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Old February 20, 2013, 08:59 AM   #1
Bart B.
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Parallax Knobs; A Misnomer???"

timelinex states:
Quote:
While the parallax knob is often used to 'focus' the target at different distances. That is not the actual purpose of the function, and is actually an incorrect use for it. The parallax adjustment is meant to put the optics in the same plane as the target. Which in turn makes your sight picture parallax free.
In the beginning rifle scopes had adjustable objectives that would focus the target image on the scope's reticule; exactly like a camera lens focuses the subject on the film. If the target image ain't focused on the reticule, it will appear to move around the reticule as the aiming eye moves around behind the scope's eyepiece. Such condition's been called parallax since rifle scopes were invented in the very late 1800's. And everything in the view of the scope will not be parallax free; only things at the range the scope's focused for.

More recently, with so many rifle shooters not understanding basic optical lens functions in telescopes, they started calling the process of adjusting the objective lens to focus on target "parallax correcting" and in order to make more sales of rifle scopes, many makers as well as dealers began calling that functional part of the scope a "parallax adjustment."

That aside, the scope only focuses things at one distance sharply on the reticule. Everything closer is focused behind the reticule (closer to the aiming eye), everything further away is focused in front of the reticule. 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. That's when the aiming point on the target, the focused target image on the reticule and the aiming eye are all exactly in line with each other.
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Old February 20, 2013, 10:46 AM   #2
timelinex
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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'll expand on it later today, I dont have time to do it right now.

Here is my original paragraph for reference:
Quote:
While the parallax knob is often used to 'focus' the target at different distances. That is not the actual purpose of the function, and is actually an incorrect use for it. The parallax adjustment is meant to put the optics in the same plane as the target. Which in turn makes your sight picture parallax free.

With iron sights, the cheek weld is even more important than with modern optics because there is no way to make it parallax free. If your cheek is in a different position than your last shot, your looking through the sights from a different place and your not aiming at the same spot any more, even if you line up the sights correctly. You can test this by using your fingers as mock sights. Line them up in front of you so that one tip is directly in front of the other. Notice the spot that you are 'aiming at' in the background. Now move your head without moving your body. After you line them up again, you are not aiming at the same thing again. This is parallax.

With the parallax option in modern scopes, that is no longer an issue as long as you adjust the setting to where moving your head around doesn't move your cross hair on the target. Once this is done, no matter where your eye is in relation to the scope, as long as you have a full sight picture, you are aiming at the same spot.

As long as your target is clear enough that you can aim at it, you can be just as accurate with it out of focus. Parallax on the other hand can be the difference between a hit and miss. Just because you focus the image using the knob, doesn't mean your parallax free.. and vice versa. Those two optical properties are mutually exclusive. Being parallax free is the more important one.

Last edited by timelinex; February 20, 2013 at 02:33 PM.
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Old February 20, 2013, 02:28 PM   #3
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I got a couple minutes so I am able to better explain....

Your assumption that the image is parallax free when the object is in focus makes multiple assumptions. The assumptions include, that the ocular focus was set correctly, that the optic system itself doesn't have major aberrations and lastly that the person looking through the optic has 20/20 vision.

For most hobby hunters/shooters, all three of those assumptions are most likely not valid.

1. Most people don't have their reticle focused 100% correctly, as evident by the repeated postings online asking how to set it, and the incorrect information that is passed on.

2. Most people do not have 20/20 vision

When someone with vision thats slightly worse than 20/20 goes to 'focus' the image using the parallax knob, he will focus it to the point that it corrects for his bad vision. This means that he will pass the 'parallax free' setting and his in focus image will NOT be parallax free. So now the user will have a clear image, but his point of aim will actually be different at every slight movement of his eye. Depending on the size of the target, the distance and the magnification used this could lead to bad accuracy and missing the target.

On the other hand, if the sight picture is set to be parallax free, even if it is out of focus, then the system will be dead on, no matter how you look through the scope.

This is the reason ALL reputable optic companies recommend you adjust the parallax out by moving your head side to side and seeing if the reticle moves. Notice that none of them say to just move the knob until the image is focused... You would think since that is the easiest solution, that they would say that if it was 100% correct.... This is probably the same reason that they moved away from calling it a focus knob and started calling it a parallax knob. That is its main function for marksmen. Not everything is a marketing gimmick. If you are looking through a telescope, the focus portion of it is more important than parallax and they call it a focus knob.

