Computer glasses are specialized prescription eyewear designed to provide clear, comfortable vision at the intermediate distance of a computer screen. This viewing range, typically between 20 and 40 inches (50 to 100 centimeters), falls between the focus of reading glasses and standard distance lenses. Relying on standard distance or near-vision glasses for computer work often forces the eyes to strain their focusing muscles. This strain leads to symptoms like eye fatigue, headaches, and blurred vision, collectively known as Computer Vision Syndrome. Calculating a specific prescription for this unique range alleviates this strain by matching the lens power to the required focus distance.
Measuring Your Optimal Working Distance
The first step in estimating your computer glasses power is accurately measuring your personal working distance. This measurement must be taken in your natural, comfortable working posture, as the lens power is specific to this distance. Sit at your desk as you normally would, with your back relaxed and your screen positioned where you typically view it.
Use a measuring tape to find the exact distance from the front surface of your eye to the center of your computer screen. This measurement should be taken in inches or centimeters while looking straight ahead. For the optical calculation, this distance must be converted into meters (100 centimeters equals one meter).
Converting Distance to Required Dioptric Power
The necessary lens power to focus at any specific distance is determined by a fundamental optical principle. The power (\(P\)) required is the reciprocal of the focal length (or working distance, \(WD\)) when measured in meters. The formula is \(P = 1/WD\).
For example, if your measured working distance is 50 centimeters, converting this to meters gives you 0.5 meters. Applying the formula, \(1/0.5\) meters requires a total lens power of \(+2.00\) Diopters. If your distance is one meter, the required power is \(+1.00\) Diopter. This calculated power represents the total focusing ability needed at that specific intermediate distance.
Modifying Standard Prescriptions for Intermediate Use
For individuals who already wear multifocal lenses and have a reading addition (ADD) power, the computer glasses prescription is calculated by modifying this existing value. A reading prescription is optimized for a close distance, usually around 16 inches (40 centimeters), which is often too strong for a computer screen. Therefore, the intermediate power for computer use is a reduced version of the full reading ADD.
A common rule of thumb is to take half of the full reading ADD power as the intermediate ADD power. If your reading ADD is \(+2.00\) Diopters, the intermediate ADD would be \(+1.00\) Diopter. Apply this calculated intermediate ADD to the spherical component (SPH) of your distance prescription. The new sphere value for your computer glasses is the sum of your distance sphere and the intermediate ADD.
The cylindrical correction (CYL) and its axis (AXIS), which correct for astigmatism, typically remain unchanged from your distance prescription. For instance, if your distance prescription is \(-1.50\) Sphere with a \(+2.00\) ADD, the new computer sphere would be \(-1.50 + (+1.00)\), resulting in a \(-0.50\) Sphere. This calculation provides the precise, reduced focusing assistance needed for the intermediate range while maintaining correction for existing refractive errors.
Lens Type Options for Computer Viewing
The calculated intermediate power can be incorporated into several practical lens designs, depending on your visual needs and work environment. The simplest option is a single-vision lens, which provides clear vision only at the specific measured working distance. This lens offers the widest, most distortion-free field of view for the computer screen, but everything beyond that distance will be blurred.
For individuals requiring both intermediate and near vision without changing glasses, an occupational bifocal or trifocal lens can be considered. An occupational bifocal places the calculated computer power in the main upper portion of the lens and a stronger near power in a small segment at the bottom for reading documents. A popular multifocal solution is the occupational or office progressive lens, which prioritizes a wide intermediate zone for the screen and smoothly transitions to a near zone for desk work. These specialized progressive lenses offer a much wider intermediate field than standard progressive lenses, but they sacrifice the ability to see clearly at far distances.