Adjusting for parallax

Another way to think about it is that the parallax adjustment puts the target image and the reticle in the same optical plane. Most scopes are pre-set to 100 yards. Anything farther out is effectively at infinity.

JoeR

Sorry this is off-topic, but the title of this thread would be a good name for the next Rush album if there is one

traveller said:

Once you hit 100yards, its influence becomes negligible.

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we got a smart cookie here

This comment is right on, after 80-100 yards you really do not need to worry about parallax

Mark75H said:

You set it for the distance you are shooting.

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True, although NOT by reading the numbers on the turret.

Mark75H said:

Its not clarity as much as reticle position inside the scope to avoid distortion at the particular target distance....

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Not "distortion but actually "displacement", but otherwise correct.

wilcam47 said:

it has nothing to do with ballistic drop....just for clarity at the particular yard setting on the dial.

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Improved clarity is incidental to reduced parallax. Target clarity is only truly improved if the shooter begins with a sharply focused reticle.

traveller said:

Once you hit 100yards, its influence becomes negligible.

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Wrong.

amoebicmagician said:

we got a smart cookie here

This comment is right on, after 80-100 yards you really do not need to worry about parallax

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Wrong.

JoeRinMD said:

Another way to think about it is that the parallax adjustment puts the target image and the reticle in the same optical plane. Most scopes are pre-set to 100 yards.

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True.

Anything farther out is effectively at infinity.

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False, unless you only shoot out to 100 yards.

tylerevans42 said:

...So parallax can be observed by the target appearing off center from my reticle by moving the orientation of my eye? Or my head completely?

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Yes, but as posted, you must slightly nod the head yes/no without disturbing the gun. Learned by practice.

smdub said:

No, move your head.

If keeping your head still, and just looking around the sight picture w/ your eyes won't do anything. Your pupil isn't moving side to side enough the TINY amount it takes to 'look around'. Plus, you're always looking right at the center of reticle when shooting anyway so that isn't a problem in real life.

When you move your head, you are displacing your pupil so that it is seeing the image more from the side, than the center. When shooting you don't typ move your head around but every time you mount the rifle, your head is in a slightly different spot each time. THAT is whats causing the problem. The parallax setting of the scope will superimpose the reticle at some distance out. This can be simulated by putting you finger up in front of your face half way betwen your eyes and monitor. Imagine this is a 50yd parallax and your monitor is at 100yds. Move you head side to side (while holding finger still) and you will see your finger 'move' across the screen. Just looking around while holding your head still does nothing. Now, place your finger right up against the monitor and repeat (both at 100yds). It doesn't move across the screen anymore does it?. Of course the scope isn't actually placing the reticle way out at your target (would take a LONG scope body) but its using optics to simulate it.

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Good post.


IMHO, for most non-technical folks to best grasp this, we should start at the beginning, so:

1) The definition of "parallax": The apparent displacement of an object due to a change in the observers vantage point. (http://dictionary.reference.com/browse/parallax

All this means is that if the person looking moves, the relationship between the object and background also changes, thus the object seems to move.

This effect is caused by, and influenced by, the distance between the object and background.

A very simple example in common context is the analog fuel gauge in a vehicle. While in the driver's seat, the eye is aligned correctly with the indicator needle/gauge face relationship and the indication is correct. If one departs the driver's seat and observes the same indication from elsewhere, the needle may not appear to provide the same indication. Driver's seat sees 1/2 tank, passenger seat sees 3/8 or 1/4. This effect is parallax in its simplest form.

2) To control parallax, we have two options:
a) Stay in the driver's seat - the reading will always be correct
b) Move the needle closer to the gauge face - reduces apparent error due to changing vantage points.

In the context of firearms optics, correct stock fit serves to keep the shooter aligned correctly...in the driver's seat, so to speak. Unfortunately, stock fit alone will only minimize, but not eliminate, the effects of parallax.

Even the mere elasticity of the face will prevent perfect alignment every time, so we must be able to adjust the reticle<>target relationship to eliminate this effect.

The parallax turret serves to move the needle (reticle) closer to the gauge face (target) to reduce or eliminate parallax error. If they are on the same focal plane, there can be no error.

