Total blindness can occur from penetrating wounds in one eye, followed by sympathetic ophthalmia in the other eye. Corneal scarring, cataracts, blood or debris in the vitreous and aqueous, all block the passage of visual information coming into the eye. If reflected light patterns do no hit the retina totally and without distortion, then visual information cannot be processed correctly. Glaucoma causes fluid pressure to push against the back of the eye. If left untreated, retinal cells begin to die in a characteristic fashion.
Visual field loses result in blind spots initially, leading to tunnel vision, and then blindness.
A cataract is a hard substance floating in a fluid medium. When cataracts are present, expect to find a decreased ability to focus. A cataract will block vision most when it is located directly along the axis of the eye. If it is on the edge of the lens, it will probably not affect vision significantly. The reduction in vision acuity caused by a cataract depends upon its density, spread, location, and whether or not there is a corresponding cataract in the other eye.