Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: Joe Barber on 12/01/2011 12:30:03
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Joe Barber asked the Naked Scientists:
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
What do you think?
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Your eyes will adapted to the task required for your species survival.
Birds often are without cones... or at least some of them.
The cones are essentially inactive after dark, so they would be useless for an owl.
So, various birds of prey would have lots of rods... for very high visual acuity, without the color capabilities from the cones.
Owls might have the same thing plus larger eyes for night-time visual acuity.
A whale would certainly be adapted to its environment, but I don't know if it has better visual acuity than a dolphin.
Your ideas about retina size sound reasonable. High density of cells would provide high acuity in normal lighting. Larger cells would be as useful as smaller cells with dark vision as you might be concerned with events of just a few photons striking the retina.
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That really is an impossible question to answer, since different eyes are adapted for different purposes.
The horse has the largest eye of all land mammals, giving it 180o vision from each eye. The eagle has probably the best eye for pin point vision and the chameleon has eyes which can move independently of each other, giving it virtual all round vision.
The Colossal Squid has the largest of all eyes at 30cms across, but what it can see in the murky, dark depths of the ocean is unknown.
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Maybe I should have been clearer. The question is about different sized eyes in different sized adult people. Consider a person who is 6-foot 4-inches tall with a proportionately larger retina than his or her friend, who is 4-foot 11-inches tall. I'm asking about an apples to apples comparison.
You could make a comparison to digital cameras with different sized sensor chips.
Is there a functional advantage in sight to being a larger person?
Thank you,
Joe
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Your typical uncorrected human eyesight is 20/20.
I don't think there is a major difference between schoolkids and adults.
Nor have I heard of a size difference. Are you sure the size of the eyes varies significantly?
It would be an easy study to conduct, but you would have to determine how to include/exclude people for glasses and focusing issues.
Perhaps the US Airforce would already has some of the data.
If there is a difference, it is also possible that the typical eyecharts aren't sensitive enough to pick it up, although I'm not sure an eyechart with smaller steps would be meaningful either.
(larger text) 20/30 --> 20/25 --> 20/20 --> 20/15 --> 20/13 --> 20/10 (smaller text)
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People requiring glassess (for distance vision rather than just reading) fall in to 2 catagories, hyper metropic, or myopic. Hyper metropic being long sighted, this being that the focusing power of the cornea and lens are not strong enough for the size of the eye, therefore bringing light to focus behind the macula, i.e the eye is too small. Myopic people have the opposite problem, where the focusing power is to great for the size of the eye, i.e the eye is too large, this obviously being known as short sighted. Also, myopic people are at greater risk of retinal detachment and somthing called myopic degeneration. Hyper metropic people are at greater risk of angle closure glaucoma.
Hope this helps to answer your question.
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[:-\]
There is, of course, a third category of people.
Those of us with egg-shaped eyes... or lenses [:-\]
Astigmatism
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"People requiring glassess (for distance vision rather than just reading) fall in to 2 catagories, hyper metropic, or myopic. "
No
People who require glasses for distance vision rather than reading (or other close work) are all myopic.
Those who need reading glasses but can see distant objects are hypermetropic.
Both groups can also have astigmatism, as can some people who are not long- or short- sighted.
Anyway, another aspect of the question relates to the physics involved. There's a theoretical limit to the resolution of the eye, or any other optical system, which arises from diffraction effects.
Given the size of the eye it would be pointless to have many more receptors because they would have to be smaller. As things are, the size of the receptor is roughly the size of the diffraction limited spot.
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I thought I would look up and see if colorblind people had higher visual acuity since they should have more rods and cones of the types that they actually have. However, I didn't see anything indicating that. Some colorblind people have other visual deficits too.
However, I did see a note that colorblind people may be able to distinguish camouflage or see through it better than normal people.
Dichromats Detect Colour-Camouflaged Objects that are not Detected by Trichromats
M. J. Morgan, A. Adam and J. D. Mollon (http://rspb.royalsocietypublishing.org/content/248/1323/291.full.pdf)
What I also noticed....
Many women are tetrachromats
And, thus, they may actually be able to see more color shades than men [xx(]
http://en.wikipedia.org/wiki/Tetrachromacy#Possibility_of_human_tetrachromats
So, when they're trying to explain the difference between violet and purple... there may be a reason.
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We discussed this question on our show
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.
Click to visit the show page for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/show/2011.01.16/) Alternatively, [chapter podcast=2986 track=11.01.16/Naked_Scientists_Show_11.01.16_7769.mp3](https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2FHTML%2Ftypo3conf%2Fext%2Fnaksci_podcast%2Fgnome-settings-sound.gif&hash=f2b0d108dc173aeaa367f8db2e2171bd) listen to the answer now[/chapter] or [download as MP3] (http://nakeddiscovery.com/downloads/split_individual/11.01.16/Naked_Scientists_Show_11.01.16_7769.mp3)