96% of all animals possess a complex optical system. Some animals in the world have something called simple eyes. Such as pit eyes, which are eye-spots that are set into a pit to reduce the angles of light that enter and affect the eye.

Complex eyes contain retinal photosensitive ganglion cells. These cells send signals along the retinohypothalamic tract to control pupillary light reflexes.

Some animals need eyes that can handle large amounts of detail and visual angles. Visual acuity is controlled by cone cells. It is often measured in cycles per degree (CPD). This measures angular resolution. This resolution in CPD can be measured by bar charts with white and black stripe cycles.

Human eyes max out around 50 CPD for resolution. A rat has about 1 to 2 CPD.

Some animals such as crustaceans or insects, have eyes consisting of an array of numerous small visual units. Rather, eyes within eyes. The resolution for these eyes can be hard to measure. But they generally have a lower acuity than other vertebrate eyes and mammals. More on this later.

Some other animals such as snakes have eyes called Pit Eyes. Pit eyes are also known as stemma. These are eye spots that are set into a pit to reduce the angles of light into the crevice. These eyes are thought to be the precursor to simple eyes. Some animals with these eyes have reduced the size of their aperture by incorporating reflective layers behind their receptor cells. Pit vipers also have eyes that sense thermal infra-red radiation. They also have optical wavelength eyes.

Pit Eyes can also contain spherical lens eyes. These eyes have higher refractive indexes in their lens. This in turn reduces blur and increases resolution in the eyes. It also decreases the focal length of the eyes, and more light to enter. These can be seen in gastropods.

Some eyes in animals have multiple lenses in their eyes. Most of the time this happens in marine animals. For example, the coppod patella has three. In its eyes, it’s outermost lens is parabolic in nature, while the others fit smoothly behind it. However, other copepods, such as the copilia, have two lenses arranged like a telescope. Multiple lenses have also been seen in eagles and jumping spiders.

In combination with these eye types that we have discussed. Some contain refractive corneas. Mammals , birds, reptiles, and other animals with this trait have vitreous fluid in their eyes with a higher refractive index than the air. They usually also have spherical lenses in their eyes that produce spherical aberrations. When these two character traits come together the lens is held together with an inhomogeneous lens material. This causes a wide field of view to develop in the animal's sight. Refractive corneas are only useful outside water. This is because the refractive index of water is often close to or the same as the vitreous fluid in the eye. So it causes the animal’s sight to diminish.

Instead of having refractive liquid behind the lens of the eye, some animals have reflective eyes. This means instead of having a lens inside their eye they have a series of mirrors to reflect the images to a central point. This means if we were to look into those animals' eyes, we would see our own reflection. Some animals that have these eyes are rotifers, copepods and flatworms. But because they are so tiny, their eyes can’t really reflect anything through them. This makes their eyesight very poor. However, some larger animals such as scallops that use reflector eyes, line their shell with them. This is so it can detect moving objects as they pass around it.

Now, back to compound eyes. Arthropods such as the bluebottle fly have compound eyes. Compound eyes have thousands of receptors to capture an image. These receptors are usually located on convex surfaces, and this points them in different directions. This gives the creature a very large view angle. So they can detect very fast movement, and the polarisation of light.

There are two main groups of compound eyes. Apposition and Superposition eyes. Apposition eyes form multiple inverted images. Superposition eyes form one large image.

Apposition eyes are the most common type of eye. They are found in all arthropod groups, and horseshoe crabs also have them. They work by gathering a number of images, eye from each sensor or eye.

Superposition eyes are harder to narrow down. They are divided into three different types:

Reflecting and Refracting: This eye has a gap in between its lens and rhabdom. It has no side walls to hold everything together. It takes light in, and then reflects it to the side angle on the other side of the eye. It is normally found in nocturnal insects. It can create images 1000 times brighter than apposition eyes.

Parabolic Superposition: This eye refracts light and uses a parabolic mirror to focus the image. It combines features from both superposition and apposition eyes.

