Optical Imperfections Of The Human

As a result of various optical imperfections of the eye, the images of objects on the retina have edges which are not completely sharp, or are to a degree totally fuzzy. As a consequence, there exists a certain limit of varying sensitivity of our eye which determines the sharpness of vision. It must be said that sharpness of vision, in and of itself, is inconstant. It has a certain variable value which depends upon the degree and the conditions under which, the optical imperfections of the eye have a noticeable effect. Therefore, the shooter must know, at least in overall features, the conditions which influence the sharpness of vision and thus the degree of accuracy of aiming.

As an optical instrument, the eye has inherent in it, the phenomena of aberration and diffraction of light.

1. Spherical aberration is a function of the eye in which rays of light falling upon the crystalline lens are refracted differently and are not focused at a single point. The extreme outer rays are refracted more strongly than the central ones (Figure A2-2). As a result of spherical aberration, a beam of parallel rays entering the eye is focused on the retina not in the form of a sharp image, but in the form of a circle of light diffusion. The size of the circle of light diffusion resulting from spherical aberration is in direct proportion to the size of the pupillary opening. It is obvious that the sharpness of the image is increased if one eliminates extreme rays. Consequently, as the pupillary opening contracts, the sharpness of the image of the object upon the retina increases.

The degree to which spherical aberration can hinder the seeing of objects sharply, and to which the sharpness of the image depends upon the size of the pupillary opening, can be convincingly shown to the shooter by means of a simple example. Small orienting marks and objects which can be distinguished only with difficulty during overcast weather become incomparably more discernible if one looks at them through a small peep hole which, in this instance, fulfills the role of an artificial pupil.

Spherical Aberration The Human Eye

Figure A2-2. Phenomenon of Spherical Aberration.

2. The phenomenon of light diffraction lies in the fact that light rays passing through small openings, particularly through the crystalline lens; seem to bend (figure A2-3) and produce on the retina an image not in the form of a single sharp point, but in the form of a circle surrounded by a number of concentric light rings of decreasing sharpness. This occurs as a result of the wave nature of light.

a. As the pupillary opening decreases, the diameter of the diffraction ring of light diffusion increases. The diffraction rings around the images have a noticeably telling effect only when there are extremely small dimensions of the pupil, and this, as we can see, is a certain opposite of the phenomenon of spherical aberration. The phenomenon of diffraction makes itself felt when there is solar illumination from the front and the sun in shining into the eyes; when there are bright patches of sunlight on the horizontal surfaces of the front and rear sights, etc.

Figure A2-3. Phenomenon of Diffraction of Light on the Pupil.

b. The operation of the eye as an optical apparatus is also harmed to a certain degree by the light diffusion occurring with in it. It is especially discernible when one views brightly illuminated objects located against a dark background. The effect of light diffusion in the form of a more or less noticeable radiation, covering the field of vision, is caused by media which do not possess absolute transparency -- the crystalline lens and the vitreous humor. The light diffusion in the optical media is responsible for the halos of light. They are especially noticeable where the targets are strongly illuminated by sunlight. In such an instance, the white background of the target casts a sharp reflection and causes a considerable light diffusion in the optical media. This causes a blinding effect. Both the bull's-eye, perceived by the eye in the form of a gray spot with indistinct edges, and the front and rear sights are perceived with unclear outlines.

c. It is obvious from what has been said that the amount of light diffusion from spherical aberration is in direct proportion to the size of the opening of the pupil, and the amount of light diffusion from diffraction is in inverse proportion to the size of the opening of the pupil, and thus it is not possible to eliminate these types of diffusion completely. As a result of this inverse dependence of the effects of aberration and diffraction upon the size of the pupil, the best conditions of sharp vision correspond to a certain average size of the pupillary opening -- a diameter of approximately 3 mm.

Taking this into consideration, depending upon the conditions of illumination which influence the size of the pupillary opening, the shooter must strive, insofar as he can, to create the most favorable conditions for the operation of the eye. He must protect his eyes from the action of light by using visors, filter type shooting glasses, or by possibly using an artificial pupil. An eye disc device is attached to the shooting glasses with adjustment for varying diameters. He must also make sure that the sights do not shine and thus produce a blinding effect upon the eye: They must be carefully and evenly blackened.

d. Brilliant sources of light harm the eye chiefly by means of the violet sector of the visible and invisible portions of the spectrum. The complete elimination of the violet sector of the spectrum is achieved by yellow, yellow-green, and yellow-orange light filters. Such light filters not only do not reduce the acuity of visibility, but, on the contrary, increase it. Type of darker glass protecting the eyes from brilliant sources of light somewhat reduce the acuity of vision. However, by having an assortment of shooting glasses of varying shades, it is possible to select and use them in such a way that the shooter's eye perceives the correct sight alignment under the brightest illumination in almost the same way that he does during overcast weather.

