# The Naked Scientists Forum

### Author Topic: why do objects appear to converge in the distance?  (Read 2558 times)

#### matisda

• Guest
##### why do objects appear to converge in the distance?
« on: 16/11/2008 10:37:38 »

Hey, my name is Matt Tisdale, I am a mechanical engineering student at
Clemson University in South Carolina, USA.

I love how I can download the past podcasts, put them on my phone's memory card and listen to them while I'm doing something monotonous.  Feels better learning than just listening to the latest puff daddy album like the rest of my friends, so keep up the good work!!

I have been wondering this for years and have never got a better answer than, "that's just the way it is, things look smaller the farther away they are."

Why is it that you can hold out your arm and pretend to crush someone's
head like a grape, but in reality you know their head is the size of an
American football?  And why the sun is huge, but so small in the sky?

Maybe not all the light is making it back to our eye, that makes sense in
the case of the sun.  But if all the light is not being reflected into our
eyes then why would you see all the details of a person just as they
"are" and they're not missing arms or legs or just all fuzzy when
they step a few feet away?

I thought it might be something to do with having two eyes and how image is being crossed by them, but I can disprove that by closing one?

So what's up?

Thanks dudes, and chicks,
Matt T

What do you think?

#### lyner

• Guest
##### why do objects appear to converge in the distance?
« Reply #1 on: 16/11/2008 22:51:26 »
Is there any more to this than that we see things in terms of angular separation - which depends upon spacing divided by distance away?

#### LeeE

• Neilep Level Member
• Posts: 3382
##### why do objects appear to converge in the distance?
« Reply #2 on: 17/11/2008 18:24:23 »
It's just down to having a non-parallel field of view.  You can change the degree of perspective by changing your field of view, by using binoculars to narrow the field of view, which flattens the perspective, or by using a wide-angle lens, which exaggerates it.

#### lyner

• Guest
##### why do objects appear to converge in the distance?
« Reply #3 on: 17/11/2008 21:00:09 »
LeeE: this is a common misconception. The perspective is not actually changed by swapping / zooming the lens. The only way to change the real perspective is to move  yourself nearer or further away.
The paths of light are the same, whatever lens you use and the same things are obscured or revealed  by the foreground objects until you actually move.

We were all astounded when we first saw (but now we're used to it) the effect of moving the film camera forwards at the same time as zooming out, so the face in the foreground stays the same size on screen. The way the perspective changes without an object changing size is very disturbing.

The foreshortening of distant images with a very 'long' lens is odd because the perspective is all wrong for the apparent distances involved. The poor brain is disturbed by conflicting messages.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### why do objects appear to converge in the distance?
« Reply #4 on: 18/11/2008 18:38:49 »
Oops yes, that was sloppy of me.  I should have clearly said that it was for the same image feature size, which corresponds to changing the distance to the target.

#### Physics Dilettante

• Jr. Member
• Posts: 21
##### why do objects appear to converge in the distance?
« Reply #5 on: 19/11/2008 18:28:47 »
I think sophiecentaur nailed it. An object's apparent size is determined by the size of the image it forms on the retina, ignoring perceptual effects, such as the famous moon illusion. The image size is a function of the angle the object subtends at the eye, and that angle decreases as the object recedes.

1. The eye's lens has a diffraction limit. Once the image reaches that limit, it can't get any smaller. (Increasing the distance further will, however, make the image dimmer because fewer photons are forming it.)

2. The eye's photoreceptors can't distinguish among images equal to or smaller than they are. This limit would apply if the lens' diffraction limit was overcome.

3. Atmospheric interference smears the photons out, making the image larger than it would be otherwise.

#### The Naked Scientists Forum

##### why do objects appear to converge in the distance?
« Reply #5 on: 19/11/2008 18:28:47 »