3.  Problems with the brain in its attempt to combine the images from the two eyes into one image with depth perception.

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Problems with Focus or Glare

Focus problems cause the image to be blurred (fuzzy overall) and may result from the incorrect shape of either the cornea or lens (as in near-sightedness or far-sightedness) or astigmatism (where the elements may be slightly warped), all of which cause a poor focus on the retina.  Most of these conditions can be improved with glasses or contact lenses that merely correct the focus of the eye.  If the cornea or lens is sufficiently irregular that glasses or a contact lens cannot adequately correct the focus (and the image remains blurred), then surgery must be performed to correct the irregularity -- either corneal surgery (sometimes requiring transplantation) or lens surgery (removing the cataract or cloudy protein within the lens and implanting a small plastic lens within the capsule to replace the protein that is removed).

Glare is created by the cornea or more often by the lens if it becomes cloudy.  As we all grow older (chronologically gifted!), the clear gel protein in the lens becomes crystallized and cloudy resulting in glare -- which we experience as a haze that washes out the contrast of the image on bright sunny days, or as flare seen around headlights at night.  Glare can be reduced with filtering lenses (yellow-amber lenses, orange "blue blockers", gray lenses that reduce the light level overall, or polarizing lenses that reduce the glare from reflective surfaces).  When glare is not reduced adequately by such lenses, then surgery (most commonly cataract surgery) is performed to replace the cloudy lens in the eye with a clear plastic implanted lens.

Rarely, glare and focus problems are caused by an iris that cannot adequately close down the opening size of the pupil.

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Problems with Image Detection by the Retina and Transmission of the Image though the Optic Nerve.

Problems of the retina and optic nerve most often cause loss of color or loss of contrast.  Most commonly the problem is not observed over the whole field of vision of the eye but rather is experienced in portions (causing smudges or "scotomas" in the visual field).  Our ability to see textures and hues is based on the contrast sensitivity of the retina -- the ability of the film in the camera to produce a good, high-contrast picture with correct colors.  It is also dependent upon the ability of the optic nerve to transmit that picture to the brain (and on the ability of the brain to "see" the image).

Blockages of blood vessels, or a "stroke" of the retina, optic nerve, or brain, can cause a defect in a portion of the visual field blurring and with loss of color and contrast.  Inflammation or degeneration of the retina can cause focal blur or loss of color and contrast.  Leakage of blood vessels in the retina (such as may occur in diabetes) or scarring of the retina not only causes smudge defects in the field of vision but also produces distortions of the retina, causing distortions in the field of vision (doors or windows have kinks or bumps).

For these conditions, focus is not the problem and glasses do not improve the vision, since the problem lies with the retina and its inability to detect and transmit the image.  The problem becomes much worse under conditions of reduced lighting (i.e. the restaurant where the lighting is romantic but you can't read the menu!).  Here the key to improving vision is to increase the lighting on the object with focal lights that don't reflect into the face and with filter lenses that reduce glare and improve contrast.  Magnification devices that magnify the image and increase light also are of some help, provided the visual field defect is not large or severe, or if it is, then devices which assist the person in using the remainder "peripheral" vision may be of help.

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Problems with Integration of the Images from the two eyes into one Image with Depth

If the brain cannot integrate the two images (one from each eye) into one image with depth perception, then the person has double vision (often noticed as a ghosting of the image or as two separate images) or problems with depth perception (pouring liquids outside of the cup, hitting the fender of the car against the garage door!).  Double vision is most often observed only when both eyes are open (and disappears when one eye is covered) and occurs either because the eyes are not lined up to look at the same object, or because one of the images is distorted or of a different size than that in the other eye.  These problems often require neurological or retinal evaluation and sometimes require surgery to correct (or if impossible to correct, then permanent patching of one eye may be required to remove the distracting second image).

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