Naked Science Forum
General Science => General Science => Topic started by: Elbe Coetzee on 30/10/2008 09:04:56
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Elbe Coetzee asked the Naked Scientists:
Hi there, Chris,
I absolutely love your program with Redi on 702. I always listen to talk radio when I drive between appointments and your program is so interesting and delightful. By the way, you have a very pleasant and easy so listen to voice, it often sounds as though you're about to smile or giggle.
I love the politeness of the English, the heritage and history of England, even the weather. My husband and I are going to live in Reading for a year and a half, we're both looking forward to the experience. Living amongst our 'own' people, Africa indeed is a tough continent!
Do 'chinese-type' people see the same 'amount' - vertically & horizontally etc. as 'big-eyed-people'?
Kind regards
Elbe Coetzee
Centurion
Pretoria
What do you think?
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The shape of the eye is the same for all, regardless of the shape the of the aperture the lids make. So, yes, they do the same visual field as Westerners.
A year and a half in Reading eh? With good behaviour and some luck you'll be out after 12 months!
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As blakestyger said, the aperture of the skin around the eye makes no difference to the height or width of the field of vision. Even the aperture of the iris makes no difference to these factors. It merely controls the amount of light entering the eye.
So you love our weather eh! Do us all a favour and take it back to Pretoria when you go home, and please bring some of your weather over with you when you come.
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Cats' eyes, on the other hand, have pupils which become vertical slits in bright light (as opposed to our round ones).
I have a theory that this is to preserve vertical resolution and also depth awareness even when the eyes are 'stopped down'.
What do you think?
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The aperture of a lens makes no difference to it's angle of view.
Look at these two pictures, they are identical in every respect, same camera, same lens (set at 50mm focal length), same camera position, same lighting, but in one shot the lens was stopped down to just f4.5 and in the other all the way down to f32.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi77.servimg.com%2Fu%2Ff77%2F12%2F57%2F44%2F24%2Fdsc_0310.jpg&hash=4b6ece9d602d1a7ec54dcdd0c1026cff)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi77.servimg.com%2Fu%2Ff77%2F12%2F57%2F44%2F24%2Fdsc_0311.jpg&hash=e3ab5eb4882c14e7e210b39a71071afd)
The only visible difference between them is in the depth of field.
The top pic, taken at f32 has a greater depth of field but has not lost any of height or width of the angle of view of the lower picture which was taken with the lens openned up to an aperture of f4.5
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A change in the height and width of the angle of view would only be affected if the distance from the lens to the focal plane and/or the size of the focal plane sensor were to be altered.
In the case of a camera, the rear element of the lens remains static, therefore the distance between the lens and focal plane is always constant. The focal plane, being either the celluloid film or digital sensor, does not change in size. The same is true of the eye, except the eye has just the one static lens element where the camera has several, and the sensor in the eye is the retina.
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I think males with heavy brows will have their field of view restricted by them.
To extend the photographic analogies above, it would be like a too-long lens hood causing vignetting.
The brow ridges of course protect the eyes from blows to the head, (why they are a male feature), but the trade-off is they do restrict the upper field of view: If a heavy-browed male wishes to look at the top shelf magazines he will have to tilt his head back when others need only move their eyes.
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If the vertical slit in cat's eyes worked like a pin-hole camera, without a lens, it would increase the horizontal resolution, but as there is a lens it shouldn't make much difference.
I say that it shouldn't make 'much' difference because the amount of spherical (do organic lens generally have spherical curves?) and chromatic aberration increases with the amount of curvature, so reducing the area of the lens that is used effectively makes the lens 'flatter' and reduces these aberrations.
The image from vertical slit irises should therefore have slightly less horizontal aberration, and slightly more vertical aberration, than a round iris. Ultimately though, the retina needs to be sensitive enough to make use of the info.
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We perceive our entire field of view as equally sharp, but only the central field (about 2o ) is sharp on the retina (fovea),
the eye moves to place different parts of the scene over this sharp central area (http://en.wikipedia.org/wiki/Saccade#Function),
the brain builds up a picture of the scene using this high-resolution information.
