Defects of Vision

4 Key excerpts on "Defects of Vision"

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  eBook - ePub

  Physics for O.N.C. Courses

  • R.A. Edwards(Author)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 22

  The Eye. Defects of Vision and Optical Instruments

  Publisher Summary

  This chapter focuses on various experimental results related to Defects of Vision and optical instruments. Image formation, in a good eye, is achieved and perfected by the action of the eye as a whole, namely, cornea, lens, aqueous, and vitreous humors. The angle subtended at the eye lens by the object is the same as that which is subtended by the image at the lens. If the eyeball is of such a length from lens to retina that parallel light from infinity is focused in front of the retina when the ciliary muscles are completely relaxed, short sight, or myopia results. Long sight or hypermetropia is a condition in which the effective power of the eye lens is too small in relation to the length so that when there is no accommodation, parallel light from infinity is focused behind the retina. Presbyopia is the loss of power of accommodation, which is normally associated with the advancing age of an individual. A photographic enlarger is a slightly less elaborate device to the projector. The photographic negative is used in place of a slide and the light-sensitive paper on which the print is to be made takes the place of the screen.

  22.1 Structure of the Eye

  The human eye (Fig. 22.1 ) contains a converging lens L of a gelatinous, transparent material which is not of uniform refractive index throughout and the surfaces of which have different curvatures. The power of the lens is controlled by the ciliary muscles M which act in order to increase the curvature of the lens surfaces. When these muscles are fully relaxed the eye is said to be unaccommodated and it is adjusted for viewing objects at a great distance. When the muscles are fully tensed the eye is fully accommodated for close vision. The point closest to the eye at which an object may be focused clearly when the eye is fully accommodated is called the near point. The distance from the eye of the near point varies considerably from one person to another but is often regarded as being at about 25 cm. The far point is at the greatest distance for which vision is clear when the ciliary muscles are completely relaxed. Ideally this point should be at infinity. The hard, opaque coating of the eyeball, called the sclerotic S , becomes transparent at the front of the eye in order that light may enter. This transparent “window” is called the cornea C . In fact most of the refraction of the light occurs at the cornea, the lens being employed largely for accommodation. Behind the cornea and in front of the lens is the aperture through which light is permitted to enter the lens. This is the pupil P , the size of which is adjusted by a diaphragm called the iris I , which is the familiar coloured part of the eye. The pupil has its largest diameter when illumination is poor, and vice versa. The space A between the lens and the cornea is filled with a salt solution called the aqueous humour . The front of the cornea is also moistened with salt solution and the action of blinking with the eyelids keeps the surface of the cornea clean. The inner wall of the sclerotic at the back of the eye forms the light-sensitive surface R on which the light entering the eye is focused to form images. This surface is called the retina and is covered with light-sensitive cells situated at the ends of nerve fibres which form a network over the surface of the retina and which all leave the the eyeball in a bundle known as the optic nerve O . This leads to the brain which interprets the image on the retina resulting in “sight” or vision. The point at which the optic nerve leaves the eyeball is insensitive to light and is called the blind spot B . In contrast to this is the fovea , or yellow spot Y , at which the retina is most sensitive. This point lies at the intersection of the principal axis of the lens and the retina. The space V between the retina and the lens is filled with a jelly-like substance called the vitreous humour

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  eBook - ePub

  Improve Your Eyesight

  A Guide to the Bates Method for Better Eyesight without Glasses

  • Jonathan Barnes(Author)
  • 2011(Publication Date)
  • Souvenir Press

    (Publisher)

  Refraction in the eye is carried out not only by the lens, but also, and more importantly, by the cornea, which is strongly curved in cross-section. The lens and cornea work together as a kind of “lens system”.

  The eyeball is only about 2.5 centimetres (1 inch) in diameter, and if focusing is to be precise this lens system must be free from flaws and very accurately positioned. Should the retina be only fractionally too far away, the focal point of rays from distant objects will fall short, so giving a blurred image. Conversely, if the retina is too close to the lens system (that is, if the eyeball is too short from front to rear), then the focal point of rays from nearby objects will fall, in theory at least, behind the retina, again producing a blurred image.

  The refractive errors resulting from these two conditions are called myopia (short-sightedness) and hypermetropia (long-sightedness ) respectively.

  A third type of error, astigmatism, arises when there is a flaw in the shape of the cornea or, more rarely, in the shape of the lens. Unless the cornea is perfectly symmetrical, it will be unequally refractive and rays in differing planes will be brought to differing focal points, producing an image that will be only partly in focus, if at all (Figure 9).

  Myopia, hypermetropia, and astigmatism can thus all be classed as refractive errors caused by malformation of the eyeball. The fourth common kind of refractive error, presbyopia (also called “old-age” sight and, confusingly, “far-sightedness”), is brought about because over the years the lens slowly loses its elasticity and hence its power to change shape during accommodation. In most people this process begins quite early in adulthood and is completed by the age of about 55 or 60, by which time all flexibility in the lens is lost.

