How eyes work

There are two types of eyes: camera-type eyes (like ours) and compound eyes (like the ones insects have)

Camera-type eyes

Most animals (birds, mammals, fish, reptiles) have basically similar eyes. They work like a camera does: light is focused from a lens onto the film. In our eyes, the film is the retina.

How the human eye works : basic version

Light enters the eye through the cornea, pupil and lens. It hits the back of the eye, which is called the retina. Special cells, called rods and cones, process the incoming light and send it as electrical impulses to the optic nerve. The impulses reach the brain and is interpreted as an image.

Eyes are just over 2 cm in diameter.

Our eyes can distinguish over 500 shades of grey and over 2.7 million colours!!

Kids health website has a good video for kids about how the eyes work

Go to Eye video

Parts of the eye & their function

Cornea: clear outer part at the front of the eye. It focuses incoming light.

Conjunctiva: tissue that covers the inside of the eyelids and the sclera (white of the eye).

Iris: the coloured part. It has muscles attached to it so it can widen (dilate) or contract the pupil.

Anterior chamber: filled with aqueous humour (clear liquid) and is between the cornea and iris.

Pupil: the black part that can adjust to let more or less light in.

Lens: curved structure that helps focus incoming light onto the retina. The lens can flatten to help focus distant objects.

Ciliary body: this structure also include the ciliary muscles that hold the lens in place and change the shape of the lens. When the muscles relax, the lens flattens. When the muscles contract, the lens thickens. This focuses close up objects. Aqueous humour is produced by cells in the ciliary body.

Sclera: the white of the eye. It gives protection as it is made up of tough connective tissue (collagen and elastic fibres)

Choroid: a layer of tissue between the sclera and retina containing lots of blood vessels.

Vitreous humour: the clear, jelly-like substance inside the eye.

Retina: is a light sensitive membrane towards the back of the eye. There are millions of special cells, called rods and cones, each with a different function.

Rods: the are about 125 million in a human eye. Rods main function is for monochrome vision in poor light (i.e. at night) and for recognising shapes.

Cones: there are 6 to 7 million cones in a human eye. There are 3 types, each sensitive to a different colour (red. green or blue). The main functions are colour perception and fine detail.

Macula: the most sensitive part or the retina. Part of the macula, called the fovea, has the highest concentration of cone cells.

The optic nerve carries the electrical impulses to the brain.

Compound eyes

Some insects and crustaceans have big bulging eyes. These are called compound eyes. Unlike human eyes, they are made of thousands of photoreptor units called ommatidia. These tiny units are packed closely together.

How compound eyes work : basic version

Light enters each ommatidium through the cornea. The ommatidium also contains pigments that can detect colour. Each ommatidium captures light from a different angle and contributes to the image the insect sees.

The head of a wasp showing three ocelli (in the middle, on the top of the head, between the two compound eyes).

There are 6 photoreceptor cells per ommatidium, arranged around a central rhabdom. The rhabdom collects information from the photoreceptor cells and that information is sent to the brain via the optic nerve.
Not all compound eyes are the same. Each insect has its own adaptations to enable it to "see" what is important to them.
The number of ommatidia vary between species too.
Each ommatidium is hexagonal so they fit closely together well (like honeycomb).
Compound eyes produce poor image resolution i.e. the image is blurry. However, they can detect fast movement around them. This is handy if you are someone else's meal!
Compound eyes are useful in flight
Flowers have UV markings on them (that humans can't see). Compound eyes enable insects to see UV light - handy if you need to find flowers for food.
Often insects have two compound eyes plus smaller "eyes" or ocelli, which are basically light & dark sensors.

Differences in animal vision

Here are some images I found showing differences between human vision and a dog, snail, gecko, giant clam & jumping spider.

Human vs Dog

Dogs only have 2 types of cone cells for yellow and blue-UV (remember, we have 3). Dogs do not colour as well as we do.

Human vs Jumping Spider

The forward facing eyes of the spider give it high-resolution vision. They also see a wider range of colours than us i.e. UV (ultraviolet) so they see more details on the petals of flowers (same with bees).

Human vs Gecko

Geckos are mostly nocturnal so need good night vision. they can see about 350 times better than us at night!

Human vs Snail

Snails have very poor eyesight. They can't see in colour or focus images.

Human vs Giant clam

Giant clams have tiny pinholes along their body. These have no lens and although they can sense different colours, they see a colourful undefined image. Clams are stationary. All they need is to be able t sense when a threat is nearby so they can close their shell or squirt a jet of water

You may like to check out the Natural History museum for more amazing animal eye facts

https://www.nhm.ac.uk/discover/amazing-eyes-vision-champions.html

Pupil shapes

Some animals have a different pupil shape to our round pupils. They still have the same function of letting light into the eye but different shapes allow light to enter from different angles. Here are 7 different pupil shapes:

Above: chimpanzee

Round pupils

found in large predatory mammal like lions, wolves and humans. We used to hunt for our food
usually day-time hunters (found in some nocturnal hunters)
eyes are usually high off the ground
field of view in narrow (eyes looking forward and on the front of the head)
have good depth perception
less peripheral vision but animals with round pupils are at the top of the food chain with less need to escape from predators

Who else has round pupils?

humans
dogs
large cats

Above: tuatara

Vertical slit pupils

found in smaller predators that ambush their prey (pounce on them) and hunt close to the ground
pupil can expand more than a round one to let more light in (ideal at night)
this pupil allows the animal to judge short distances very precisely

Above: horse

Horizontal slit or oblong/rectangular pupils

belong to prey animals (i.e. might be eaten by other animals)
excellent wide field of view as the eyes are on the side of the head
good field of vision to see in front and behind to both look for predators and check for obstacles when fleeing

Horizontal slit pupils

belong to prey animals reptiles & amphibians
the pupil can open wider to let in plenty of light, whether it is day or night.

Left: octopus

Crescent pupils

this shaped pupil decreases the light distortion caused by water
may boost contrast
provide a wide field of view.

Left: Triactus, a species of catfish

W-shaped pupils

possibly a modified horizontal slit pupil
in low light, the pupil opens up to a more circular shape
this shape enables light to enter the eye from many directions.

Vertical beaded pupils

in bright light, these vertical pupils can decrease to very thin vertical slits with multiple pinholes ("beads")
each bead works together to help depth perception
found in animals that stand not too far off the ground

Left: gecko

Who else has vertical slit pupils?

small cats
foxes
crocodiles
some reptiles

Who else has oblong pupils?

goats
sheep
horses
deer/antelope
cattle

Who else has horizontal slit pupils?

frogs
toads
snakes
octopi

Who else has crescent pupils?

stingrays
catfish
flatfish

Who has W-shaped pupils?

Cuttlefish are the only animal with these pupils

Who has vertical beaded pupils?

geckos
some fish