slightly out of sync with each other.
I could focus the image as sharp as can be and then I would have to roll the parallax adjustment about 3 degrees counterclockwise to eliminate all parallax.
It was very consistent and I learned that with that specific scope, I focused it, then went counterclockwise until the image just started to go soft and that was right where my parallax error was at it's lowest.

I now teach people NOT to focus their scopes, but to adjust them for minimum parallax error.
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Old February 26, 2013, 09:36 AM   #30
geetarman
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Quote:
I had a nightforce scope that had the focus and parallax slightly out of sync with each other.
I am not sure if I would go that route. You may be using your scope at a variety of ranges. I think the most important thing when you first mount the rifle is a crisp reticle.

Unless your eye changes or you get very tired, that would be the constant.

The variable would be the target.

I have the 8-32 NightForce and the one thing that bothers me is getting just the right adjustment for the reticle. You can make several turns of the eyepiece without a great deal of change in the clarity of the reticle.

What I have noticed is that regardless of range or magnification, the target image parallax is minimized when the target image is just slightly "soft."

I think we are talking about essentially the same thing but approaching the solution from a different angle.
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Old February 26, 2013, 11:11 AM   #31
Bart B.
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AR15Barrels, that issue of focus and parallax mismatched can happen. The cause is the range scale on the adjustment's not exact for the actual objective lens' focal length.

Let's say the scope's optical system designer chose the objective (front) lens system to have a 200mm focal length. There's a tolerance in making lenses; ones of that type can easily have a few millimeters spread. The mechanics of the focus system are exact; a given threads per inch of the adjustment moves the lens mount exactly the same for all scopes with that design. So the range marks can be put on each and they'll all be perfect for the designed lens focal length.

But the one in your scope is different. You set your range focus at 200 yards but the objective lens actually focuses that target in front of the reticule if it's actally 198mm, behind the reticule if that's a 202mm lens.

Depending on how much error the objective lens' focal length is off from spec, the amount of parallax will vary.

Note that if you're teaching people to not focus their scopes, they'll never be able to adjust parallax. These terms both relate to the same thing. It's physically impossible for the reticule be properly focused on by the eyepiece and when the range focus is on the reticule, to have any parallax at the target range. If I understand what you're saying, I think the scope's eyepiece wasn't focused perfectly on the reticule but instead in front or behind it.

What did you use to "focus" the scope on the target? I ask because all the scopes I've seen only have one adjustment to focus at the target range and it cannot be the eyepiece.

What make and model of scope do you have? This interests me very much and I'd like to look into it deeper.....thanks.
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Old February 26, 2013, 01:13 PM   #32
geetarman
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Bart,

Maybe it is my old eyes ( probably ) but it seems that years ago, it was a piece of cake to get the cross hair nice and sharp. I have an OLD Weaver K4 that is just as bright and sharp as the day I got it.

Beginning with an old Redfield 3-9 Widefield, I was never able to get that cross hair the way I wanted it. I had another 6-18 that was just as bad if not worse.

I have a new reissue Weaver K4 and it has a duplex instead of the old cross hair and you can get that one pretty sharp.

The NightForce is a nice scope but the cross hair never seems just right. It is close but when I start to focus it, it seems as though the adjustments are TOO fine and you can't find that "sweet" spot from coming from infinity to "in focus" and driving through to be out of focus. Normally, on the older scopes, I would go from out of focus to in focus and then to out of focus and reverse direction to get the "right" spot.


Even years ago when I was working, K and E usually had pretty crisp and black crosshairs. There were some Brunson alignment telescopes that were very good and others, all the same model, were not.

Without a clear and black cross hair to get your initial line of sight, it was difficult to set the line of sight and if the crosshair was not really clear, a plunge and rotate to measure the centering of the optical axis and mechanical axis was almost impossible.

Have you experienced similar?
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Old February 26, 2013, 06:18 PM   #33
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Trying to read and absorb what has been said.

Would it be correct to say that with front objective adjustable scopes, like my Nikon Monarch UCC 5.5-16.5x44AO, that the front objective would focus the target on the reticle plane, and the rear objective would focus the reticle image on your eye?

I actually work on some pretty advanced optical systems, but we probably use different terms. I am trying to understand and relate this parallax, still reading so might be back to ask questions.
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Old February 26, 2013, 06:21 PM   #34
Bart B.
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geetarman, what most folks do that often doesn't keep the reticule in sharp focus is twofold.

