Eyewear

	Eyewear   |   Bifocals   |   Visual after effects

Eyewear
Monofocal lenses worn by people with adequate accommodative ability pose few, if any, problems for display terminal users. Some people, however, need multiple lens segments in their glasses to enable them to hold their heads in a comfortable position when viewing a display. Problems may be encountered by such people when using multifocal lenses. Some of the options available include reading glasses (which are bifocals in the top and bottom halves) and different types of multifocals where the division between sections of different focal lengths has been "smoothed" out. Each of these has advantages and disadvantages. VDT users who might benefit from eyeglasses for near point tasks should know the options available to them so that, with professional assistance, they can select a solution most satisfactory to them.

Differences in refractive prescriptions may result in considerable changes in the range of comfortable viewing. The range of comfortable viewing changes rapidly as the positive strength of the lens is increased. It is important that the lenses not be too strong for the intended visual tasks. The following table shows the range of acceptable focus (in steps on +/-0.5diopter) for various strengths of near-point lenses.

A person's ability to make the lens of the eye stronger will allow objects to be seen closer than the near range, as shown above. That same function will also reduce the furthest point in the range. When people converge their eyes to focus on a near point, lenses automatically accommodate, even if that accommodation is not required. The amount of accommodation varies among individuals. Any amount of accommodation will shorten the far distances shown.

Reading glasses are monofocal lenses with a correction factor that is designed to optimize focusing at a predetermined near distance. They may provide an excellent field of view for near work but have the disadvantage of degrading the image of distant objects. This type of lens normally would be recommended for users who work at near-point tasks for long periods of time where a large field of view is helpful. The disadvantage is that the glasses must be removed and possibly replaced by another pair when distant objects need to be seen clearly.

In addition to monofocal lenses, there is a wide variety of multifocal lenses available.

Fields of view of different lenses.

Figure 31 shows a representation of the fields of view and the type of distortion inherent in each. Note the distortion caused by the blending of the two lens powers in the progressive-addition bifocal (e).

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