Non compound eyes are the opposite of compound eyes. These are very simple lens eyes that can be found in vertebrates, cephalopods, annelids, crustaceans and cubozoa. More on this later.

There are smaller categories of compound eyes. One of them being the Strepsipteran compound eye. This eye is made up of a bunch of simple eyes clustered together. Because each eye is a simple eye, each one forms an inverted image. These images are then combined in the brain to form one big image. Honey bees and other insects share these eyes.

Some insects have a single lens compound eye. This is a mix between a superposition and a multi-lensed compound eye.

The pigmentation of animal eyes is always hard to understand. But the process is usually similar to how our eyes get their color. Pigment molecules will vary from species to species. It can also be used to define the distance between different animal groups, and when they broke away from each other.

Opsins are pigments involved in photoreception. Melatonin pigments are usually used by the eye as a shield to block photoreceptor cells from light.

Opsin comes in two different forms., and has two different purposes when it comes to vision. C-opsins are associated with ciliary photoreceptor cells. R-opsins are associated with rhabdomeric photoreceptor cells. Ciliary photoreceptors are UV-sensitive and mediate sight in non-directional UV light. Eyes with rhabdomeric photoreceptor cells are designed to avoid UV light. The cells do this by determining the incoming ratio of blue to UV light, and then tell the creature what to stare at.

Vertebrates usually have ciliary cells with c-opsins in them. Invertebrates have rhabdomeric with r-opsins. However, different invertebrate species can also have c-opsins in their eyes.

Now let’s talk about some specific eye types per animal.

Eagles:

Eagles are known for their extreme sense of vision. They have excellent long distance vision, and can see eight times as far what a human can. Eagles can also quickly shift focus in their eyes, and create “zooms” to see camouflage animals. They can also see a wider range of colors than humans, such as colors in the UV spectrum. However, their vision fails them during the dark as they don’t have good night vision.

Owls:

These birds are basically night eagles. They have great night vision. Their eyes don’t sit at an angle in their face, so their head is forced to rotate, and face directly forward. This makes their vision a bit binocular. Their eyes can’t move, or roll around in their sockets either. So the full rotation of an owl’s head is locked in at 270 degrees.

Owl eyes contain more rods than humans, and this makes them more light sensitive. Their irises can widen to make sure all light comes into contact with their retinas. Because their irises can adjust, they are not limited to only seeing in the dark. They can also see during the day, but their vision is blurry and they can’t see colors well.

Owls and other night animals have reflective surfaces behind their retinas known as the tapetum lucidum. This layer allows light to reflect back into the animal's eye even if it has already passed through. This can give the animal a second chance to see movement it might have missed.

Mantis Shrimp:

These animals are considered to have one of the most complex eyes in the animal kingdom. Humans usually have three types of cones in our eyes. These cones allow us to see the color spectrum from red to violet. Mantis shrimp have 16 different types of cones. We still don’t know to this day what a mantis shrimp's full range of color is. As most of the purposes of these cones, have not been identified. But we do know that they have an advanced color recognition system.

Their eyes can move independently of each other, and in no way does this compromise their vision. Their color receptors allow them to pick up small changes in color. This helps them evade predators.

Sheep and Goats:

These animals have some very unique eyes. Their pupils are shaped as horizontal lines. Pupil shape helps the animal customize it’s sight for a specific environment. Daytime hunting animals usually have forward facing eyes with round pupils. Small animals that hunt during the day and night usually have vertical slit pupils to help them with depth perception.

Meanwhile, goats and sheep have pupils in the shape of lines because they graze out in the open and need to be always alerted of predators. Their wide and narrow pupils give them a wider field of vision when they lower their heads to eat.

Some general animal eye facts:

Tarsiers have the largest eyes in the animal kingdom relative to their body size.
Trilobites were one of the first animals to develop complex, compound eyes.
Some species of dragonfly can have more than 28,000 lenes per compound eye. This is more than any other living creature. They also have an almost complete 360 degree vision as well.