Do large eyes see better than small ones?

16 January 2011

Question

Dear Naked Scientists,

Do large eyes see better than small ones?  Does size matter (in eyes)?

There are three possible answers:
1) There are more light gathering rods and cones of someone with a larger retina than a smaller one, which would result in a higher resolution image in the brain for a larger retina.
2) The number of cones and rods is the same on a small or large retina, but each cell is larger and therefore gathers more light, resulting in better low-light vision for someone with a large retina.  Or
3) The number and size of light gathering cells is roughly the same for adults with larger or smaller retinas, but there are larger gaps (and more supportive cells) between rods and cones in larger retinas.  This would result in equivalent eyesight among adults with different sized retinas.

Which possible answer is correct?

Thank you very much,

Joe

Answer

Sarah - Well, I think the idea behind - if you have a bigger eye, you want to get as much light onto your retina as you can, so you can get a more detailed picture of the outside world. So you often see large eyes in nocturnal animals that rely on sight, things like aye-ayes and bush babies, or fish, or indeed cephalopods like giant squid in the deep sea where it's constantly dark. So, either they can be predators and they need to see their prey or they need to be determining distances to jump between branches, that sort of thing. It's kind of not necessarily just a question of how big the eye is, but you also need to think about what you're doing with it. So, if you have a large pupil, you'll need to have a larger eye or a deeper eye in order to get the focus depth right. Because if you have a big aperture and you just let the light into the eye, then you need to make sure that it's deep enough in order for you to get the focal length, otherwise you'll have a big blurry picture. So, you still need to have the focusing power of a lens going on there. So if you can't focus the light, it's quite useless. Also things with bigger eyes that need to see more often have a larger optic lobe in their brain because they need more processing power, and that sort of thing. Now, this is just the case for simple eyes here, we're not talking about compound eyes which are the sort of things that invertebrates like insects and crustaceans have. Although I do know that mantis shrimps have the most complex eyes of any invertebrate and they have 16 different types of light receptors, whereas we only have four.

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