The lens is focused in the eye by a muscle which pulls on the strands or ligaments that suspend the lens behind the pupil to change its shape. The retina in each eye sends the image it generates through the optic nerve to the brain where the images are combined into one image with the perception of depth. Problems with the detection of the image by the retina or its transmission to the brain through the optic nerve that cause smudges in the field of vision. Problems with the brain in its attempt to combine the images from the two eyes into one image with depth perception. Most of these conditions can be improved with glasses or contact lenses that merely correct the focus of the eye.
Glare is created by the cornea or more often by the lens if it becomes cloudy.
Rarely, glare and focus problems are caused by an iris that cannot adequately close down the opening size of the pupil. 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. Inflammation or degeneration of the retina can cause focal blur or loss of color and contrast. 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.