Parallax error itself is detrimental to precision. The blurry target image that is incidental to parallax error really isn't the issue at all. The issue occurs when you chase the displaced reticle image and move the gun away from the actual position on target. The target image must remain stationery if you wish to aim at the same place each time.


In optics, there is the term "focal plane", which is where the image is at a certain distance that is correctly focused. Then, as an accessory to this, "depth of field" defines the near<>far limits of acceptable focus of an object's image. Think of it as the "plus/minus" tolerance on either side of your focal plane.

Depth of field varies with magnification. As magnification increases, the depth of field is reduced and becomes shorter. This makes target focus, and if another object (reticle) is involved, parallax, a very critical and unforgiving adjustment.

At lower magnifications such as 2 or 3x, our depth of field will encompass average shooting distances without noticeable limitations. As magnification increases, depth of field is reduced and we will find some objects to be outside of our focal plane. This becomes a focus problem, and if another object (like a reticle) is involved, a parallax problem too.

Scopes of less than about 10x have a relatively long depth of field and if they are preset at the factory for 100-150 yards (most are), they will typically perform acceptably at normal shooting distances.

The parallax error in a fixed-objective scope is typically most apparent at closer distances, as pointed out in several posts above. Because this error (in a preset scope) is most prevalent up close, firearms intended for close range use will require special settings. This is what drives the manufacture of special ".22 scopes" and "muzzleloader scopes" and "shotgun scopes". Aside from some having BDC reticles for a given application, .22 scopes are often preset for 50 yards and the MZ/shotgun scopes for 75.

So, anyway, your rifle scope operates as two halves...two completely independent systems, that must interface correctly for proper performance.

The first half is closest to your eye. This first half is comprised of the ocular assembly and adjuster, the rear length of tubing, the turret housing & turrets, and most important, the reticle.

We adjust the first half of our scope to match the reticle focus to our individual eyesight. Normal vision usually puts this adjustment at mid-range, but this can vary with misc. scope design factors. In any case, the reticle lies at a specific physical and optical distance from the shooter's eye and is focused by moving the ocular adjustment. To make this adjustment, it is best to "turn off" the other half of the scope by placing a light colored obstruction out a few inches in front of the objective. This eliminates clutter and draws the eye to the reticle alone.

Once the reticle is focused correctly, this sets up our first focal plane and the reticle is presented to the eye with no eyestrain or distortion.

Then, we can address the second half of the scope. The second half begins on the other side of the turret housing, with a length of tubing that connects to the objective housing (if present - some are straight tubes). This section serves to bring the target image to the reticle. That is its entire reason for existence - bringing the target image to the same focal plane as the reticle. If two objects are on the same focal plane, there can be no parallax error.

What makes focus incidental to parallax is the state of your ocular adjustment for the reticle. If the reticle is not on the same focal plane as your eye, it will not be focused and will either appear blurry or your eye will bend over backwards to make the image work. If we present the target on the same focal plane as the mis-focused reticle, it too will be blurry, even if properly adjusted to be parallax free.

In a low powered, non-adjustable scope, this adjustment is made at the factory and the objective lens is locked in. A collimater is used to set the parallax so that the target image at the desired distance is erected on the same focal plane as the reticle.

In higher magnification scopes (over about 10x), our depth of field is so reduced, we cannot simply set a compromise distance and be able to function correctly at various ranges. For this reason, a user adjustable system is employed. Older scopes are "front focus", with a rotating ring out on the objective housing. Newer scopes are most often "side focus", with the parallax adjustment on the left side. Regardless of the control location, both systems allow the user to move the objective (front) lens in and out until the target image is correctly presented.

In lower magnification scopes, the depth of field is long and so the adjustment is less critical. A low magnification scope and/or a lack of clear understanding of the function/effects of parallax can lead to the assumption that there is a range limit at which parallax is no longer apparent.

As magnification increases and depth of field is reduced, this adjustment becomes very critical. It is quite easy to see parallax at 1,000 yards and further. If incorrectly adjusted, the reticle can seem to move 1 to 2 FEET across the target area. When the parallax adjustment is correctly made, the target and reticle are on the same focal plane and will appear to be as one.