First, one needs to back the eyepiece out just 1/3 turn at a time then look at some distant point at least 200 yard away to let your aiming eye focus at infinity. Then look into the scope with it pointed at a clear sky. If the reticule ain't tack sharp immediately, quit looking through the scope. Continued staring through an out of focus eyepiece makes the eye try to focus; eye strain't not good for sharp vision. Readjust the eyepiece then refocus your aiming eye at a distant target. Finally look again into the eyepiece. Again, the reticule should be sharp instantly. Repeat if needed.

Second, the most common problem with some folks not seeing the reticule sharp all the time is the eyepiece wasn't focused right to begin with. They eye's trying to get sharp focus and older one have a harder time with it.

But sometimes, old eyes just don't stay focused that well, sad but true.
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Old February 27, 2013, 04:13 AM   #35
AR15barrels
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Parallax Knobs; A Misnomer???"

Quote:
Originally Posted by Bart B. View Post
AR15Barrels, that issue of focus and parallax mismatched can happen. The cause is the range scale on the adjustment's not exact for the actual objective lens' focal length.

Let's say the scope's optical system designer chose the objective (front) lens system to have a 200mm focal length. There's a tolerance in making lenses; ones of that type can easily have a few millimeters spread. The mechanics of the focus system are exact; a given threads per inch of the adjustment moves the lens mount exactly the same for all scopes with that design. So the range marks can be put on each and they'll all be perfect for the designed lens focal length.

But the one in your scope is different. You set your range focus at 200 yards but the objective lens actually focuses that target in front of the reticule if it's actally 198mm, behind the reticule if that's a 202mm lens.

Depending on how much error the objective lens' focal length is off from spec, the amount of parallax will vary.

Note that if you're teaching people to not focus their scopes, they'll never be able to adjust parallax. These terms both relate to the same thing. It's physically impossible for the reticule be properly focused on by the eyepiece and when the range focus is on the reticule, to have any parallax at the target range. If I understand what you're saying, I think the scope's eyepiece wasn't focused perfectly on the reticule but instead in front or behind it.

What did you use to "focus" the scope on the target? I ask because all the scopes I've seen only have one adjustment to focus at the target range and it cannot be the eyepiece.

What make and model of scope do you have? This interests me very much and I'd like to look into it deeper.....thanks.
I know about reticle focus, scope image focus and parallax.
In the particular nightforce scope I was talking about above, my reticle was perfectly focused.
It was just that particular scope (I owned 3 of them at the time) which had the parallax and the image focus out of sync.
My other two would put the image in focus and the reticle parallax free at the same adjustment.

When I teach new precision rifle students, I have them first focus their reticle using the sky as the background.
Then I teach them to adjust the parallax for focus first (gets you close) then continue adjusting so that when they move their eye behind the rifle without moving the rifle, they do not see any reticle movement on target.
Some scopes will coincide image focus with parallax.
Other scopes will not.

Shooters looking for precision grouping will do better with soft focus and zero parallax error than they will do with sharp focus and parallax error present.
The parallax markings on the scope are rarely correct.
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Old February 27, 2013, 08:33 PM   #36
Bart B.
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AR15, thanks for your commenst and explanations. But there's still something about it that doesn't seem right.

It's physically impossible (optical laws of refraction of light rays as they travel through one medium to another) for a rifle scope to have the target image focused exactly on the reticule and the aiming eye seeing the reticule in sharp focus and not have both target image and reticule as sharp as can be as seen by the human eye.

For a given focal length of the combined lenses in front of the reticule, there's only one finite point where a target at a given range will focus sharply. Same for the eyepiece and the human eye; there's only one finite point in front of the eyepiece lens that the human eye can naturally focus on.

If you're referring to the side adjustment (focus or parallax, whatever one chooses to name it) being set to a range but the target image from that range isn't in sharp focus, then I understand what you're saying.
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Old February 27, 2013, 09:33 PM   #37
Aberration
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Actually its quite possible and very well documented. I could go into a lot of aberrations that would degrade your resolution yet be in focus. Most of the aberrations will only start having an effect at the sub-micron level, but there is plenty still. Spherical and Chromatic would be the the most obvious. Even at the mean focus, you are going to have a bell curve of that focus depending on the size and position in the field of view, and of each color. Better glass, better coatings, different internal gas and pressure...

There are reasons that in general there are cheap and expensive scopes.

In my field of occupation I believe what we call this parallax is telecentricity?
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Old February 27, 2013, 09:48 PM   #38
Bart B.
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Abberation, I understand what you're referring to:
Quote:
Even at the mean focus, you are going to have a bell curve of that focus depending on the size and position in the field of view, and of each color.
Which is why different glass types (different refraction indexes), shapes and coatings are used in each element of doublets and singlets, even triplets, so each color band's focused closer together on the optical axis and eliminate as much of field curvature as possible at the edge of the field of view..