This phenomenon is experienced by many shooters all the time. You focus you scope to be crystal clear and then your friend sits behind your scope and starts playing with the parallax knob because the image is blurry to him. You sit back behind the scope and now its blurry to you. Its not magic. Focus is a based on the persons own visual acuity while there is only ONE correct setting on the knob for that particular distance that your target is at. So in this instance, one of you are wrong, or even both of you are wrong. This experience alone should show you that your argument that parallax and focus is the same thing is inherently wrong in REAL life scenarios.
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Old February 20, 2013, 02:33 PM   #4
Bart B.
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timelinex, range focus and parallax adjustment are both the same thing on scopes that have a "parallax adjustment."

If the range focus is out of adjustment and the aiming eye's off the optical axis of the scope, there'll always be parallax.

If you're referring to the eye piece focus on the reticule being off, then the reticule will be out of focus but the target image will be in perfect focus if the range focus is where the eye piece focus is.
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Old February 20, 2013, 02:39 PM   #5
timelinex
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I've explained my stance as best as I could, including real world examples that prove my stance.

Maybe someone else will step in and explain their stance and it will clear it up for one of us.
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Old February 20, 2013, 03:06 PM   #6
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Timelinex. From my personal experience with side focus knob scopes (multiple leupold Mark 4s) on multiple rifles you are correct. Just because the target is in focus doen't mean the scope is parallax free. Since I learned that you adjust the side focus knob until the crosshairs don't move on the target, not when the target is perfectly in focus, my groups have gotten much better in local bench rest matches.

BTW your explaination is the best I have come across.
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Old February 20, 2013, 03:35 PM   #7
Bart B.
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For rifle scopes that have an adjustable focus for target distance (many have range marks on that adjustment), where does that adjustment focus a 100 yard target at inside the scope when it's set for 100 yards?

A. First focal plane only.

B. Second focal plane at the reticule.

C. Both first and second focal plane but not on the reticule if the eyepiece is not adjusted right.

D. Only at the focal plane of the eyepiece lens regardless of how the eyepiece is adjusted.
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Old February 20, 2013, 05:39 PM   #8
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timelinex, your explanations' one of the best I've seen. You know more about rifle scope optics than most.

But I made no assumptions. The image is parallax free when the scope's focused on the target. That puts the target image plane on the scope's reticule. It mattes not about optical abberations as that only defines the clarity of the image; how detailed the focus it or the focal lengths of all the individual lenses. And the scope's eyepiece can be removed and no human eye behind it and the objective lens stuff still focuses the target on the reticule. Focusing the scope on the target only involves those parts of the scope from its reticule forward. The scope's eyepiece makes up for the human eye not being able to focus on the reticule that's only 4 to 5 inches away from it; it's just a magnifying glass.

All the eyepiece does is compensate for the aiming eye's vision regarding its focal length. It's job is to correct the aiming eye's optical properties so the reticule is sharp; that's focusing the reticule on the eye's retina. It matters not what vision numbers the aiming eye has; 20/20, 10/20 or 40/20 without correction. The eyepiece will correct for that.

It's best that when someone gets a rifle scope, the first thing they do is look through it at the clear blue sky then focus the eyepiece so the reticule's very sharp. If the reticule ain't sharp the instant one looks into the scope aimed at a clear blue sky, adjust the eyepiece until it does.

My original comments related to the front to back movement of scope lenses focusing the target image on the reticule. Parallax is a situation one one of the optical devices in the aiming system is off the scope's optical axis at right angles to it. That's the human eye not aligned with that axis and the scope's not focused on the target focusing it on the reticule. Nothing in the scope moves anything at right angles to the optical axis. E and W adjustments move the optical axis in the scope for getting zeros and do nothing focus wise.

Rifle scopes focusing for target distance are exactly like camera lenses focusing for subject distance. One puts the focused image on a reticule and the other puts it on a sensor (chemical or photocell). Rifle scope's eyepiece does the same thing as a SLR camera's eye piece; focuses the target/subject image on the human eye's retina correcting for the human eye len's focus characteristics.