Well, this is the way I like to think of it.

E.Shell said:

True, although NOT by reading the numbers on the turret.Not "distortion but actually "displacement", but otherwise correct.
Improved clarity is incidental to reduced parallax. Target clarity is only truly improved if the shooter begins with a sharply focused reticle.
Wrong.Wrong.

True.False, unless you only shoot out to 100 yards.
Yes, but as posted, you must slightly nod the head yes/no without disturbing the gun. Learned by practice.
Good post.


IMHO, for most non-technical folks to best grasp this, we should start at the beginning, so:

1) The definition of "parallax": The apparent displacement of an object due to a change in the observers vantage point. (http://dictionary.reference.com/browse/parallax

All this means is that if the person looking moves, the relationship between the object and background also changes, thus the object seems to move.

This effect is caused by, and influenced by, the distance between the object and background.

A very simple example in common context is the analog fuel gauge in a vehicle. While in the driver's seat, the eye is aligned correctly with the indicator needle/gauge face relationship and the indication is correct. If one departs the driver's seat and observes the same indication from elsewhere, the needle may not appear to provide the same indication. Driver's seat sees 1/2 tank, passenger seat sees 3/8 or 1/4. This effect is parallax in its simplest form.

2) To control parallax, we have two options:
a) Stay in the driver's seat - the reading will always be correct
b) Move the needle closer to the gauge face - reduces apparent error due to changing vantage points.

In the context of firearms optics, correct stock fit serves to keep the shooter aligned correctly...in the driver's seat, so to speak. Unfortunately, stock fit alone will only minimize, but not eliminate, the effects of parallax.

Even the mere elasticity of the face will prevent perfect alignment every time, so we must be able to adjust the reticle<>target relationship to eliminate this effect.

The parallax turret serves to move the needle (reticle) closer to the gauge face (target) to reduce or eliminate parallax error. If they are on the same focal plane, there can be no error.

Parallax error itself is detrimental to precision. The blurry target image that is incidental to parallax error really isn't the issue at all. The issue occurs when you chase the displaced reticle image and move the gun away from the actual position on target. The target image must remain stationery if you wish to aim at the same place each time.


In optics, there is the term "focal plane", which is where the image is at a certain distance that is correctly focused. Then, as an accessory to this, "depth of field" defines the near<>far limits of acceptable focus of an object's image. Think of it as the "plus/minus" tolerance on either side of your focal plane.

Depth of field varies with magnification. As magnification increases, the depth of field is reduced and becomes shorter. This makes target focus, and if another object (reticle) is involved, parallax, a very critical and unforgiving adjustment.

At lower magnifications such as 2 or 3x, our depth of field will encompass average shooting distances without noticeable limitations. As magnification increases, depth of field is reduced and we will find some objects to be outside of our focal plane. This becomes a focus problem, and if another object (like a reticle) is involved, a parallax problem too.

Scopes of less than about 10x have a relatively long depth of field and if they are preset at the factory for 100-150 yards (most are), they will typically perform acceptably at normal shooting distances.

The parallax error in a fixed-objective scope is typically most apparent at closer distances, as pointed out in several posts above. Because this error (in a preset scope) is most prevalent up close, firearms intended for close range use will require special settings. This is what drives the manufacture of special ".22 scopes" and "muzzleloader scopes" and "shotgun scopes". Aside from some having BDC reticles for a given application, .22 scopes are often preset for 50 yards and the MZ/shotgun scopes for 75.

So, anyway, your rifle scope operates as two halves...two completely independent systems, that must interface correctly for proper performance.

The first half is closest to your eye. This first half is comprised of the ocular assembly and adjuster, the rear length of tubing, the turret housing & turrets, and most important, the reticle.

We adjust the first half of our scope to match the reticle focus to our individual eyesight. Normal vision usually puts this adjustment at mid-range, but this can vary with misc. scope design factors. In any case, the reticle lies at a specific physical and optical distance from the shooter's eye and is focused by moving the ocular adjustment. To make this adjustment, it is best to "turn off" the other half of the scope by placing a light colored obstruction out a few inches in front of the objective. This eliminates clutter and draws the eye to the reticle alone.

