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Visual Perception
Published in Robert W. Proctor, Van Zandt Trisha, Human Factors in Simple and Complex Systems, 2018
Robert W. Proctor, Van Zandt Trisha
For nearsightedness, or myopia, the eye is too long, resulting in a focal point that is in front of the receptors when the lens is relaxed. For farsightedness, or hyperopia, the eye is too short, resulting in a focal point that is behind the receptors when the lens is fully flexed. As people become older, the speed and extent of their accommodation decrease continually. With age, the lens becomes harder and less responsive to the pulls of the muscles in the eye, so accommodative ability decreases and essentially all people become hyperopic. This condition is called presbyopia, or old-sightedness. The near point can increase from as close as 10 cm for 20-year-olds to as far as 100 cm by age 60. Presbyopia can be corrected with reading glasses or bifoc als, which typically are not prescribed until age 45 years or older. A person can have perfect vision in all other respects but still need reading glasses to compensate for the decreased accommodative ability of the lens.
The Visual Environment: Measurement and Design
Published in R. S. Bridger, Introduction to Human Factors and Ergonomics, 2017
The near point of vision is the closest distance at which an object can be brought into sharp focus. A 16-year-old can focus on an object less than 10 cm in front of the eye. However, the lens loses elasticity with age and, in practice, this results in a reduction in refractive power. By the age of 60 years, the near point may have receded to 100 cm. This is why older people often need reading glasses or, for example, have to hold the paper at arms length, when reading a newspaper. By the age of about 50 years, the lens has only about 2 D of accommodation left. After this, it can be regarded as completely non-accommodating—a condition known as presbyopia. The result of presbyopia is that the eye becomes focused at a fixed distance, which varies between different people depending on the characteristics and condition of their eyes. Frequently, the fixed viewing distance in the presbyopic eye is intermediate between the previous near and far points and the person has to wear bifocal lenses; the upper part is set for distant vision and the lower part for near vision (mainly for reading). Bifocal or trifocal lenses can restore a kind of stepwise accommodation to the presbyopic eye. In practice, if workplaces are adequately lit, the depth of field of the eye is increased and the net effect is to lower the requirements for accommodation. This explains why good lighting is important in all facilities used by older people. A more detailed discussion of accommodation may be found in Miller (1990).
Corneal onlays and inlays
Published in Pablo Artal, Handbook of Visual Optics, 2017
Presbyopia is a condition of the eye associated with aging where the accommodative mechanism of the eye, consisting of the crystalline lens and ciliary muscle, is no longer able to increase the power of the eye to view near objects. Patients who do not require glasses to see distance (emmetropes) usually just use reading glasses to correct presbyopia. Patients who are myopic, hyperopic, or astigmatic (ametropes) generally use glasses to correct their distance vision with an addition (bifocals) or multifocal contact lenses in order to see clearly at near. Monovision is an approach where one eye is corrected for distance and the other eye is corrected for near. Monovision can be achieved with contact lenses, corneal refractive surgery (PRK or LASIK), or IOLs. A multifocal IOL is another option for the correction of presbyopia, either to replace a cataractous lens or as a refractive (clear lens) exchange procedure.
Effect of unifocal versus multifocal lenses on cervical spine posture in patients with presbyopia
Published in International Journal of Occupational Safety and Ergonomics, 2019
Rami L. Abbas, Mohamad T. Houri, Mohammad M. Rayyan, Hamada Ahmad Hamada, Ibtissam M. Saab
Presbyopia is one of the many conditions that may contribute to bad posture through habits. Presbyopia is an age-related, visual impairment condition, caused by the hardening and inelasticity of the lens of the eye. This inelasticity prevents the lens from being able to accommodate and refract light that focuses it on the retina. Ultimately, this condition causes blurry vision when looking at an up-close image [3]. Many postural problems are associated with presbyopia. Additionally, the increase in the use of computers and visual display units has increased visits of patients to various health care providers, particularly those with computer vision syndrome. Computer vision syndrome is a group of problems associated with eyes and vision that results from prolonged computer and cell-phone usage [4]. The prevalence of musculoskeletal impact on the cervical spine associated with the use of visual display units has been discussed recently, with the focus being on resolving the body's adoption of bad postural habits and poor ergonomics [5].