The optics of the eye

Object distance (u):100 3000 mm

Eye Diopter (1/f): m-1

Correction: m-1

Eye Diameter: mm

Image distance (v): mm

Myopia (Nearsightedness)

Hyperopia (Farsightedness)

Accommodation Presbyopia Myopia Hyperopia

The eye is an adaptive optical system comprising of a cornea and a lens.

  • Accommodation is the process by which the shape of the lens can be altered to change its power when the eye needs to focus at different distances. In this tutorial we will see how eye optics and anatomy combine to focus the image on the retina (blue). We will start with an image that is far from the observer's eye. Light rays from such a distant object are parallel and are focused on the retina. Move the object closer by changing the value of the Object distance slider. At a close distance the focus moves beyond the retina.
  • Eye diopter determines the accommodation power of the eye. Change it to get a sharp retinal image. Now move the object far away. What happened to the image?

    Eye defects:

  • Presbyopia: a diminished ability to focus on proximal objects. As the age progresses, the power of accommodation decreases due to weakening of the ciliary muscles and hardening of the lens. Click on Presbyopia button. Try to focus on a near-by object. Can you correct the condition? What lens do you need? Do you need the correction for distal objects?

  • Myopia (Nearsightedness): impaired vision where a person sees clearly near objects, but distant objects are blurred. The defect is caused by the elongation of the eyeball or excessive curvature of the cornea. Select Myopia (Diopter). Note the change in the minimal eye diopter. Can you focus the distant object now? Do the same test for Myopia (Eye Diameter). Can you see a difference between an eye that has good focusing abilities (Emmetropia) and an eye with presbyopia? What corrective measures would you need to focus on far-away objects?

  • Hyperopia (Farsightedness):it is the opposite of shortsightedness and it is a defect in vision where a person sees near objects with blurred vision, but distant objects are clearly visible. The defect occurs when the eyeball is too short or when the focal length is too great. Repeat the manipulations done above. How does presbyopia affect the condition?


    The thin lens equation: 1/f=1/u+1/v

    Where f is the focal length of the lens, u is the object distance, and v is the image distance. (Typical units: mm). The 1/f is the power of lens which is measured in diopters (units: m-1)

    After entering into the eye through the cornea, light is refracted by the cornea and lens. The transparent front part of the eye, the cornea plays the most important part in refraction. While the cornea contributes most of the eye's focusing power, its focus is fixed. The curvature of the lens, on the other hand, can be adjusted to "tune" the focus the image on the retina in the back of the eye depending upon the object's distance. The iris is controlling the amount of light rays enter into its opening which is called pupil by it’s aperture stop mechanism. This aperture stop is a very important component of an optical system, affecting a wide range of optical processes. Subsequently the light beam projected to the inner layer of the eye, the retina , which is an extension of the central nervous system. the image on the retina is inverted - like a camera and is connected to the brain by the optic nerve.

    Near-sightedness, also known as short-sightedness and myopia, is a condition of the eye where light focuses in front of, instead of on, the retina. Severe near-sightedness increases the risk of retinal detachment, cataracts, and glaucoma. The underlying mechanism involves the length of the eyeball growing too long or less commonly the lens being too strong. There is tentative evidence that the risk of near-sightedness can be decreased by having young children spend more time outside.

    In some countries, the prevalence of myopia is reaching epidemic proportions:>

    Far-sightedness, also known as hyperopia, is a condition of the eye in which light is focused behind, instead of on, the retina. As the condition worsens, objects at all distances may be blurry. Often it occurs when the eyeball is too short, or the lens or cornea is misshapen. Far-sightedness primarily affects young children, with rates of 8% at 6 years and 1% at 15 years. It then becomes more common again after the age of 40, affecting about half of people. Far-sightedness can have rare complications such as strabismus and amblyopia. At a young age, severe far-sightedness can cause the child to have double vision as a result of "over-focusing". The signs and symptoms of far-sightedness are blurry vision, headaches, and eye strain.
  • Composed by Alon Poleg-Polsky, 2019