RGPM1A, when there's no movement of the target realtive to the reticule after you've focused the scope but it's not sharp, that's because the scope'e eyepiece isn't set right for your vision; corrected or otherwise. Take your scope outside and look through it at the clear blue sky then focus the eyepiece so the reticule's very sharp. If the reticule ain't sharp the instant you look into the scope aimed at a clear blue sky, adjust the eyepiece until it does. Then put it back on the rifle and focus parallax free on a 100 yard target. The target image should be a sharp, well defined image.
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Old February 21, 2013, 12:59 AM   #9
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Bart and timelinex,

I want to thank you both for this thread, as it has most definitely cleared up things for me that people have explained but I just haven't understood.

I think you are both getting at the same thing, just slightly differently from one another. You are both very knowledgeable in this field, and it definitely shows!

Thanks again
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Old February 21, 2013, 09:25 AM   #10
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Holy smokes!

Look at the 50 pound brains in this room.

Man you guys have explained things so very well, so that even I can understand it better.

Next question, if one does not have a parallax adjustment, does this mean he has an inherently inaccurate optic?
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Old February 21, 2013, 12:23 PM   #11
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stubbicat, you're just fine without a range-focus/parallax adjustment. With the lower power scopes (typically 6X and under), it's not a big problem as they're usually focused at about 100 yards. While they may well have a 1/2 MOA error with ones eye at the edge of the field of view when the target's 50 yards away, you'll miss your aiming point by only 1/4 inch.

I got to thinking last night that it would be nice if rifle scopes had an autofocus system like modern cameras. With present technology with such things, a 10X scope would need a lens at its front at least 3.5" in diameter for it to work. If on a 20X scope, the front lens needs to be 7" or more. Imagine how high the aiming eye would have to be off the butt stock to look through that scope. To say nothing of the mount height above the bases to let that scope clear the barrel.

I've contacted several scope makers over the years suggesting they get a qualified technical writer to put instructions in print and pictures so folks could easily understand what they need to do for best scope performance. None of them's done it as far as I can tell. Nightforce has pretty good instructions:

http://nightforceoptics.com/pdf/OwnersManual.pdf

But they messed up in the section on adjusting the eyepiece to focus on the reticule:
Quote:
If adjustment is necessary, follow the steps outlined for the type of Nightforce riflescope you have. Due to the way the human eye focuses, best results are usually obtained by turning the eyepiece inward until the reticle is slightly blurred then moving it outward until sharp focus is obtained. Refer to Figure 1.
It's not about the way human eyes focus. It's the way the eyepiece lens focuses. It's identical to a camera lens depth of field; things are in good focus twice as far beyond the focus distance than in front of it. The eyepiece lens' job is to make the reticule appear at infinity so your eyes will be relaxed and have a clear view of it. When you move the eyepiece, its focus point moves with it and you need to put that point right on the reticule.

Such errors in explaining optics makes all sorts of folks start thinking incorrectly. And that leads to getting confused with other things.
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Old February 21, 2013, 12:33 PM   #12
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They're getting close to what you describe BartB. Seems like with all that on there it wouldn't be hard to add autofocus.
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Old February 23, 2013, 09:10 AM   #13
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Thanks BartB.

I have a 2.5 to 10x scope w/o a parallax adjustment built in. I have no idea where the focus of the objective is set, but boy if I am suffering a parallax issue, it doesn't seem to affect the accuracy of the rifle mounted underneath.

Next time I guess I'll pony up the additional shekels and purchase a scope with the parallax adjustment.

I also have a 4.5 to 14x Nikon with parallax knob on the side of it. I wonder whether the lines scribed on the knob are reliable for setting the focus? Or whether I should futz with it until I get it perfect or something as judged by the Mark 1, Mod 0, eyeball.

Thanks so much for your input.
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Old February 23, 2013, 10:24 AM   #14
Bart B.
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No sight on a rifle effects its accuracy. Fired when held in a machine rest withouit sights, a rifle still shoots bullets into some sort of group. All the sight does is help one get the groups centers at the point of aim.

