Eyesight is the most commonly referenced element of vision. Eyesight, clinically known as visual acuity, is a measure of how clearly we see a standard target. Provided eye health is intact and strong, the only obstacle to clear sight is the Refractive State (‘RS’, commonly referred to as Refractive Error, ‘RE’). Some people are nearsighted (clear and easy sight up close), others farsighted (strained and difficult sight up close), some have both (one eye nearsighted, one eye farsighted). Astigmatism, can occur in nearsighted and farsighted eyes alike and is uniquely difficult to live with in its uncompensated state.
Refractive State is determined entirely by genetics and environment. That is, our eyes are pre-disposed genetically to grow to achieve a certain size and shape. Our focusing system (ciliary body) can tug and pull on the lens in only a limited way to provide only a small amount of focusing power, but overall it’s the eye’s shape that determines how light is focused on the receptor cells of the retina. Refractive State is a measure of how the eyeball can naturally focus light, if it can focus light at all for practical use. As a rule, bigger numbers equal bigger problems for the owner of the eyes.
Visual acuity is is measured using a variety of standardized eye charts, usually at a distance of 3m (10-ft) or 6m (20-ft), and (hopefully) at near distance as well (30-40cm, 12-14-in). This requires people to identify optotypes (letters/symbols) to the testing agent/clinician. In other words, visual acuity (VA) is a subjective test perhaps as much as an objective one.
The Refractive State (RS), on the other hand, can be measured objectively, without the client’s input, using a variety of automated tools, or even very simple tools like the ophthalmoscope and retinoscope pictured above. Since these measures are objective, requiring limited input from clients and patients, refractive state may be determined in people of all ages and in a variety of cognitive states or with perceptual challenges.
In the end, clarity of sight is determined by three factors (assuming good health and proper alignment of the eyes as a paired set, or team.) 1) The Refractive State, 2) The nature and distance of the visual task/object, and 3) The quality of optical compensating lenses at hand. So, for example, a nearsighted person will have no difficulty with a near task, but the same task will be more strenuous for the uncompensated farsighted or astigmatic eye. The best outcomes for clients involves a combination of objective and subjective measures in a clinical procedure called the subjective refraction. This is generally what is provided by the doctor as the ‘eyeglasses prescription’, more or less – the details of the prescription and how to turn it into optical compensation can become rather involved.
This series takes a look at refraction using paper ‘strips’ from auto-refractors, automated tools for providing objective measures of an eye’s refractive state.
This one procedure most often takes only a matter of a minute to perform for both eyes and yet it uncovers a wealth of information about each eye, the eyes as a paired set, and what this means for the user. Most strips offer additional information captured at the same time but these won’t be considered in this general discussion about Refractive State.
- The auto refractor strip and results are not intended to be a glasses prescription, nor should a glasses prescription ever be generated in an automated way for optimal results in therapeutic situations.
- Therapists are encouraged to ask for copies of ophthalmic prescriptions as well as a recent auto refractor strip for study. This series will offer insight into how the information can be implemented in important ways to achieve best clinical outcomes and patient satisfaction.
- Spectacle prescriptions are not equivalent to contact lens prescriptions.
- Since optics drive neurological and sensory responses, attention to detail is critical. For example, in important clinical cases (TBI, learning concerns, dyspraxia, vestibular problems, others) self-measurement for online glasses purchases is almost always a serious error in the making.