3. Near sightedness, farsightedness and astigmatism: The optical imperfections of the eye also include nearsightedness, farsightedness, and astigmatism, the existence of which also hinders the correct focusing of the optical system of the eye and the obtaining of sharp images of objects upon the retina.

a. If the eye is constructed in such a way that rays of light entering it in a parallel beam are focused exactly on the retina without any effort at accommodation, we say that the eye is a normal one. (Figure A2-4(a)).

b. The eye is nearsighted if the rays entering it in a parallel beam are focused in front of the retina (Figure A2-4(b)). Nearsightedness is caused either by the fact that the eyeball is excessively long from front to back or by the fact that the eye has great refracting force, or by a combination of both factors. Nearsightedness can be corrected comparatively easily by means of eye glasses. Many pistol shooters suffer from nearsightedness, but this defect of vision, after being corrected by the proper choice of eyeglasses; does not prevent them from achieving record-making competitive results.

Figures A2-4. Schemes Showing the Refraction Rays in the Eyes: {a) Normal, (b) Nearsighted (c)


c. The eye is farsighted if the rays entering it are focused in back of the retina (Figure A2-4(c)). This can occur either as a result of the eye having a weak refracting force, or that the eye is too short from front to back or by a combination of both factors. In such an instance, in order for the rays to focus upon the retina, they must enter the eye in a converging beam, and therefore a farsighted eye sees near objects worst of all. This type of eye is harder to correct but eyeglasses help it overcome the difficulty. Shooter's suffering from farsightedness will see the sights very poorly. The characteristic complaint of farsighted persons is that the rear notch seems to fuse with the front sight.

d. An indistinct, hazy image of objects on the retina can also result from astigmatism of the eye.

(1) An eye in which the refracting surfaces of the cornea and crystalline lens do not have a perfectly spherical form is called astigmatic. When an eye is astigmatic, parallel rays entering the eye cannot produce an exact focused image on the retina. The refraction of the light rays at various points in the eyeball occur at different angles. As a result, the eye possesses not a single principal focus, but several foci, which are located at various distances from the retina; that is why the image on it is indistinct and uneven (Figure A2-5). It must be said that certain strange phenomena frequently observed under practical conditions of marksmanship (for example, when two shooters are firing the same pistol, with an identical sight setting, and the centers of impact differ sharply from one another) are, in all probability, connected with astigmatism of the eye.

Figure A2-5. Scheme showing the Refraction of Rays in an Astigmatic Eye.

(2) One can easily be convinced of the existence or absence of astigmatism by making use of a circle chart (Figure A2-6). For this purpose it is necessary to look with one eye from the distance of best vision (approximately 10 feet) at a disk on which concentric circles are drawn. If the person has astigmatism, only certain areas of the disk will be clearly visible and the remaining areas will seem hazy.

Astigmatism Chart Filetype Png

Figure A2-6. Chart for Discovering Astigmatism.

4. Correction of Defects: If even insignificant defects in vision are discovered, it is necessary to wear corrective eyeglasses when firing, since the excessive accommodation of the eye resulting from aiming will greatly fatigue vision and this can lead to a still greater decrease in its accuracy. It must also be kept in mind that when eyeglasses for firing are chosen by the ordinary method, that is, in an optometrist's office, the choice is not completely satisfactory for the shooter. It is very desirable when selecting the lenses to check them immediately on the pistol range, to make sure that one can see well the sight alignment located at arm's length distance away from the eye. This selection is linked not so much with the determination of the lens dioptrics, as with the determination of the quality of their grinding, since all defects will make themselves known quickly during such a check.

When wearing shooting glasses (including those with filter lenses) it is necessary to make sure that the line of sight runs perpendicular to the surface of the lens and through its center, since the central portion of the lens is usually ground considerably better and therefore has less distortion of the vision. To hold the lenses perpendicular to the line of sight requires no change in the ordinary level placement of the head when assuming the firing position.

It is necessary to dwell on one more peculiarity of our eyes which has tremendous importance in aiming -the existence of monocular and binocular vision.

1. Vision with one eye is called monocular and vision with two is called binocular. The fact that a person has two eyes does not always mean that he also has binocular vision.

There are instances when the eye which has the poorest vision is not included in the act of vision and the person actually uses just one eye, the better one. The dominance of one eye over the other also occurs, even when both eyes possess identical sharpness of vision. The eye that the person prefers to use is called the dominant, or directing eye. There exists a very simple method by which one can determine which of the eyes is the stronger.

2. In order to determine which is the dominant eye the shooter must hold his hand out slightly making a ring out of the fingers and thumb, and look through it in such a way that some small object can be seen by both eyes (Figure A2-7). Then, by alternately closing one eye then the other, it is necessary to see whether the object stays within the ring or leaves it. The dominant eye is the one with which the shooter sees the object as un-shifted, remaining in the ring. In most people the dominant eye is the right one.

3. The protracted work of one eye (for example, by laboratory assistants, microscopists) contributes to the fact that the eye used becomes the dominant one. This naturally pertains also to shooters, who, when aiming, use one eye. The right eye is, in the overwhelming majority of cases, their dominant one.

4. In the past the shooter has usually been instructed to squint his left eye and aim the pistol at the target with his right eye. During subsequent instruction it is no longer necessary to continue this device of closing the eye, since it has its major shortcomings, which are attested to by numerous instance of medical research.

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  • belladonna
    How to test astigmatism?
    9 months ago

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