Digital screens have become an unavoidable part of daily life, but prolonged exposure often leads to Computer Vision Syndrome (CVS) or digital eye strain. Symptoms include dry eyes, headaches, blurred vision, and neck pain, largely because the eyes must constantly work to focus on the screen at an intermediate distance. The correct strength of computer glasses is determined by matching the lens power to your specific working distance, which differs from the distance used for reading or driving. Finding the appropriate lens power significantly reduces eye fatigue and improves visual comfort during long hours in front of a screen.
How Computer Glasses Differ
Standard reading glasses are designed for viewing close objects, typically 12 to 16 inches away. This short focal length is too strong for a computer screen, which is usually positioned in the intermediate vision range of 20 to 40 inches. Computer glasses are optimized to bring this intermediate range into clear focus.
The fundamental difference lies in the optical power, measured in diopters, needed to correct the focal point. Since the computer screen is farther away than a book, the required optical power is less than the power in reading glasses. Using full-strength reading correction forces the eyes to over-accommodate, causing muscle strain and eye fatigue. Computer glasses use a weaker lens power, allowing the eye muscles to relax.
Calculating Your Working Distance
Determining the correct strength for computer glasses begins by accurately measuring your personal working distance. This is the distance between your eyes and the computer screen while you are seated in your usual working posture. Sit naturally, with your back resting against the chair, and use a ruler or measuring tape to find the distance from the bridge of your nose to the screen. This measurement directly translates into the required lens power; shorter distances require stronger lenses, and longer distances require weaker lenses. The typical intermediate range is 24 to 32 inches, but if you use multiple screens, measure the distance to the one you focus on most often.
Determining the Required Lens Strength
The required lens strength, expressed in diopters (D), is inversely related to your working distance when measured in meters. For example, a working distance of 0.5 meters (about 20 inches) demands a lens power of +2.00 D (1 divided by 0.5). If your working distance is 0.8 meters (about 31.5 inches), the required power is +1.25 D (1 divided by 0.8).
Using the ADD Power Subtraction Method
A common method to find the approximate strength is to subtract a specific amount of power from your existing reading glass prescription, known as the “ADD” power. This is because the ADD power is typically calculated for the closer reading distance of 14 to 16 inches. Opticians often calculate the computer prescription by taking half of your ADD power and adding that value to your distance prescription’s sphere power. For instance, if your ADD power is +2.00 D, your intermediate correction would be +1.00 D stronger than your distance prescription.
The subtraction method provides a quick approximation for self-purchasing over-the-counter computer glasses, which are often sold in 0.25 diopter increments. For those who wear no distance correction, the calculated diopter value is the strength needed. If you already use reading glasses, try subtracting 0.50 D to 1.00 D from their power to find a suitable intermediate strength.
When to Consult an Optometrist
While self-calculations provide a starting point for purchasing over-the-counter computer glasses, they are only approximations. Consulting an optometrist is highly recommended for obtaining the most accurate prescription. A professional eye exam ensures the lenses account for astigmatism or other complex refractive errors. An eye care specialist can also determine the best lens type for your work environment. Options include single-vision lenses, which offer clear focus at one fixed distance, or occupational progressive lenses, which provide clear vision for the screen and surrounding objects. Professional guidance is important if you have a complex prescription or if self-calculated strengths do not relieve digital eye strain symptoms.