This is something I've been wondering about for a long time. As an Art teacher I teach linear perspective to my students. That's the effect that makes parallel lines moving away from us appear to converge until they 'vanish' at a particular point, and objects appear to get smaller as they get further away. That apparent diminution happens in a predictable, measurable way. What I wonder is, why? What is it about the way we see that makes that appear to happen? Is it to do with the way our eyes are constructed? I assume it has something to do with light travelling in a straight line, but ... It really bugs me at times because I can't get my head around why something should look small just because it's far away. I've thought maybe it's got to do with field of vision and relative sizes and that kind of thing, but I'd really like to know waht are the mechanics of the effect. Anybody help?
Posted by mr_porteiro_head (Member # 4644) on :
It happens because things get smaller apart the further they are from your eye. Two parallel lines appear further apart when then are closer to you, and closer together they further they are. As the distance to those lines approaces infinity, the apparent distance between them approaches zero.
Posted by MEC (Member # 2968) on :
It's part of depth perception. Some other things that are similar are Texture gradients, overlaps, and motion parallax.
Imagine two spheres, one farther than the other, then imagine them emmiting themselves at all angles. As the emittions aproach you they spread out more, the farther one spreads out more than the little one when they reach your eyes, makeing you see it smaller.
Or like this, the same spheres, but imagine cones coming out of your eyes and reaching the outside of each sphere, the farther cone would be narrower.
[ December 06, 2004, 02:43 PM: Message edited by: MEC ]
Posted by mr_porteiro_head (Member # 4644) on :
I have no idea what you just said, MEC.
Posted by MEC (Member # 2968) on :
I meant that linear perspective is one of the ways we percieve depth, the other things i mentioned are other ways.
Overlap(Interposition) - when a closer object covers a farther one.
motion parallax - farther objects move slower
Posted by Cashew (Member # 6023) on :
MEC I'm with Mr Porteiro Head Thanks for the answers guys, but they don't address 'Why?' Why do things look smaller when they're further away? What's actually happening to create that effect? Maybe I'll reread the other posts to see if I can understand them better ...
Posted by MEC (Member # 2968) on :
Less photons reach your eyes than when it is closer.
Posted by Cashew (Member # 6023) on :
MEC - that sounds like something I can understand! Can you expand on that, PLEASE?
Posted by kaioshin00 (Member # 3740) on :
The cornea focuses light rays through the pupil. So my guess is that ttems closer to you have a smaller span of light waves and thus need to be compressed less, whereas objecsts far away have a very wide span of light waves and need to compressed more?
Posted by MEC (Member # 2968) on :
The photons from the object are emmitted in everydirection. Your eye collects the photons that hit it. Since the photons from the object spreads out the farther it is, less of it hits your eyes.
Posted by mr_porteiro_head (Member # 4644) on :
It stems from the fact that all light entering your eye has traveled in a straight line that ends at your eye.
Let's assume that your eye is a point. Let's say that to lines go out side-by-side from this point. At 1 meter away from your eye, they are 10 cm apart. But when they are 10 m away from your eye, they are 1 m apart.
But to your eye, those lines at those points would appear exactly the same, because the your line-of-site would be exactly the same.
Posted by Cashew (Member # 6023) on :
MEC - And that, along with the effect of the layer of air between our eye and the object, would also explain why we see less detail as well?
[ December 06, 2004, 03:04 PM: Message edited by: Cashew ]
Posted by mr_porteiro_head (Member # 4644) on :
Sometimes, but generally no. You can assume that there is always some "fog" in the air, and that things away will always be dimmer, but that is generally not why you can see less detail. It's usually just because you can't see as much detail when it's smaller.
Posted by Bokonon (Member # 480) on :
Actually, I think (if I remember my Sense & Perception class right) it's a mix of the physics of the eye, and the construction of the "optical" pipeline in the brain, which in cludes shortcuts that are, or have been, advantageous to have at some point.
Number of photons has to do with the intensity (brightness) of an object, not distance.
-Bok
Posted by MEC (Member # 2968) on :
I meant the number of photons, as if they were set a distance from eachother. if they were all grouped together in one spot, it would have that effect.
Posted by Bokonon (Member # 480) on :
I think that because objects that are closer tend to hit larger areas of your retina, your brain convinces itself that an object is closer than an object that takes up less space. There is a ton of wiring and interpolations going on in your optic system that create a bunch of exceptions based on context, type of object you are viewing, etc... Liek the illusion that the moon is larger/closer when at the horizon than when straight above. That illusion is a mic of context (comparing the retinal area the moon occupies compared to things that seem similarly far away (trees, hills, houses at the horizon), AND the fact that our brain perceives the sky as a somewhat flattened bowl, with "straight up" being perceived as closer than the "horizon". Therefore, the moon is taking up the same space on the retina when at the horizon than at "straight up", but since the horizon is "further away", your brain perceives the moon to be larger than it is at other points in the sky. Same amount of photons hitting your eye, the whole time (baring a lunar eclipse, by the earth or clouds ).
-Bok
Posted by Bokonon (Member # 480) on :
MEC, I still don't understand what you mean, unless you are agreeing that you mean that it's the photons hitting the retinal nerves (which will be more for close objects, and less for far objects).
It isn't the photons, it's the retina. Otherwise, what would it mean if you had a bright light far away that sent the same net amount of photons to your eye as a dimmer, closer light?
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