All Eyes

Ridgemonkey Winner

Q I spotted this picture of a fish with freaky eyes on the web. Why do some fish have bigger eyes than others? Chris Harrison, via email.

A Just occasionally you see a fish picture that really screams out at you, and there is no doubt that the one shown here screams louder than most.

Fish eyes are similar to ours, apart from the fact that they have a spherical lens, which better interprets the passage of light through water, a far denser medium than air.

The retina of a fisheye is not too dissimilar to the human retina, being multi-layered and lined with a network of photoreceptive rod and cone cells. Rod cells are for monotone vision only, and they simply measure the quantity of photons hitting the back of the retina – the level of illumination. They provide high visual sensitivity in low-light conditions, but with a loss of definition, resulting in blurred images. They give no sense of colour, and images are in shades of grey only.

Cone cells give much higher resolution than rod cells, and allow for detailed colour vision in a well-illuminated environment, by analysing and interpreting the different wavelengths of the colour spectrum (red, orange, yellow, green, blue, indigo and violet), which comprise white light. But, as I always say, it’s science, so it’s not quite as simple as that.