How Does the Eye Focus?

How Does the Eye Work?

Light is divergent in nature. Which means that any light that enters into our eye is “out-of-focus” and needs to be brought back into FOCUS. The cool thing about the eye is that the front one-third of the eye has the potential to converge light into a perfect focus onto light sensor cells that are layered on the internal back two-thirds of the eye. These sensor cells then send the accumulation of details regarding the light that was received from the world around us to the brain to be processed… and then we “see”.

Parts of the Eye and Vision

When divergent light enters the eye, it first passes through the tears. A healthy tear layer provides stable optics for light to freely pass without being disturbed. 

Then light passes through the cornea. The cornea is made up of transparent collagen and handles the majority of bending (from 65-75%) light back into focus. 

Beyond the cornea, light freely passes through aqueous humor which nutrifies and oxygenates tissues within the eye without a blood supply (such as the backside of the cornea). 

Then light passes through the pupil which is actually just a hole for light to pass through the center of the color part of our eye, the iris

Behind the iris is the flexible crystalline lens, which helps fine tune the focus of light the rest of the way (the remaining 25-35%) with the help of connected ciliary muscles

Before light reaches the back of the eye (light’s final focus destination), it also passes through the vitreous humor, which is a gel composed of 99% water and 1% other stuff (sometimes this 1% wrinkles and we see it as a floater in our vision when conditions are right). 

The final point where light must focus within the back of our eye for our best vision potential is called the fovea. The fovea is our sharpest area for vision, and it is the center portion of a zone called the macula. The macula is filled with color sensitive light receptor cells (photoreceptors) called cones, and it, in turn, is part of the entire photoreceptor membrane called the retina

The area of the retina that surrounds the macula is mostly made up of photoreceptors called rods, which are primarily responsive to non-specific-color light and motion. As the cones and rods receive details about light and motion around us, it relays the information out of the back of the eye along nerves that collectively form the optic nerve

The optic nerve then crosses behind the eye at the optic chiasm (in the brain near the pituitary gland) and then into two shared optic tracts, which ultimately bring the light information to the visual cortex which lies the region in the back of the brain’s cerebellum called the occipital lobe.

Refraction of Light

If light is bent (refracted) correctly into the back of our eye and onto our fovea without glasses, it is called emmetropia. If light is incorrectly focused (termed refractive error) then we may need glasses to fine tune the focus point, or we use our eyes to refocus light to the fovea when possible. If light focuses in front of the fovea, we call it nearsightedness (aka: myopia), and when it focuses behind the fovea, we call it farsightedness (aka: hyperopia/hypermetropia).

When we are fully corrected (naturally or with corrective lenses) to see far away (beyond about twenty feet), our focusing muscles (ciliary muscles) that are attached to our lens are relaxed and our lens is in a “flatter” position. 

When light is moved within twenty feet, the divergent nature becomes more apparent to the eye as the focus point moves behind the fovea. In response to this divergence and shift of the focal point, the ciliary muscles flex and the lens will become “rounded” to refocus the light back to the fovea. 

The closer the object, the more the muscles must flex, and the rounder the lens must become to maintain focus. Obviously, the closer the object, the greater the focus requirement, and the greater the strain of the individual to maintain focus. That is why holding a book or phone too close for long periods often produces eye fatigue. The ciliary muscles are straining to focus the lens.

