When a distant image is focused perfectly on the back of the retina when accommodation is at rest this condition is termed emmetropia. The eye functions like a camera – it has an optical system (the tear film cornea aqueous humor lens and vitreous humors) and a light-responsive film (the retina). The image of an object is in focus on the retina when the optical power and the length of the eye are equal. The length of the eye is fixed but the power of the eye can be changed by a process called accommodation during which the ciliary muscle within the eyeball contracts causing a change in shape of the lens and a consequent change in optical power.
When a distant image is focused perfectly on the back of the retina when accommodation is at rest this condition is termed emmetropia. More commonly referred to as 20/20 vision it is when there is a harmonious blending of the cornea curvature the shape of the eye lens and their distances from each other and the retina. Vision is clear and your eye functions in its ideal state.
If however there is a mismatch between the power of the eye’s optical system and the length of the eyeball then the eye is said to have a refractive error. This error can take the following forms:
- Myopia (nearsightedness) occurs when the focal point of light entering the eye is at a virtual distance in front of the retina. Images from afar appear blurry.
- Hyperopia (farsightedness) occurs when the focal point of light entering the eye is at a virtual distance behind the retina. Images up close appear blurry.
- Astigmatism occurs when the cornea is misshapen like a football. Vision might be blurred or distorted at all distances and those affected might see objects that appear on a tilt stretched or condensed.
- Presbyopia is a condition that results from aging. The progressive loss of elasticity of the lens results in the eye becoming less capable of focusing on close objects. Tasks like reading small print and computer work become more difficult. Distance vision however is usually unaffected.
You can find out your refractive error (if you’re not an emmetrope) by receiving your yearly eye examination.