While water is a better medium for the transmission of sound than air it is a much worse medium for the transmission of light. Basically water absorbs light. This means that the further a ray of light travels through water the weaker, or more attenuated it becomes.
While we who live in the air can often see for several kilometres, sometimes even more than 20 km, a fish is lucky if it can see for 50 metres and generally it can see for far less than this. In muddy, algified or turbulent water visibility can often be measured in mere centimetres. Furthermore, because water absorbs light, the amount of light available for fish vision steadily decreases the deeper you go. At depths of between 150 and 750 metres even the clearest water becomes a twilight zone, and below 1,000 metres you are in the realm of constant night. Nevertheless many fish some, or all of, their lives at much greater depths than this.
As you know visible light is composed of a range of wavelengths, with violet being the shortest and red being the longest. Water absorbs the red end of the spectrum more easily than the blue end, at a depth of only 1 metre 25% of the red light entering the water has already been absorbed. What this means in practice is that the deeper you go the less color you can see, and below 100 metres there is no real color vision at all.
For sharks and Rays this is not a problem as they do not have color vision anyway. The eyes of shallow water species are adapted to have a maximum sensitivity to light of around 500 nanometres, and the eyes of those species that live in deeper waters are adapted to 475-480 nanometres. Most Teleost fish however have color vision, and the eyes of the Trout (Onkorhynchus mykiss) have three sensitivity peaks at, 455, 530 and 625 nanometres.
It should be no surprise then that sight is often far less important to many fish, as a means of perceiving the world around them, than sound, touch, taste (chemistry) and the lateral line system.
Despite all this most fish have good eyes, the exception to this rule being the Hagfish in whom the eyes are vestigial. The eyes of fish are in fact very similar to our own and those of other vertebrates. The main differences being: 1) They have no lachrymal glands (tear ducts), living in water which is constantly washing their eyes they have no need of them; 2) They have no eyelids, although some species do have extensions of the skin that cover part of the eye, and some sharks have a nictitating membrane which can be pulled down over the eye. Scientists believe that this is mostly to protect the eye during feeding.
The basic teleost eye, see diagram, like all vertebrate eyes, consists of an outer case called the sclera, which is only transparent in front of the lens (when it is called the 'cornea'). The cornea has a constant thickness, unlike ours which is faintly lens-shaped, hence it does not refract (change the path of) light rays passing through it.
On the inside the 'sclera' lies another layer of tissue called the 'coroid' layer, and inside this there is the 3rd layer, this is the light sensitive part o the eye, called the retina. The internal part of the eye is divided into two sections by the lens and the iris. The small part of the eye in front of the lens is filled with a fluid called the 'Aqueous Humour' and the space behind the lens, which makes up most of the body of the eye is filled with a fluid called the 'Vitreous Humour'.
The iris of the fish's eye is normally immovable in Teleosts, and very slowly moveable in Elasmobranchs, (sharks and rays), it takes about one hour to change from its most open to its most closed. In comparison the iris of mammals can open or close in a matter of minutes.
The lens of a fish's eye is purely spherical, unlike ours, and it has a refractive index (light bending ability) of 1.65; this is higher than that of any other group of vertebrates. Furthermore the lens is fixed in its shape, meaning its shape cannot be adjusted to facilitate focusing on nearer or more distant objects.
When a mammal wants to change the focus of its eye from a nearby object to a distant one it uses special muscles to change the shape of the lens, thus changing the part of the light entering the eye which will be land perfectly on the retina. Because the lens of a fish's eye is fixed in shape it cannot do this, so instead it moves the lens backwards or forwards.
In teleosts the lens is pulled back towards the retina by muscles called retractors, whereas in sharks and rays it is pulled away from the retina by muscles called protractors. Naturally, when these muscles are relaxed the lens returns to its normal position.
An interesting aspect of the design of the fish's eye, and its placement on the side of the fish's head is that it is bifocal. This is because, while the fish's lens is spherical the eye itself is elliptical. This means that light entering from some directions has a short journey to the retina than light from other directions. What this means to the fish is that its eyes are close focused to light from in front, but distanced focused to light from the side, or from behind.
Also, because the lens is spherical it tends to protrude through the iris, and this combined with the whole eye tending to protrude from the fish's head, and the normal sinuous nature of a fish's movement means that fish often have pretty good all round vision.
Whilst most fish have their eyes placed on the upper side of their heads, some interesting variations have evolved. A deep sea fish called Opisthoproctus soleatus has telescopic eyes that are placed on its head so that they look straight up towards the sky. The Mud Skippers, Periopthalmus sp., have their eyes placed on prominent turrets allowing good all round vision. When they are out on the mud they can also withdraw their eyes into the turret to clean and lubricate them.
The Four-eyed Fish (Anableps tetrophthalmus) has eyes that rise above its body, each eye is divided into two parts with the upper part being adapted for seeing in the air and the lower part being adapted for seeing under water (like whirly-gig beetles of the genus Gyrinus). In the young of Idiacanthus fasciola the eyes are found on the end of stalks projecting from the upper sides of the head. Finally many of the deepest living fish, and those that live deep in caves have very small and often non-functional eyes.
The retina of the fish's eye is made up of 'rods' and 'cones', the rods detect only the presence or absence of light and the cones detect colour. As I said above, most fish, except for sharks and rays can detect colour. Rods are much more common than cones, and usually there are 4 or 5 rods for every nerve cell, whereas cones normally have only one per nerve. The ratio of rods to cones is very variable between species, but as a general rule, the deeper a fish lives the less cones it has.
Within the cones the light sensitive chemicals, the ones that actually catch the light and convert it into an electrical impulse, Rhodopsins in fresh water fish and Porphyropsins in marine fish, however fish that live below 500 metres often Chrysopsins instead as these chemicals are particularly sensitive to the blue end of the light spectrum.
In most Teleosts a posterior part of the choroid layer is enlarged and filled with blood vessels, this is called the 'Falciform Process'. The nerve that carries the electrical impulse from the retina to the brain is called the 'optic nerve', and the part of the brain that receives this data is called the 'optic lobe'.
Many Teleosts and most Elasmobranchs possess a 'Tapetum lucidum', a layer of reflective material at the back of the eye that sends light back passed the rods and cones. In Elasmobranchs the tapetum lucidum is found in the choroid layer of the eye and is composed of guanine crystals. In Teleosts the tapetum lucidum is located in the retina (behind the rods and cones) and is either composed of a) guanine crystals as in Bream (Abramis brama) and Anchovy (Anchoa mitchilli), or b) a melanoid lipid as in Garfish (Lepisos teidae) and Catfish (Siluridae).
In some fish other parts of the body may be sensitive to light. In Scyliorhinus species, some other juvenile fish, and in some sharks, the 'Pineal Gland' (in the centre of the forehead) is light sensitive. Larval Sea Lampreys have light sensitive cells in their skin, even in their tail.
Some fish, such as Rainbow Trout and Goldfish, and possibly many others, can detect UV light.
Many fish, even when unmoving have a greater monocular field of vision than man, but much less binocular vision.
In some deep water fish the eyes are very large, with large lenses focusing on small retinas. This greatly increases the eyes sensitivity to light, in Myctophium rissoi the diametre of the the anterior part of the eye is equal to 50% the length of the head of the fish.