One factor is the transparency of the eye structures in the line of sight. Any disturbance in the clarity may impair best visual acuity or cause problems with glare or cause other visual distortions and distractions.
Cataracts cause a clouding with distortions and lack of clarity in the lens of the eye.
This is a very common condition with over one million surgical procedures for cataracts performed each year in the United States. The anatomy of the eye is similar in many ways to a camera. First, light passes through the cornea, the clear portion of the outer eye that contact lenses are placed on. This area is sensitive to touch and is similar to a filter placed over a camera's lens. It usually remains clear unless there is an infection with scarring, trauma or vision correction surgery with complications.