Dogs can get cataracts in their eyes just like humans can. Cataracts in dogs affect a structure in the eye called a lens. The lens is the globoid structure in the eye filled with a clear jelly-like substance. The lens is very important for sight, as it focuses light onto the retina to create an image that is in turn recognised by the brain. In a cataract, the normally clear lens undergoes changes that cause it to become opaque. The more opaque the lens becomes, the less it is able to fulfil its role in sight.