The Ergonomic Eye — Optical Distance & Ergonomics Calculator

A diopter is the unit of lens power: it equals 1 divided by the focal length in metres. A +2.00 D lens bends light to a focus 0.5 m away; the sign shows the direction — minus lenses spread light out (myopia), plus lenses bring it together (hypermetropia and reading adds).

Myopia and hypermetropia are about the eye’s overall focusing power being too strong or too weak for its length. Presbyopia is age-related loss of the eye’s ability to change focus, unrelated to SPH sign. Astigmatism means the cornea or lens curves more in one direction than another, so light forms two focus lines instead of one point.

It depends on your spherical equivalent power and an assumed 4.00 D of accommodation reserve, which is an average for a young adult and normally declines with age (this is exactly what ADD compensates for). Real focusing ability also varies with lighting, fatigue, pupil size and individual anatomy, so treat the numbers as a useful estimate, not a diagnosis.

No — accommodation amplitude falls fairly predictably with age, from roughly 10–14 D in children to near zero by the mid-50s. We use 4.00 D as a rough working-age average so the tool gives one consistent baseline; your own reserve could be noticeably higher or lower.

Minus (concave) lenses diverge light before it enters a myopic eye so it focuses on the retina instead of in front of it; plus (convex) lenses do the opposite for hypermetropia. Cylindrical lenses correct astigmatism by adding power in only one axis, and prism lenses bend the whole image sideways to help two eyes aim at the same point.

The eye’s length, corneal shape and lens flexibility all shift gradually with age, growth or health changes, which is why SPH, CYL and ADD values commonly drift over the years. Regular eye exams (typically every 1–2 years for adults) catch these shifts before they meaningfully affect daily vision.

OSHA (U.S. Occupational Safety and Health Administration) and CCOHS (Canadian Centre for Occupational Health and Safety) are government agencies that publish evidence-based workplace ergonomics guidance. Their posture and joint-angle figures come from broad population studies, so they’re a reliable baseline independent of any one person’s eyesight.

Current clinical research has not shown that blue-light filtering lenses meaningfully reduce digital eye strain compared with ordinary lenses; the strain is mostly caused by reduced blinking, poor distance and small text. Blue-light coatings may still help with sleep quality if screens are used late at night, but they are not a substitute for the ergonomic basics in this tool.

High-index lenses help stronger prescriptions (roughly beyond −4.00 D or +4.00 D) because standard plastic gets noticeably thick at that power. Computer/office lenses suit ADD wearers and heavy screen users by widening the intermediate focus zone, while astigmatism benefits most from precise cylinder/axis grinding rather than any special coating.

It is SPH plus half of CYL, a single number that represents the average focusing power across all directions of an astigmatic eye. It is the standard way opticians approximate "overall" prescription strength when a quick single figure is more useful than the full three-number astigmatic prescription.

Your far point is the farthest distance you can see sharply, and your near point is the closest; the span between them is your clear-vision range. As a quick rule of thumb for short-sightedness, the far point in metres is about 1 divided by the minus power: −2.00 D is roughly 50 cm and −4.00 D roughly 25 cm. Enter your prescription above to see your own estimated range.

A common guideline is roughly an arm’s length, about 50–100 cm (20–40 in) depending on screen size and text size, with the top of the screen at or slightly below eye level. The right distance also depends on your prescription, so this tool adjusts its suggestion to your estimated clear-vision range instead of giving every user the same number.

Every 20 minutes, look at something about 20 feet (6 metres) away for at least 20 seconds. It gives the focusing muscles a break from close work and reminds you to blink. Research on its effect is limited, but it is a low-effort habit that many eye-care professionals recommend for digital eye strain. The reminders in this tool can time it for you.

It is free, with no login or account. The calculations run in your browser, and the tool makes no network requests to send your prescription anywhere. Your history, streak and reminder settings may be saved in your browser’s local storage on your own device, and you can clear them any time through your browser’s site data settings. It is an educational estimate and does not replace a professional eye exam.