Of course, if the scope doesn't always keep its adjustable optical axis the same direction and amount relative to the bore axis, then that degrades the rifle-ammo accuracy that one will see using the scope to aim it.

You can learn where the non-range focusing scope's parallax free at by mounting the scope in a fixed position then looking through it with a small monocular or even one side of a pair of binoculars. Move that around looking through the eyepiece at some distant target and whatever range the target doesn't move relative to the reticule is the range it's focused at. Doing this will also check an adjustable scope's range markings are accurate. Some are way off. A Nightforce scope I checked out some years ago focused at 150 yards when its objective lens barrel was set to infinity.
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Old February 23, 2013, 10:41 AM   #15
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Quote:
If adjustment is necessary, follow the steps outlined for the type of Nightforce riflescope you have. Due to the way the human eye focuses, best results are usually obtained by turning the eyepiece inward until the reticle is slightly blurred then moving it outward until sharp focus is obtained. Refer to Figure 1.
Bart,

Now I am confused. For years the process I used for setting up scopes was to first focus the eyepiece. The way I was taught was to turn the eyepiece inward until the reticle was out of focus. The next step was to look at a neutral background and begin turning the eyepiece the opposite direction a half turn or so and to remove your eye from the scope. The idea was to present a sharp reticle when mounting the rifle so as to prevent your eye from "accommodating" the out of focus condition and introducing eyestrain.

As soon as the reticle was presented nice and sharp, lock the lock ring and then you could focus on the target and bring it into the focal plane of the reticle.

The side focus knobs work pretty well for me. The older Weavers I have had a focus ring on the end of the tube with yardage marked on them from somewhere around 50 yards to infinity.

When I used to hunt groundhogs with that scope, I would go to a prospective area and set my focus with the focus ring until the background was clear. I would then move my eye a bit sideways and up and down and look for relative movement between the greenery and the crosshairs. If I saw that, I would tweak the focus ring until it stopped but I did NOT adjust the eyepiece.

Today, most of my scopes have the focus on the side and my drill is to first make sure the reticle is clear, Then I dial in the magnification I want. My Nightforce is an 8-32 so there are a lot of options.

Once that is settled, I focus on the target and pay no attention to the range markers but as soon as the target is clear, I move my eye a bit side to side or up an down looking for that "relative" movement and adjust until it goes away or is minimized.

I generally start from infinity and work back until focus is achieved and go slightly past that point until the target starts to blurr and then go back in the opposite direction until a clear target is achieved and then quit.

I always try to take up any slop in the same direction. Learned that from over 40 years working in a metrology lab and playing guitar ( NEVER tune down to pitch. . .ALWAYS go flat and tune UP to pitch ).

Same thing occurs when adjusting the mechanical axis and optical axis of a theodolite or jig transit. You always want to be increasing tension on a screw adjustment for final tweaking to get all the slop out of the adjustments.

I think you are saying pretty much the same thing. If I have missed something, please feel free to school me. I am always ready to learn something new.
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Old February 23, 2013, 03:42 PM   #16
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geetarman, you've got it right. Nightforce's instructions saying screw the eyepiece in 'till the reticule's fuzzy is the same as 'till the reticule's out of focus. Same as an 8 ounce glass with 4 ounces of water in it; some say it's half full, to others it's half empty.

The reason the eyepiece needs to be backed out unitl the reticule's sharp and clear is the "in focus" range is about twice as far beyond the focus point as it is in front of it towards the shooting eye. So using this eyepiece focus method, the eye piece ends up where it'll make the reticule sharp as the aiming eye goes through normal changes.

After the eyepiece is set, don't move it unless your eye changes. Focusing the scope at different ranges involves moving the lenses in front of the reticule.

