Parallax explained
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Parallax is the relative movement you see between the cross hairs and the object you are aiming at when you try to line up the two.
Most hunting scopes are set by the factory to be parallax free at 100 yards (European 100m) but you'll find that this does change due to expansion and contraction of the scope due to temperature changes. Probably at 70f or so, you'll find parallax in most scopes to be 0 @ 100 yards but don't hold your breath.
The easiest way to see the effects of parallax is to have your gun secured on a table or bench in some sand bags. While looking through your scope, align the cross hairs on an object. Now bag the gun in so it can't move and try not to touch it when you start looking through the scope. Move your head back and fourth while looking through the scope. If you see the cross hairs shifting position or scanning in your view, you have parallax.
Now the effects of parallax can be dealt with to a degree by keeping your head in the exact same position each time you pull the trigger. If you shift your head (eye) to a new position, you bullet will strike in a different location.....
In walks...parallax adjustment. The idea being that "you" can move your lens (through an adjustable objective (A/O) or side focus (Sf)) yourself to reposition it and eliminate that relative movement when viewing. These scopes often come with a scale to get you in the ball park, but you will always need to do the manual check - gun bagged, and see if you have any relative movement between the reticle and object you are aiming at. Zero movement and you are parallax free. Any movement and parallax is present.
There are probably more accurate and shorter descriptions by the pros but this is the ball park story....
Most hunting scopes are set by the factory to be parallax free at 100 yards (European 100m) but you'll find that this does change due to expansion and contraction of the scope due to temperature changes. Probably at 70f or so, you'll find parallax in most scopes to be 0 @ 100 yards but don't hold your breath.
The easiest way to see the effects of parallax is to have your gun secured on a table or bench in some sand bags. While looking through your scope, align the cross hairs on an object. Now bag the gun in so it can't move and try not to touch it when you start looking through the scope. Move your head back and fourth while looking through the scope. If you see the cross hairs shifting position or scanning in your view, you have parallax.
Now the effects of parallax can be dealt with to a degree by keeping your head in the exact same position each time you pull the trigger. If you shift your head (eye) to a new position, you bullet will strike in a different location.....
In walks...parallax adjustment. The idea being that "you" can move your lens (through an adjustable objective (A/O) or side focus (Sf)) yourself to reposition it and eliminate that relative movement when viewing. These scopes often come with a scale to get you in the ball park, but you will always need to do the manual check - gun bagged, and see if you have any relative movement between the reticle and object you are aiming at. Zero movement and you are parallax free. Any movement and parallax is present.
There are probably more accurate and shorter descriptions by the pros but this is the ball park story....
I'm gonna try and give a functional example of the effects of parallax. This is way way easier for me to explain with a drawing, but I don't have an image host.
1) Point each of your index fingers up in the air in front of your face.
2) Put one finger about 12" from your face, and the other about 6" from your face. Close one eye. Make sure your fingers are in a straight line with respect to your line of sight. (IE your 6" finger is directly behind your 12" finger)
3) Move your head WITHOUT moving your hands one iota. You should see that your 6" finger no longer covers your 12" finger. This is because of the distance between them.
That's the main problem with your eye being off centre in a non-parallax adjusted scope.
To put my example into real world terms, your 12" finger might represent the focal point of the image coming into your scope, while your 6" finger would represent the reticle in your scope.
A parallax adjustment would make your fingers, or realistically the focal point and the reticle, be in exactly the same place, thus making side to side shift of your eye irrelevant. (Line your fingers up as before, but make them touch.)
It also happens to fix up the focusing issues inherent in a human's eye. (IE we can only focus on one thing at a time)
As I said, easier to explain with visual aids.
I'll draw it out if anyone wants it. *Edit: and is willing to host it
1) Point each of your index fingers up in the air in front of your face.
2) Put one finger about 12" from your face, and the other about 6" from your face. Close one eye. Make sure your fingers are in a straight line with respect to your line of sight. (IE your 6" finger is directly behind your 12" finger)
3) Move your head WITHOUT moving your hands one iota. You should see that your 6" finger no longer covers your 12" finger. This is because of the distance between them.
That's the main problem with your eye being off centre in a non-parallax adjusted scope.
To put my example into real world terms, your 12" finger might represent the focal point of the image coming into your scope, while your 6" finger would represent the reticle in your scope.
A parallax adjustment would make your fingers, or realistically the focal point and the reticle, be in exactly the same place, thus making side to side shift of your eye irrelevant. (Line your fingers up as before, but make them touch.)
It also happens to fix up the focusing issues inherent in a human's eye. (IE we can only focus on one thing at a time)
As I said, easier to explain with visual aids.
I'll draw it out if anyone wants it. *Edit: and is willing to host it
Last edited:
103159 explained it pretty well.
When the target and scope are in a fixed position, and you move your eye slightly, if there in any apparent movement of the crosshairs on the target, that movement is the parallax.
It is not evident at lower powers. It is more evident at very close ranges as most fixed power scopes have the parallax adjusted for 150 or 200 yards.
When the target and scope are in a fixed position, and you move your eye slightly, if there in any apparent movement of the crosshairs on the target, that movement is the parallax.
It is not evident at lower powers. It is more evident at very close ranges as most fixed power scopes have the parallax adjusted for 150 or 200 yards.
From John Williams II (USO, R.I.P.)
Whenever rifle scopes are discussed, a topic that frequently arises is parallax. There seems to be a great amount of misunderstanding and confusion concerning this subject. Parallax can be defined appropriately to rifle scopes as; the apparent movement of objects within the field of view in relation to the reticle.