  Figure 9: The principle of astigmatism

  It is unusual for only one of the four common types of refractive error to be present in any given case. Most myopes, for example, have some degree of astigmatism also, and as a myope or a hypermetrope ages his condition is likely to be complicated by advancing presbyopia.

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  eBook - ePub

  Ophthalmology Secrets E-Book

  • Janice Gault(Author)
  • 2022(Publication Date)
  • Elsevier

    (Publisher)

  Fig. 3.3 A, The secondary focal point of a myopic eye is anterior to the retina in the vitreous. B, In hyperopia, the secondary focal point is behind the retina.

  Source: (From Azar DT, Strauss L: Principles of applied clinical optics. In Albert DM, Jakobiec FA, editors: Principles and practice of ophthalmology, vol 6, ed 2, Philadelphia, 2000, W.B. Saunders, pp 5329–5340.)

  4. What is the far point of an eye?

  The term far point is used only for the optical system of an eye. It is the point at which an object must be placed along the optical axis for the light rays to be focused on the retina when the eye is not accommodating.

  5. Where is the far point for a myopic eye? A hyperopic eye? An emmetropic eye?

  The far point for a myopic eye is between the cornea and infinity. A hyperopic eye has its far point beyond infinity or behind the eye. An emmetropic eye has light rays focused on the retina when the object is at infinity.

  6. How do you determine which lens will correct the refractive error of the eye?

  A lens with its focal point coincident with the far point of the eye allows the light rays from infinity to be focused on the retina. The image at the far point of the eye now becomes the object for the eye.

  7. What is the near point of an eye?

  The near point is the point at which an object will be in focus on the retina when the eye is fully accommodating. Moving the object closer will cause it to blur.

  8. Myopia can be caused in two ways. What are they?

  • • Refractive myopia is caused by too much refractive power owing to steep corneal curvature or high lens power.
  • • Axial myopia is due to an elongated globe. Every millimeter of axial elongation causes about 3 D of myopia.

  9. The power of a proper corrective lens is altered by switching from a contact lens to a spectacle lens or vice versa. Why?

  Moving a minus lens closer to the eye increases effective minus power. Thus, myopes have a weaker minus prescription in their contact lenses than in their glasses. Patients near presbyopia may need reading glasses when using their contacts but can read without a bifocal lens in their glasses (see question 45). Moving a plus lens closer to the eye decreases effective plus power. Thus, hyperopes need a stronger plus prescription for their contact lenses than for their glasses. They may defer bifocals for a while. The same principle applies to patients who slide their glasses down their nose and find that they can read more easily. They are adding plus power. This principle works for both hyperopes and myopes.

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  eBook - ePub

  Handbook of Visual Optics, Two-Volume Set

  • Pablo Artal, Pablo Artal(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  21  Peripheral aberrations Linda Lundström and Robert Rosén

  Contents

  21.1 Introduction 21.2 Peripheral optical errors 21.2.1 Oblique astigmatism 21.2.2 Defocus due to field curvature 21.2.3 Coma 21.2.4 Transverse chromatic aberration 21.3 Measuring peripheral optics 21.3.1 Traditional techniques: Subjective refraction and retinoscopy 21.3.2 Foveal refractometers: Additional requirements for peripheral measurements 21.3.3 Lab-based systems 1: The double-pass technique 21.3.4 Lab-based systems 2: Wavefront sensing 21.3.5 Future systems for complete image quality evaluation 21.4 Population data on peripheral optical errors 21.4.1 Refractive errors over the peripheral field and for different types of ametropia 21.4.2 Wavefront aberrations over the peripheral visual field 21.4.3 Image quality over the peripheral visual field 21.4.4 Peripheral variations with age and accommodation 21.5 Effect of peripheral optical errors on vision 21.5.1 Optical effects on peripheral vision 21.5.2 Application 1: Central visual field loss 21.5.3 Application 2: Myopia development References

  21.1 INTRODUCTION

  Our peripheral vision is essential for many daily tasks, such as locomotion and detection. However, the human eye is optimized for central vision, both neurally and optically, and similar to man-made optics, the optical errors of the eye generally increase with incident angle. Peripheral visual function therefore depends on a combination of optical and neural factors. The peripheral visual field is defined as the area in object space that is not imaged to the macula lutea of the eye, that is, typically angles equal to or larger than 7°–10° off the visual axis. The monocular visual field stretches out to around 90° temporally, 60° nasally, 70° inferiorly, and 50° superiorly (Millodot 1997). This chapter starts by presenting the different types of optical errors that are often found in the periphery. An overview will then be given on how the optics can be measured and described as well as population data. The last part of this chapter discusses how these off-axis errors can affect our visual performance.

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