Factors Affecting the Eye’s Focusing Ability

  • Short Axial Length of Eye: Causes light to focus behind the fovea… aka: Hyperopia
  • Long Axial Length of Eye: Causes light to focus short of the fovea… aka: Myopia
  • Asymmetric Corneal Shape: Causes light to be stretched across two focal points instead of one… aka: astigmatism
  • Warped Cornea Surface: does not allow for even focus of light on the fovea, as in keratoconus
  • Growth: Increases in the eye’s front-to-back measurement (axial length) often causes the fovea to grow beyond focus point, contributing to myopia. This is especially apparent in rapidly growing children
  • Presbyopia: Around the age of forty and continuing until about the age of fifty-three, the lens slowly hardens and loses its ability to fine tune focus to the fovea. This happens to everyone, everywhere.
  • Diabetes/Systemic Changes: any disorder that can affect the body may also affect focus (though commonly temporarily). Diabetes is a common cause of “hyperopic shifts” in vision, but sickness, pregnancy, weight loss, and chemotherapy are common factors that disrupt the focus of light in our eyes.
  • Sleeping in Contact Lenses: The cornea needs oxygen from the air to breathe when the eyes are open, and oxygen from the blood vessels in the lids when our eyes are closed. Contacts limit the amount of oxygen to our eyes, especially during sleep, causing a pH change in the cornea and a shift of water balance. This shift can disturb the refractive properties of the cornea, and disturb focus.
  • Cornea Swelling: Eye infections, allergic conditions, and even excess swimming can cause the cornea to swell which will change the optics of the cornea, and alter focus of light.
  • Medicines: especially nerve and pain medicines in excess can affect our fine tune focusing of our lens of our eye. The majority of the usable part of our brain is tuned into eye function in one way or another. So if you mess with the brain, expect that the eye may follow.
  • Dry Eye: a common condition that causes the eye to water (think of it this way… “when there is a fire, the sprinklers turn on” – our eyes water in response to dryness). It is neither the cornea nor the lens that affects focus, instead it’s the tears. The tears are the first thing that light passes through as it enters the eye. If the tear is not stable (as with MGD: Meibomian Gland Dysfunction associated with aging), then light focus will be disturbed

Vision Correction Options

  • Glasses
    • Plus (+) lenses move focus points forward to the fovea
    • Minus (-) lenses move focus points backwards to the fovea
    • Add Power: provides Add-itional plus (+) power for people who have trouble with presbyopia
  • Standard Soft Contact Lenses & RGP’s (Rigid Gas Permeables)
    • Plus (+) lenses move focus points forward to the fovea
    • Minus (-) lenses move focus points backwards to the fovea
    • Add Power: provides Add-itional plus (+) power for people who have trouble with presbyopia
  • Refractive Surgery
    • LASIK: (Laser-ASsisted In situ Keratomileusis) a laser surgery option that shaves the of the cornea stroma after a flap of the cornea epithelium and a thin slice of the stroma are pulled aside
    • PRK: (PhotoRefractive Keratectomy)a laser surgery that shaves the cornea stroma after the cornea epithelium was brushed aside 
    • SMILE: (SMall Incision Lenticule Extraction) a laser surgery that shaves the cornea stroma without disturbing the central corneal epithelium
  • RLE: (Refractive Lens Exchange) a surgery that removes the natural crystalline lens of the eye and replaces it with a biocompatible lens that corrects light onto the fovea – similar to cataract surgery that is performed when the crystalline lens becomes “dirty”.
  • Scleral Lenses: a contact lens that vaults the cornea and rests on the conjunctiva overlying the sclera to create a new optical combination of tears and cornea to assist with cornea issues such as severe dry eye and corneal ectasia (corneal thinning).
  • Ortho-K: (OrthoKeratology) a method of temporarily reshaping the natural shape of the cornea to refocus light onto the fovea in lower powered myopes.
  • Myopic Management: an ever growing strategy they may utilize multifocals, contact lenses, drops, and/or lifestyle intended to slow the rate of the eye’s axial length during developmental years and therefore reduce the progression of myopia and complications.
  • Dry Eye Management: intended to stabilize the tear layer and increase comfort and vision utilizing various methods
    • Artificial Tears: offer 10-20 minutes of immediate tear stabilization
    • Nighttime Ointment: offer overnight moisture lock to heal the eye during sleep
    • Lid Scrubs: offer a chance to remove components from the lids and lashes that may disturb the tears
    • Hot compresses: provide a “melting” of the oil glands so they flow more freely and increase vascularity around the oil glands for long term health
    • Fish Oil: provide long term support through Omega-3 that both increases vascular flow around the oil glands and causes a change in the oil itself so it can flow more freely
    • Punctal Plugs: used for the less common water deficient forms of dry eye

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