The fovea comprises less than 1% of retinal size but takes up over 50% of the visual cortex in the brain.
http://en.wikipedia.org/wiki/Fovea
The distribution of neurology in retina and brain has evolved to match the limitations of the lens, (poor edge resolution / chromatic abberation).
Spherical "abberation" is not a problem in the eye because the retina, (analogous to film in a camera), is hemi-spherical, (unlike flat film).
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Spherical "abberation" is not a problem in the eye because the retina, (analogous to film in a camera), is hemi-spherical, (unlike flat film)
Doh! - of course - thanks.
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Just to follow up on this question. If all eyes see about the same field of vision, evolutionarily speaking, why are larger eyes generally more attractive in most cultures around the world than otherwise?
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Having large eyes, relative to the size of the face, is a feature of infants.
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The emotional response to large eyes so readily triggered that the large eyes of a cartoon animal can trigger
an emotional response in some people: it's not a human, it's not even realistic, but still some people will go "awhh".
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If the vertical slit in cat's eyes worked like a pin-hole camera, without a lens, it would increase the horizontal resolution, but as there is a lens it shouldn't make much difference.
'Fraid not: the bigger vertical aperture would lead to a higher acuity in the vertical.
"Angle of first diffraction zero is inversely proportional to aperture"
This applies to a hole or a lens. The lens merely changes the position of the image. But I wasn't concerned with the diffraction limit.
What I was referring to is the depth of focus which would be more sensitive when focusing on horizontal edges (i.e. vertical spatial frequencies) then on vertical ones.
Or are we agreeing? Can't be sure.
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I think we're disagreeing [:D]
There's no diffraction in a pin-hole camera - the size of the 'pin-hole' you'd find in a pin-hole camera is zillions of times larger than the wavelength of light.
If resolution increased with aperture size in a pin-hole camera you could do away with the aperture entirely - it would be just like just holding the film towards what you were trying to photograph, without using a camera at all.
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Diffraction does noticeably soften the image with large f-numbers, e.g. f22 on a 50mm lens, i.e. a 2.3mm aperture.
So the diffraction with a pinhole (say 0.25mm) is severe: the resolution of pinhole images is very poor because of diffraction.
(Pinhole images do have "infinite" depth of field though)
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http://www.pinholephotography.org/
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Well, I've got to admit that I was only aware of the 'circle of confusion' issues and didn't know about the diffraction effects.
http://en.wikipedia.org/wiki/Pinhole_camera#Selection_of_pinhole_size (http://en.wikipedia.org/wiki/Pinhole_camera#Selection_of_pinhole_size)
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The optimum aperture for a typical camera lens, (i.e. which produces the sharpest overall image), is usually f8-f11. Using a smaller aperture, say f22, reduces the resolution from this optimum, i.e. small apertures (large f-numbers) soften / blur the image by diffraction.
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I think males with heavy brows will have their field of view restricted by them.
My pa, who was doing a PhD (or possibly it was a post doc) using lasers had to have a visual field eye test. The optician got all worried that his visual field had been damaged until he observed that the thing at the top of what he could see was undoubtedly eyebrow so probably the effect was physiological rather than pathological...
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My pa, who was doing a PhD (or possibly it was a post doc) using lasers had to have a visual field eye test. The optician got all worried that his visual field had been damaged until he observed that the thing at the top of what he could see was undoubtedly eyebrow so probably the effect was physiological rather than pathological...
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My pa, who was doing a PhD (or possibly it was a post doc) using lasers had to have a visual field eye test. The optician got all worried that his visual field had been damaged until he observed that the thing at the top of what he could see was undoubtedly eyebrow so probably the effect was physiological rather than pathological...
@antony96 , welcome to the forum.
Can I ask why you have copied a previous post and posted it as your own?
Did you mean to quote it and then comment? If so you will find a quote function in the ‘actions’ drop down top right of the post.
I hope you enjoy the forum and are able to participate in any threads that interest you.