Parallax Suppression with a Target Rifle Aperture Sight

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Parallax Suppression with a Target Rifle Aperture Sight Page 2. Figure 1 Typical aperture sight, The shooter looks through the aperture in the rear sight with the eye. perhaps 40 mm behind the sight observes the front sight and aims. the rifle until the top of the post is located on the desired location on. the target 1,Averting parallax shift the conventional wisdom. Although the rear sight aperture is quite small in diameter because the. eye position is fairly close to it the field of view seen through the. aperture is fairly wide Differences in eye position behind the aperture. will result in the tip of the front sight being seen at different locations. within that field Conventional wisdom holds that this leads to. different points of aim This is attributed to the well known. phenomenon of parallax shift2, Users of these sights are often urged to strive for an eye position that. puts the apparent location of the top of the front sight in the center of. the field of view seen through the rear sight This is described as. maintaining proper sight alignment, In a typical target shooting setting the target is a dark circular disk whose. apparent size is very nearly the same as the apparent width of the front sight post. and the rifle is sighted in so that the appropriate aiming situation is with the target. apparently sitting just on the top of the front sight post. Parallax shift in this context can be thought of as a change in the apparent. alignment of two objects at different distances from the observer as the point from. which the scene is viewed is changed, Parallax Suppression with a Target Rifle Aperture Sight Page 3.
The width of the front sight typically subtends an angle similar in size. to the target on the order of 6 minutes of arc MOA By contrast the. field of view through the rear aperture for the case of a 1 mm. aperture and 30 mm of eye relief3 is approximately 1 9 degrees or. 115 MOA Thus the width of the target and front sight only cover. about 5 of the width of the field of view This can make precisely. centering the front sight within the aperture seem like a daunting task. prone to error, A common admonition to novices using aperture sights concerned. with the need to make this alignment is that the eye will naturally. center the post in the aperture Still much effort is usually spent. insuring proper centering of the front sight, This figure illustrates the recommended sight alignment centering of. the front sight post tip in the field of view of the rear sight and sight. picture target atop the front sight and centered with it. Figure 2 Recommended sighting situation,Parallax shift the reality. In actual practice we find that if we look through an aperture sight. with the aim of the rifle fixed perhaps it is held in a sighting vise and. move our eye from side to side we see almost no change in the. relative locations of the tip of the front sight and the target what. shooters call the sight picture Essentially the phenomenon of. parallax shift does not appear, On the other hand if as a check on our observations we raise our. eye so that we look completely over the rear sight and adjust the. pointing of the rifle so that again the tip of the front sight aligns with. the target we find that when we move our eye from side to side by. the same amount we did before we do see a definite shift in the. relative locations of the tip of the front sight and the target This is in. fact parallax shift at work,The distance the eye is behind the rear sight.
Parallax Suppression with a Target Rifle Aperture Sight Page 4. What is happening here Does the simple rear sight have some power. to suppress the effect of parallax shift Well yes We will shortly find. out how this works,OPTICAL BACKGROUND, First a little background on some optical principles that are involved. The entrance pupil,In photographic optics, In photographic optics we draw heavily on the concept of the. entrance pupil of a lens This is defined as the virtual image from in. front of the lens of the aperture stop The aperture stop is an opaque. plate with an opening which serves to throttle the light entering the. lens as a component of obtaining proper exposure Typically the size. of this opening can be varied today usually with an iris. arrangement In all but primitive box cameras the aperture stop is. located inside the lens between certain of its elements. If we look into a typical camera lens from the front we can usually. see the iris But of course we don t really see it What we see is its. virtual image created by the working of the lens elements in front of. the stop What we see may not be the same size nor in the same. location along the lens axis as the physical aperture stop But it is by. definition the entrance pupil, The entrance pupil has two significances to us First it is the port. through which the lens collects light and its size thus influences. exposure Secondly it is the peephole through which the camera. sees the world and its location thus determines the camera s point of. perspective as we will discuss shortly,In the human eye. The concept of the entrance pupil is directly applicable to the human. eye Again we have here an iris mechanism from which the ones. used in photography take their name and its virtual image from the. front of the eye as it is seen through the eye s lens is the eye s. entrance pupil, The human visual system automatically varies the size of the opening.
in the iris and thus the size of the entrance pupil in order to. accommodate the wide range of scene luminance brightness in. which the eye must function rather like automatic exposure control in. a camera Typically for the brightest scene environment the diameter. of the entrance pupil is about 2 mm and for the darkest scene. perhaps complete darkness the diameter is about 8 mm. Parallax Suppression with a Target Rifle Aperture Sight Page 5. The point of perspective, When we deal with parallax matters we are concerned with the. vantage point from which our camera or a human eye or a human. eye plus a rifle sight views the scene This is called the point of. perspective of the optical system and in fact is important not only to. matters of parallax but also of perspective itself which we won t be. dealing with here, If we consider direct human vision one eyed in a large view of life it. is not hard to decide where the point of perspective is It is where. the eye is, But as we consider such things as small eye movements and the. precise alignment of small objects we have to get more specific. Then we normally treat the center of perspective as being at the. center of the eye s entrance pupil, This at first seems a little over simplistic After all the eye doesn t. just regard the world through the center of the entrance pupil but. rather through all of it So shouldn t we consider that the eye has. many vantage points one for every imaginable location within the. entrance pupil Absolutely But how can we deal with this in optical. calculations and the like, To help us grasp this dilemma let s do a little thought experiment on.
