Computer screens usually are positioned 20 to 26 inches from the user's eyes. This is considered the intermediate zone of vision — closer than driving ("distance") vision, but farther away than reading ("near") vision.
Most young people wear eyeglasses to correct their distance vision. reading glasses are prescribed to correct near vision only. And bifocals prescribed for those over age 40 with presbyopia correct only near and far. Even trifocals and progressive (which do have some lens power for intermediate vision) often don't have a large enough intermediate zone for comfortable computer work.
Without computer eyeglasses, many computer users often end up with blurred vision, eye strain, and headaches — the hallmark symptoms of computer vision syndrome. Worse still, many people try to compensate for their blurred vision by leaning forward, or by tipping their head to look through the bottom portion of their glasses. Both of these actions can result in a sore neck, sore shoulders and a sore back.
Though they sometimes are called "computer reading glasses," it's best to call eyewear designed specifically for computer use "computer glasses" or "computer eyeglasses" to distinguish them from conventional reading glasses. Computer glasses put the optimum lens power for viewing your computer screen right where you need it for a clear, wide field of view without the need for excessive focusing effort or unhealthful postures.