You know about the mechanical axis and optical axis of a theodolite or jig transit? I laid out the 1000-yard rifle range north of Byers, CO, in early 1985 using one of the first electronic distance meters made. 'Twas a Hewlett-Packard one that the plant were I worked invented in the late 1960's. They kept that one for employees to use for whatever reason. What a marvelous machine and it worked well in the 5 below zero weather with the external battery pack connected.

http://amhistory.si.edu/surveying/ob...dnumber=748833

The project for it was code named "Bear" as it was a real bear to get everything worked out to fit in the small case parts. But it sure was in demand; the Loveland HP plant had problems keeping up with orders. One of the engineers on the project had a cousin who was a game warden in Colorado: he asked him if he could have the DOW folks put 3-corner reflectors on the elk so folks could get accurate ranges to them during hunting season.
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Old February 23, 2013, 04:05 PM   #17
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Bart,

The parallax knob does in fact put the sight picture in the same plane as the reticle. I guess I was wrong, by listing all the assumptions you made, because as you explained, it all goes back to one assumption. that your eyepiece is adjusted correctly.

I know I adjust all of my scopes to me, however if I'm using a friends rifle, I don't mess with his settings, and vice versa. So that is why I stand by my initial statement that fully focused and parallax free isn't the same thing and you can have one without the other. That is why if its an important shot, I always check the parallax (moving head side to side) as opposed to relying on the focus.

To the others, worrying about their scopes not having parallax. Depending on the application and magnification , its not a big deal. Parallax control isn't as crucial for low magnification optics. If I remember correctly, the effect is multiplied by the amount of magnification. So you will have 8 times the parallax error at 24x than at 3x.

Lastly, all the shortcomings of a scope without these features can be overcome by excellent marksmen fundamentals. Be 100% consistent with your cheek weld and body position and parallax is a non issue. The correction is a crutch for when your not 100% consistent. There are iron sights shooters that have no such thing as parallax correction and can out shoot most of us with our fancy scopes....
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Old February 23, 2013, 04:38 PM   #18
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Quote:
if I'm using a friends rifle, I don't mess with his settings
That is a very good point. I do not mind if someone changes mine to suit their vision but I never change the settings on a scope belonging to someone else.

Funny thing about that. I shoot with an LEO a lot and I sometimes get to shoot his service rifle. His eyes are quite different than mine and when I look through his scope, I really have a poor picture. The reticle is WAY out. Consequently, I can never shoot his rifle as well as he does. Because of his line of work, I would never change his eyepiece. Yet when he shoots my 700P, he always says the picture is fine and he is able to shoot about as well as I do with it. Might be the age difference in that his eyes can better accommodate the difference in the sight picture. I don't know.
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Old February 23, 2013, 04:48 PM   #19
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Bart,

I spent a lot of time in Dimensional Metrology and worked on a lot of Brunson jig transits and transit squares, bore sight telescopes, autocollimators and theodolites.

You must be gifted of extreme patience to be good at it as anything you do imparts heat into the system which immediately begins to soak into the setup and starts the inevitable drift.

I spent 40 years with Boeing and did a lot of calibration. It is a game for young folks with really good eyes. By the time you really get good at it, your eyes begin to give you problems. At least mine did.

I enjoyed all aspects of metrology but I am not the least bit concerned about it now.

I try to incorporate the things I learned into my shooting and attention to detail is one of the things that pays pretty good dividends in working up loads as well as consistency in shooting.

The more variables you can control, the less there are to ruin your day.
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Old February 23, 2013, 06:06 PM   #20
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For those wanting to use someone's rifle scope and not change its eyepiece focus, you can put a corrective lens of the right diopter on it. Probably not more than a .25 or .33 diopter corrective lens of either + or - correction. Lenscrafters made a -.25 diopter one for me years ago that I put in place with a Butler Creek scope cap after removing its lid and hinge. It changed the magnification about 10% less but at least I could use that scope without moving its eyepiece.

You can also increase the power of a scope by putting in a + diopter lens on the eyepiece. The eyepiece has to be screwed in further and it reduces the eye relief. I changed a 16X scope to a 25X one that way; eye relief went from 3 inches down to 2 inches; just fine for a smallbore match rifle.
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Old February 23, 2013, 06:48 PM   #21
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Re: Parallax Knobs; A Misnomer???"

Just played.around a bit outside. Pointing at clear sky and adjusting the parallax knob, the reticule was sharp 90% of the time. Moved it down and across the yard, 50-60ft and it was blurry, adjusted from there to point where stuff wasn't moving around when head was moving...will this be OK at 100+ yards?
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Old February 23, 2013, 07:22 PM   #22
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Quote:
Pointing at clear sky and adjusting the parallax knob, the reticule was sharp 90% of the time
I think you are going about it wrong. If your scope has a focus ( parallax ) adjustment on the side, leave it alone for now.