parallax Imagine that in front of our eye at a distance of say 10. inches we have a vertical pencil Behind that at a distance of say 20. inches we have a vertical fat stick of chalk A plane passing through. the axis of these two cylinders also passes through the center of the. entrance pupil of our eye Thus we can say simplistically that the. two cylinders are aligned from a vantage point at the center of the. entrance pupil, But recalling that the entrance pupil has a substantial size doesn t the. eye really see multiple images from different vantage points all across. the entrance pupil one image in which the two cylinders are aligned. and many other images in which they are misaligned by differing. amounts in various directions, Now suppose that the eye is precisely focused on the pencil For our. present purpose we can look at this as meaning that multiple images. of the pencil which we can think of as being observed from multiple. vantage points across the entrance pupil all coincide on the eye s. retina and thus form a unitary image of the pencil which we consider. to be sharply focused, Parallax Suppression with a Target Rifle Aperture Sight Page 6. Now what about the chalk The eye is not precisely focused on it. That means that the multiple images of the chalk which again we. can think of as being observed from multiple vantage points across the. entrance pupil do not coincide on the eye s retina Thus they. collectively form a spread out or blurred image of the chalk. But we still must ask are the images of the pencil and the chalk. aligned Suppose that the sharp image of the pencil seems centered. between the blurred left and right outlines of the blurred image of. the chalk Thus we conclude yes the pencil and chalk are aligned. That finding is of course consistent with the concept that the point of. perspective must be considered to be at the center of the entrance. BACK TO OUR APERTURE SIGHT, The rear sight of the aperture sight and the shooter s eye collectively. form an optical system What is its entrance pupil Well there are two. candidates One is the entrance pupil of the eye and indeed no light. can enter the eye except through it and no part of the scene not. visible through it can be observed The second entrance pupil is the. aperture of the rear sight Again no light at least none relevant to. looking at the front sight can enter the eye except through it and no. part of the scene not visible through it can be observed In fact it is. the common area of the two individual entrance pupils their geometric. intersection that becomes the entrance pupil of our optical system. Assume for the moment that the sight aperture is smaller than the. eye s entrance pupil and is located wholly within it but not. necessarily centered within it Thus the intersection of the two pupils. is the sight aperture alone and it is our entrance pupil. Where is the point of perspective of this optical system the location. from which it seems to view the scene At the center of the entrance. pupil which we have just concluded is the sight aperture. Now if we move our eye from side to side a little bit our first thought. is that the point of perspective would change But since the point of. perspective is at the center of the sight aperture and that is fixed the. point of perspective does not move, And this is why there is essentially no manifestation of parallax shift.
with modest amounts of eye movement in an aperture sight. One of the authors Burdge who first noticed this phenomenon has. dubbed this the parallax suppression afforded by an aperture sight. Incidentally this parallax suppression is similar in nature to that. provided by the more complex AimPoint sight used by the military. Parallax Suppression with a Target Rifle Aperture Sight Page 7. without the added benefit of the single focal plane that sight. provides of course With the AimPoint sight the location of the. aiming dot within the sight does not affect the Point of Impact and. the sight is considered parallax free But we see now that we enjoy. this same benefit in the basic aperture sight,EXTENDED EYE MOVEMENT. If we move the eye more than a certain amount the edge of its. entrance pupil begins to encroach on the sight aperture We now have. a new entrance pupil the geometric intersection of our two individual. pupils no longer circular and with its center no longer at the center of. the sight aperture, As this happens the point of perspective does move and parallax. shift begins to appear However the total movement of the center of. perspective is not as great as the total movement of the eye Thus. although we no longer have full parallax suppression we have parallax. EFFECT OF ENVIRONMENTAL BRIGHTNESS, The brightness luminance of the environment of course affects the. diameter of the eye s entrance pupil Thus in a darker environment the. amount of eye shift permissible before parallax suppression degrades. into parallax reduction is greater than in a brighter environment. Typically the rear of the rear sight has a dull black finish said to. reduce glare and to enhance the contrast with which the front sight. and the target are seen But fortunately this also reduces the average. brightness seen by the eye and causes the eye s pupil to enlarge This. of course enlarges the range of eye movement for which we have full. parallax suppression, In some precision sights this effect is enhanced by way of an eye. cup surrounding the aperture disk proper,DECREASE IN TARGET BLURRING.
In a camera if we decrease the size of the aperture we increase the. depth of field the range of object distances surrounding the. distance for which perfect focus is attained for which the object will. be imaged with what we choose to consider negligible blurring. Looking at this from the other direction if we decrease the size of the. aperture we decrease the amount of blurring for an object at some. particular distance that is not the distance of perfect focus. Similarly when we use an aperture sight and the sight aperture takes. over the role of entrance pupil from the entrance pupil of the eye. Parallax Suppression with a Target Rifle Aperture Sight Page 8. reducing the size of the entrance pupil we enjoy reduced blurring of. Parallax Suppression with a Target Rifle Aperture Sight Robert J Burdge and Douglas A Kerr P E Issue 4 May 30 2007 ABSTRACT With an aperture sight often used on target rifles the shooter looks through a small hole in a metal plate that is mounted on the rear of the rifle near the shooter s eye observes a front sight which is typically a small vertical post located near the front of

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