But there's still only one point among all those different colors focused at different distances from the lens that's the "focus" point as seen by the eye.
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Old February 28, 2013, 01:02 AM   #39
AR15barrels
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Parallax Knobs; A Misnomer???"

Quote:
Originally Posted by Bart B. View Post
AR15, thanks for your commenst and explanations. But there's still something about it that doesn't seem right.

If you're referring to the side adjustment (focus or parallax, whatever one chooses to name it) being set to a range but the target image from that range isn't in sharp focus, then I understand what you're saying.
Nightforce NXS scopes do not have yardage markings on the parallax knob.
You just adjust by eye.
Nightforce is usually pretty good about getting the target image in focus while the reticle is parallax error-free.
This particular scope was an anomaly in my experience.

Schmidt and Bender do have distance markings on the parallax knobs.
Schmidt and bender optics are very good quality and the scopes have a very narrow depth of focus.
It's common to be able to use a Schmidt and bender scope as a rangefinder by sweeping through the focus (parallax adjustment) and finding the near and far limits of where you see sharp focus.
For example, a target placed at 300yds may show sharp image focus from 290 to 310 on the parallax knob.
You can reliably say that target is at 300yds after racking though the focus and splitting the focus range in the middle.
Now, the scope will not always be parallax error free at the 300yd marking through.
You still have to bob your eye around behind the scope and remove the parallax error that way.
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Old February 28, 2013, 08:31 AM   #40
Bart B.
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AR15, thanks a bunch for the following:
Quote:
For example, a target placed at 300yds may show sharp image focus from 290 to 310 on the parallax knob.
You can reliably say that target is at 300yds after racking though the focus and splitting the focus range in the middle. Now, the scope will not always be parallax error free at the 300yd marking through. You still have to bob your eye around behind the scope and remove the parallax error that way.
Now I know why there's parallax after you've first focused the eyepiece to see the reticule clear and sharp then focused the scope sharply on that 300 yard target.

The mechanics of the knob moving lens mounts inside the scope are exactly the same from scope to scope of that model. A given change from infinity to some shorter range will move those mounts exactly the same for each of them. And each ones movement for a given range change is based on each lens group moving based on the optical design specs for the focal length of each group. But lens making machines don't make each one of a given spec exactly the same; their focal lengths vary by a few percentage points.

For a 15X scope, the combined focal length of the lenses in front of a second plane reticule will be 22.50 inches for an eyepiece focal length of 1.5 inches (typical for scopes with a 3 inch eye relief). If the actual focal length of those front lenses is actually 22.40 inches, that'll change the focus point for a 300 yard target about 1/10th of an inch. That image will be focused about 1/10th inch in front of the reticule and parallax will easily be seen. If the actual focal length was 22.60 inch, the target image would be focused about 1/10th inch behind the reticule.

(Note that the physical length of a telephoto lens is less than what a long lens of the same focal length will be, Rifle scopes use telephoto lens systems so one with a 22.5 inch focal length may be only 8 inches long physically. Such is the magic of optics.)

With these normal variables for a given zoom range in a scope, there'll be less error at the lower magnifications, more error at the higher ones. This is why cameras had autofocus stuff put in them. The fixed mechanics of split image rangefinders in them were not exact; long telephoto lenses were often out of focus 'cause their focal lengths were a few percentage points off from what the rangefinder's mechanics were made for. Some rangefinder cameras had interchangeable cams cut for exact focal lengths (marked on the lenses as accurately measured) so long lenses would focus correctly on film.
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Old March 1, 2013, 01:22 AM   #41
AR15barrels
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Parallax Knobs; A Misnomer???"

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Originally Posted by Bart B. View Post
With these normal variables for a given zoom range in a scope, there'll be less error at the lower magnifications, more error at the higher ones.
This is exactly what I have observed.

And I happen to work for a lens company...
We would never accept a lens assembly group that was off by a few percent.
We would only accept a group that was off by a few hundredths of a percent.
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Old March 1, 2013, 01:45 PM   #42
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AR15, I used to shoot smallbore matches with a guy that worked for Redfield at their Denver plant. He said once that lots of scope company practices are to sort each batch of lenses into those with a very small tolerance to be used on high dollar scopes. Lenses with larger tolerances were put in lower priced ones.

Some years ago, Lones Wigger (Olympic and other Int'l Gold Medalist) brought me one of Redfield's new zoom target scopes to test on my collimator. When zoomed from low to high power then back to low, the reticule moved in a 1.25 MOA figure 8 about the collimator reference. It also had about 1/2 MOA slop in reticule position from simulated recoil. No wonder he didn't like using it for even light recoiling .22 rimfire match rifles.
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