I think my pet has cataracts??
The lens has numerous lens fibres and is composed primarily of protein and water. Its function is to act as the focusing structure within the eye. To simplify things, lets think of the lens like a peanut M&M which has three layers: an outer candy shell, a chocolate layer, and a peanut in the centre. The lens then has an outer layer called the capsule or 'bag' which is equivalent to the candy shell; inside of the capsule is the cortex which is like the chocolate and in the centre of the lens is the nucleus, like the peanut. As we/our pets age, the lens fibres continue to grow. Because it would not be healthy for the lens to become much larger, as the fibres grow they instead become more compact in the centre of the lens, called the nucleus. this happens more significantly starting at age 5 in our pets and age 40 in us. This change causes a sublte 'blueness' to the lens and can appear like a circle within the lens. In people when this occurs we call it presbyopia; which describes increased difficulty for the lens to focus on near objects. This is usually when we require the need for bifocals. Due to differences in how people and pets focus or accommodate, our pets do not experience the same difficulty in focusing and therefore we do not notice significant vision problems in our pets with nuclear sclerosis and don't have to go find them bifocals.
Cataracts on the other hand represent a true opacity within the lens and can interfere and affect vision. The most common cause of cataracts in dogs is an inherited or genetic predisposition and the second most is diabetes. This is different than in cats where the most common cause is secondary to inflammation inside the eye called uveitis. Depending on the degree of cataract the appearance can be variable, ranging from a pinpoint white spot to more diffuse white or greying to the lens. Obviously the more of the lens involved the more pronounced the vision deficits.