The first thing is to get the crosshair in focus. This needs to be done carefully so that it is nice and sharp the moment you mount the rifle.

The easiest way I know to do it is to take the eyepiece. . .not the focus knob ( parallax ), and turn it clock wise all the way in until the crosshair is well blurred.

After you have done that, aim the scope at a neutral background. An off white wall or a berm at the range will work also.

Begin turning the eyepiece counter clockwise until the cross hair starts becoming clear. As you get close, take your eye off the scope. This will prevent or minimize what we call "accommodation." Accommodation is what occurs when your eye tries to make something that is a little out of focus to be in focus and contributes to eyestrain.

Look through the scope again and continue to turn the eyepiece counter clockwise until the image is sharp when you first look through it. Half turns of the adjustment are normal here until the crosshair is clear and crisp at the moment you look through the scope. It may take a little trial and error of moving too far and having to start all over. That is ok. The goal is to get the crosshair clear first.

Once you are satisfied, lock the lock ring and leave it alone. If you wear corrective lenses, make your adjustment with them on. . .UNLESS you shoot without glasses. For many years that is what I did. At the range I frequent, you MUST have glasses on all the time.

If you shoot with regular shooting glasses ( no correction ) it makes no difference.

Now that you have the cross hair focused, look through the scope at the target you want to shoot.

Select your magnification if you have a variable and set your focus knob ( parallax ) adjustment to infinity and then bring it back until the target is clear.

If you move your eye slightly side to side or up and down, you should see NO relative movement between the target and the cross hair.

If you DO see movement, parallax is present and should be corrected out by further manipulation of the focus ( parallax ) knob.

This MIGHT mean a slight degradation of the target definition.

For hunting purposes, you will have to wing it. You will have to generalize a lot to make the shot and you will not have time to tweak any adjustments.

For target shooting, you generally have all the time in the world to get things set.

One thing to remember though. . .once the crosshairs are in focus for YOU, you should leave it alone unless you are changing from shooting with glasses to shooting without them.

When I used to hunt groundhogs, I wore glasses so I could look for movement around the edges of the fields. When I saw movement. . .off came the glasses and up came the binoculars. My binoculars and rifle scope were all adjusted for me to see clearly without my glasses. Anyone else using my rifle would have a problem.

If you wear glasses and your vision is 20/20 corrected with glasses and you use those glasses to set up your rifle, anyone with 20/20 uncorrected vision should be able to pick up your rifle and do pretty well with it as long as the rifle fit them like it fits you.

Sorry this is so long and hope it helps.
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Old February 24, 2013, 02:10 AM   #23
Fire_Moose
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Join Date: January 16, 2013
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Re: Parallax Knobs; A Misnomer???"

Now I'm confused. LOL.

I was using the eyepiece adjustment. Thought THAT was the parallax. Parallax is the infinity-200 yard knob?
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Old February 24, 2013, 02:32 AM   #24
Fire_Moose
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Re: Parallax Knobs; A Misnomer???"

And I switch between glasses and contacts. That means I can set it up with either and it will be the same or really close to the other?
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Old February 24, 2013, 06:49 AM   #25
geetarman
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[QUOTE]I was using the eyepiece adjustment. Thought THAT was the parallax. Parallax is the infinity-200 yard knob? [QUOTE]

I think that is correct. What scope do you have?

Quote:
That means I can set it up with either and it will be the same or really close to the other?
I am not sure what you mean by this. If you have a large correction for your vision and your rifle is set up for viewing with you wearing your glasses, you will have a problem trying to use it without your glasses.

If your vision is corrected to 20/20 with glasses and your scope is set up that way, then anyone with uncorrected 20/20 vision and those whose vision is corrected to 20/20 should be able to use your scope without further adjustment as long as the person wearing glasses is using them when he uses your rifle.

Put another way, if the scope is set up for someone with 20/20 vision, the scope does not care if you wear glasses or not as the assumption is that the eye viewing the target is corrected to 20/20.
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Last edited by geetarman; February 24, 2013 at 06:56 AM.
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