Seals have well developed underwater vision. This is important, because seals spend a lot of time under water where there is little light. A Seals retina, which is a lot like a cat’s retina, has a so-called tapetum. The tapetum is a reflecting layer immediately behind the retina of the eye, which reflects extra light into the retina. A seal’s lens is convex, which is an evolutionary adjustment and makes it possible to see clearly under water. Seals have bad eyesight above water. It can be compared to that of a very nearsighted human. It is likely that seals can see different colours. However, this has not been scientifically proven yet.
The inside of a seal’s eye is like that of a human being. A seal, like a human, has a pupil and a lens as well as a retina. Even so, there are great differences between a seal eye and a human eye. This is primarily because of the seal’s well-developed underwater vision. Unlike seal’s eyes, our eyes are not adjusted to see clearly under water.
Above water, a seal’s pupil is hard to distinguish. It is not round, but appears as a very thin line. Under water, a seal’s pupil becomes round like ours. Many creatures that need good sight underwater have round lenses, like marbles. Seals have this too. The lenses of land animals such as human beings are usually flatter.
A seal’s iris is always brown, or almost brown. Seals have larger blood vessels in their eyes than humans. This keeps the temperature in their eyes stable when they dive deep. A seal’s retina is also different to that of human beings. A seal has photoreceptor cells in its retina (rods and cones) that process images into signals (like a photo camera) which are sent to the brain (just like when you send photo’s to your computer by using a phone-line or internet cable).
The ‘cones’ make it possible to see colours. Humans have three cones to see colours, seals only two. You could say that seals are slightly colour-blind in comparison to people. Dogs and cats cannot see colour so well either. Seals do not experience problems 
with this at all, because under water they do not need to see that many colours.
Seals have reflecting eyes. They have an extra eyelid that reflects light, just like cats do. If you shine a torch straight into their eyes, their eyes will reflect the light. However, seals eyes do not reflect as strongly as cats eyes. Animals with an extra eyelid have better vision at dusk. All nocturnal animals have a reflecting layer like this.
A seal’s eyesight is not really very good. Above water it sees very little; it could do with a pair of strong glasses. It probably makes more use of its whiskers than its eyes to help it catch fish. Its whiskers have nerves at the base of the whiskers, which allow it to feel whether there are fish nearby.