In a telescopic sight, parallax occurs when the “primary image” of the object is formed either in front of, or behind the reticle. If the eye is moved from the optical axis of the scope, this also creates parallax. If the primary image is formed on the same focal plane as the reticle, or if the eye is positioned in the optical axis of the scope, then there is no parallax, regardless of the position of the primary image.
High magnification scopes, or scopes for long range shooting, where even slight sighting errors would be serious, should be equipped with a parallax adjustment. This adjustment of the objective part of the optical system would ensure that the target can be brought in the exact focal plane of the reticle at any distance. Tactical style scopes are not usually supplied with parallax adjustment because the exact range of the target can never be anticipated. Scopes of lower magnification are not usually supplied with parallax adjustment either, because at lower powers the amount of parallax is so small as to have no importance for practical, fast target acquisition.
THERE ARE TWO FACTORS WHICH CAUSE AND DETERMINE THE AMOUNT OF PARALLAX IN A RIFLESCOPE:They are;
1. The distance of the target to the objective-- The objective lens forms a primary image of the subject being viewed and subsequent components invert the image, and there is no parallax. The actual position at which the image is formed is dependent on the distance the target is from the objective. Closer targets are formed farther away from the objective and farther targets are formed closer to the objective. Since the reticle is in a fixed position within the scope housing, the image is not always formed in the same plane as the reticle and, hence parallax.
2.The distance the eye can move from the optical axis of the scope---, is determined by exit pupil size. There is no parallax, at any distance, as long as the eye is lined up exactly with the optical axis of the scope. An exit pupil small enough to do this would be impractical. is important to know that in every scope, there is some parallax. It is also important to know that in every scope, there is some one shooting distance in which there is no parallax. In most rifle scopes this one point of no parallax is usually placed at a suitable mid-range point in the scopes’ focal range.
In lower quality scopes, there are other sources of parallax. If the reticle is not precisely placed the correct distance from the objective, the distance of no parallax will be exaggerated. Reticles that are not securely mounted and allowed to move even a few thousandths of an inch, will always have changing amounts of parallax. Parallax is also caused by optical deficiencies in the objective, either by design or manufacture. If spherical or astigmatic aberrations have not been corrected, images will form a considerable distance from the reticle. If you see a scope in which the apparent movement of the reticle compared with the image viewed is different from when you move your eye up and down than when you mover your eye side to side, it is because of a bad objective. No adjustment of the scope will eliminate these faults or optical deficiencies.
You can check the parallax of any scope by sighting an object at normal shooting distance (not indoors), by moving your eye side to side (or up and down), as far as you can, keeping the sighted object within the field of view. The apparent movement of the reticle in relation the target is parallax.
Whenever rifle scopes are discussed, a topic that frequently arises is parallax. There seems to be a great amount of misunderstanding and confusion concerning this subject. Parallax can be defined appropriately to rifle scopes as; the apparent movement of objects within the field of view in relation to the reticle.
In a telescopic sight, parallax occurs when the “primary image” of the object is formed either in front of, or behind the reticle. If the eye is moved from the optical axis of the scope, this also creates parallax. If the primary image is formed on the same focal plane as the reticle, or if the eye is positioned in the optical axis of the scope, then there is no parallax, regardless of the position of the primary image.
High magnification scopes, or scopes for long range shooting, where even slight sighting errors would be serious, should be equipped with a parallax adjustment. This adjustment of the objective part of the optical system would ensure that the target can be brought in the exact focal plane of the reticle at any distance. Tactical style scopes are not usually supplied with parallax adjustment because the exact range of the target can never be anticipated. Scopes of lower magnification are not usually supplied with parallax adjustment either, because at lower powers the amount of parallax is so small as to have no importance for practical, fast target acquisition.
1. The distance of the target to the objective-- The objective lens forms a primary image of the subject being viewed and subsequent components invert the image, and there is no parallax. The actual position at which the image is formed is dependent on the distance the target is from the objective. Closer targets are formed farther away from the objective and farther targets are formed closer to the objective. Since the reticle is in a fixed position within the scope housing, the image is not always formed in the same plane as the reticle and, hence parallax.
2.The distance the eye can move from the optical axis of the scope---, is determined by exit pupil size. There is no parallax, at any distance, as long as the eye is lined up exactly with the optical axis of the scope. An exit pupil small enough to do this would be impractical. is important to know that in every scope, there is some parallax. It is also important to know that in every scope, there is some one shooting distance in which there is no parallax. In most rifle scopes this one point of no parallax is usually placed at a suitable mid-range point in the scopes’ focal range.
In lower quality scopes, there are other sources of parallax. If the reticle is not precisely placed the correct distance from the objective, the distance of no parallax will be exaggerated. Reticles that are not securely mounted and allowed to move even a few thousandths of an inch, will always have changing amounts of parallax. Parallax is also caused by optical deficiencies in the objective, either by design or manufacture. If spherical or astigmatic aberrations have not been corrected, images will form a considerable distance from the reticle. If you see a scope in which the apparent movement of the reticle compared with the image viewed is different from when you move your eye up and down than when you mover your eye side to side, it is because of a bad objective. No adjustment of the scope will eliminate these faults or optical deficiencies.
You can check the parallax of any scope by sighting an object at normal shooting distance (not indoors), by moving your eye side to side (or up and down), as far as you can, keeping the sighted object within the field of view. The apparent movement of the reticle in relation the target is parallax.