Once the reticle is focused correctly, this sets up our first focal plane and the reticle is presented to the eye with no eyestrain or distortion.

Then, we can address the second half of the scope. The second half begins on the other side of the turret housing, with a length of tubing that connects to the objective housing (if present - some are straight tubes). This section serves to bring the target image to the reticle. That is its entire reason for existence - bringing the target image to the same focal plane as the reticle. If two objects are on the same focal plane, there can be no parallax error.

What makes focus incidental to parallax is the state of your ocular adjustment for the reticle. If the reticle is not on the same focal plane as your eye, it will not be focused and will either appear blurry or your eye will bend over backwards to make the image work. If we present the target on the same focal plane as the mis-focused reticle, it too will be blurry, even if properly adjusted to be parallax free.

In a low powered, non-adjustable scope, this adjustment is made at the factory and the objective lens is locked in. A collimater is used to set the parallax so that the target image at the desired distance is erected on the same focal plane as the reticle.

In higher magnification scopes (over about 10x), our depth of field is so reduced, we cannot simply set a compromise distance and be able to function correctly at various ranges. For this reason, a user adjustable system is employed. Older scopes are "front focus", with a rotating ring out on the objective housing. Newer scopes are most often "side focus", with the parallax adjustment on the left side. Regardless of the control location, both systems allow the user to move the objective (front) lens in and out until the target image is correctly presented.

In lower magnification scopes, the depth of field is long and so the adjustment is less critical. A low magnification scope and/or a lack of clear understanding of the function/effects of parallax can lead to the assumption that there is a range limit at which parallax is no longer apparent.

As magnification increases and depth of field is reduced, this adjustment becomes very critical. It is quite easy to see parallax at 1,000 yards and further. If incorrectly adjusted, the reticle can seem to move 1 to 2 FEET across the target area. When the parallax adjustment is correctly made, the target and reticle are on the same focal plane and will appear to be as one.

Well, this is the way I like to think of it.

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Wow. Thank you sir. I appreciate your advice/knowledge

eventhorizon said:

Sorry this is off-topic, but the title of this thread would be a good name for the next Rush album if there is one

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I agree

Three Cheers for Ed. Thanks for taking the time to explain Parallax in a way that I can understand!

Ed, do you teach classes at Central Virginia Tactical?

I shoot Field Target Air Gun competitions where you use the Parallax to range find. You put a tape on your parallax wheel and then mark it with distances. You shoot at knock down targets with 1/4 inch size to 3 inch kill zones from 10 to 55 yards away. It is amazing how accurately you can range find using the Parallax at those distances. I range find at 12 power but some have 40 power scopes that have such a small depth of field they can range within inches out to at least 55 yards. I don't think this adds much to this discussion but I thought mention it.

Thanks again Ed.

justeric said:

...Ed, do you teach classes at Central Virginia Tactical?

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Yes, I do all the teaching. Vern has been retired for a few years now and only shoots occasionally these days.

I shoot Field Target Air Gun competitions where you use the Parallax to range find. You put a tape on your parallax wheel and then mark it with distances. You shoot at knock down targets with 1/4 inch size to 3 inch kill zones from 10 to 55 yards away. It is amazing how accurately you can range find using the Parallax at those distances. I range find at 12 power but some have 40 power scopes that have such a small depth of field they can range within inches out to at least 55 yards. I don't think this adds much to this discussion but I thought mention it....

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Actually a salient point. We used this technique to range groundhogs in the 70s and it was very often better than our SWAGs. As long as the scope tube is short (temperature expansion coefficient of aluminum tubing - heat moves the objective lens out) and the control is field calibrated the way you do it, it should be (and obviously is) a workable method. We failed to field calibrate ours back then because we believed in the factory marks, LOL.

As you point out, the reduction in depth of field can be quite sharp at higher magnifications. I occasionally run my Leupold Gen II Mildot equipped spotter at 40x, but between the narrowed field of view, exaggerated mirage/haze and the touchy parallax, I usually use it down near 25-30x. The parallax isn't critical in a spotter unless using it to range targets, and of course isn't even an issue at all if the spotting scope doesn't have reticle, but can be very distracting if you're used to having to tune it out.