Q: I’m studying units of measurement in my science class. Do eye exams use the metric system?
A: For the most part worldwide, yes. We use millimeters for most eyeglasses measurements, and for some biometric measurements. Lens powers are noted in diopters, which are derived from focal length measurements in meters. But the imperial system is still often referenced for visual acuity. For example, 20/40 means that the resolution at 20 feet is comparable to 40 feet in someone with normal human acuity.