What are the different eyesight measures and how are they related to each other?

Question

While searching about laser surgery for my myopia, I came across different system to measure the quality of individuals' eye sight. When speaking about quality of eyesight, I am referring to the classic test where one has to read smaller and smaller symbols (usually letters or numbers).

I found four different measures:

1. One is a scale from 0 to 1.0+, where the average/normal eyesight is 1.0. People can be above but I don't know how much higher than average.
2. Another scale from 0 to 20+, the average between 20. I guess that this one is proportionally related to the first one.
3. I saw negative integers (like -5,-2) sometimes, but I didn't understand how they work exactly. I guess they measure how bad your eyesight is compare to average.
4. My optometrist used dioptries.

My question: what are the (most used) measure of the quality eyesight and how do they relate to each other?

Answer 1

First of all, one has to distinguish between visual acuity (VA), which is a measure for the maximal possible resolution your eye-brain-system can achieve, and the refractive error, which measures the deviation of the optical system of your eye from emmetropy (=perfectly balanced optics, sharp focus on retina without any glasses) in diopters of spheres and cylinders and thus determines what corrective glasses you need to wear in front of your eye in order to achieve your maximum possible visual acuity.

Visual acuity can be measured without corrective glasses, i.e. the "native" VA of your eye, or with your best corrective glasses, which then gives you the value of your "Best-corrected visual acuity" (BCVA), and only this value is useful for comparison purposes, e.g. for driver's licenses (because errors that can be corrected by glasses also are to be corrected by law, as this is rather easy to do for everybody), and also for scientific evaluation of eye performance.

This also explains a common misconception: When somebody is very near- or farsighted (myope or hyperope), he has to wear glasses with a high (absolute) value of diopters, e.g. -7 dpt. But if he reaches 20/20 vision with his glasses on (again, BCVA), then to an eye doctor, this will matter the most; for the glasses are neglegible in comparison to "real" eye diseases which can impair your eye function and lower the BCVA your eye can reach. Many people incorrectly compare their eye functions by comparing the amount of diopters in their glasses, yet this doesn't really say anything about the maximum resolution their eyes have when wearing their best glasses. Refractive errors can be corrected by glasses, contact lenses and laser surgery, but the maximum visual acuity an eye-brain-system is able to achieve can not be altered in any (simple) way.

Now, when visual acuity is measured, a full visual acuity, i.e. "normal", or 100%, or any way you'd like to name it, has once simply deliberately been determined by a minimum angle of resolution of 1 arc minute, and the charts that are used for testing it have letters that correspond with this resolution. 1 arc minute of resolution corresponds to being able to separate two points with 2,91 cm between them at a distance of 100 m. Now note that this does not necessarily correspond with what most people are able to see; as said previously, the definition of 100% visual acuity was deliberate. The capability to distinguish points with 1 arc minute of space between them has been defined as 20/20 (or 6/6) vision in Snellen charts, 1.0 vision in decimal charts (conversion table), and later 0.0 logMAR, which is the logarithm of the minimum angle of resolution and which has become the gold standard in measuring and comparing visual acuity (see p.13 for conversion chart), but is predominantly used for scientific purposes and less in clinical environments.

This is basic knowledge in optics, optometry and ophthalmology. Wikipedia describes the correlations between the terms pretty well.

Answer 2

The 0 to 1 scale is simply a decimal expression of the 20/20 (Imperial) or 6/6 (metric) measure of visual acuity. It's related to the smallest gap size someone can see on the Landolt C chart.

Dioptres are not a measure of eyesight quality per se. Rather, they're a measure of the focal length of the lenses needed to bring your eyesight to normal. An eyeglasses prescription might specify two or three such measures to correct various aberrations (eg. "-5 diopters; -1 diopter @ 180), in such a case, the first one is a spherical correction for distance vision, while the second is a cylindrical correction for astigmatism and the third (in the rare case that it's present) is a prism correction for alignment problems. Eyesight expressed as a single diopter measure refers to the correction needed for distance vision.

Your "negative integers" method may be diopters again: a diopter measure can be either positive or negative, depending on what vision problems it's correcting.

I can't find a "0 to 20+" scale for visual acuity.

Answer 3

There are several ways eyesight is measured. The diopter (dioptre) measures lens power, which is in turn correlated with focal length in meters. For farsighted prescriptions, there's a plus sign in front of the number, for nearsighted prescriptions, a minus sign is in front of the number of diopters. As for measuring actual visual acuity, the Snellen chart uses a ratio of the distance to the chart (6 meters or 20 feet in rooms long enough to accommodate this) and the line on the chart with the smallest letters that one can see. The baseline for 20\20 or 6\6 sight is the eighth row of letters on the Snellen chart, and the threshold for needing corrective lenses is typically 20\30 or 6\9 , but then again, at what point someone begins needing glasses or contact lenses when their vision is 20\30 or worse is dependent upon the activity and environment (in addition to local laws, especially when operating motor vehicles and vessels).