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More About Light The vertebrates and some of the larger molluscs, such as the squid, have developed a camera-like eye with only one large lens. This eye gives a more precise image than the compound eye. It has been estimated that the compound eyes of a honey bee have only 1% of the efficiency of human eyes.
The camera-like eye is able to control the amount of light entering it. It does this with a structure called the iris. In the light the iris is large and the light can only enter the eye through a small, black circle in the centre of the iris, called the pupil. In the dark the iris becomes smaller and the pupil becomes larger. This allows more light to enter the eye. Nocturnal animals, such as the owl, have very large pupils.
The positions of the two camera-like eyes on the head are not the same for all vertebrates. Humans have three-dimensional vision, also called binocular vision, because the right and the left eyes see the same things but from slightly different angles. Birds, however, only have two small areas of binocular vision but, because of the position of their eyes, they are able to see all around them at the same time. The camera-like eye contains two kinds of light receptor cells called rods and cones. The rods are used in night vision and form a black and white image. The cones are used for daylight vision and, in many animals, form a coloured image. The sharpness of the image is dependent on the number of cone cells present in the eye.
The hawk's eye has many more cone cells than the human eye and its vision is very sharp; it is eight times sharper than human vision. The elephant's eye, however, has very few cones. The elephant can only manage to see blurred objects which are near to it and it has very poor distance vision.
The large eyes of the owl cannot be moved up and down or from side to side. The owl has to move its whole head to see to the side. Fortunately, its head is able to move more than 1800 in either sideways direction, so the owl can turn its head to look directly behind it!
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