There's some cool gifs like these out there demonstrating the importance of the focal
length of a lens in distorting objects and the scene.
But the focal length doesn't really have anything to do with getting this kind of distortion.
And the people who made these gifs are godless lying savages.
When you change your the focal length by changing your lens or zooming in, a zoom lens is just
a lens that can change focal lengths.
It looks like this.
Everything’s sort of staying the same.
Everything is the same proportional size and shape.
Angles are staying the same.
We’re not seeing anything new.
It's exactly what it would look like if you just blew up a photo.
Nothing about that photo changes.
If we sort of constantly framed a scene to constantly keep a subject the same size as
we zoom in, as we changed the focal length.
This is what that would look like.
Looks like nothing really.
Zooming doesn’t affect how the scene looks because we’re not getting any new information.
It’s still the same light rays coming in at the same angles.
We’re just sort of collecting a wider or narrower profile of that light.
So the only change is that everything together appears smaller or larger.
Now compare that to moving.
Now it looks like everything’s changing.
Size, shapes, proportions, the angles that we see, we can start to see behind stuff.
At every point in space we’re getting a new profile of light rays.
We’re getting new perspectives on the scene.
If we constantly cropped a subject to keep then the same size as we moved forward, this
is what that would look like.
OK, now we're getting this distortion.
If you look on the left as we move in and out.
It maybe doesn’t look all that weird.
Whereas if you look on the right it’s obviously stretching and squishing.
But that exact same distortion, that perspective distortion, is happening on the left.
We’re just so used to things distorting as we move around, that we usually don’t
even notice it.
Unless if it’s on the right here where we’ve kept it the same size so it doesn’t look
like it’s moving.
OK but if I really resized that little square, this is what that would look like.
Ok, our sensors are too weak and we’re not getting enough information to see what we’re
looking at.
So to get this shot, we have to collect more information, by changing the lens or zooming
in.
And then constantly change the zoom as we move.
So yes the lens is an important part of this.
And if you're a camera operator, focal length is the thing you’re going to think about
because you probably already know about walking.
But you don't even need a camera to get this kind of distortion.
For example hold up a mug or something and with your thumbs try to sort of mark off where
you can see the edges of the handle.
And then move it away from you.
The handle will sort of shrink by more than your thumbs and the cup as you push it forward.
And the handle will grow by more than your thumbs and the cup and you bring it back.
I made this other thing that might make it easier to see.
As it moves forward, the black tab thing shrinks and you can see more of the white.
OK, you can make the world stretch and shrink by moving things closer or farther from your
face.
The role focal length plays is just framing the scene to keep the subject the same size.
So that we can get the distortion of moving without looking like we’re moving.
If we could just resize these shots without losing quality, it would accomplish the same
thing as zooming.
This kind of effect is called a Dolly Zoom.
If you've ever seen a shot that was like "I think we should get off the road" and then
the road starts going "aaaaaaaaaa", that’s a dolly zoom.
What's going on is that as move forward, as we dolly in, objects closer to us look like
they're becoming larger relative to objects farther away.
As we dolly out, you know, they’re getting smaller.
But their proportions are becoming more similar.
They look more and more the relative sizes they would if they were standing right next
to one another rather than spaced apart.
If we maintain a zoom to keep things that are a certain distance away from the camera
the same size, we can see what’s going on a bit easier, and it makes it look everything
is shrinking and enlarging based on their distance from that certain point we're focussing
on.
But why do the proportions change like that and why does stuff get smaller as it goes
off into the distance anyway.
“These are small.
But the ones out there, are far away.
Oh Forget it!”
It feels like a fundamental property of how the universe works.
But it's really just a side effect of the way our eyes and cameras collect light.
Let's say there was a big ol’ lens that only allowed light through it if it came in
straight through the plane of the lens.
And it blocks out light from any other angle.
Light would hit an object.
Scatter in many directions.
But only light going straight on would make it through the lens and hit the sensor.
And maybe it would create an image that looks something like this.
With an object farther back.
Same thing, light hits it, scatters in many directions, and only the light that goes straight
through the lens makes it to the sensor.
But the image it creates would have the same proportions as an object closer to the lens.
Because we’re only dealing with light moving in straight lines.
So distance shouldn’t have any impact on the proportions of the image that that light
creates.
You know, how far away is this third guy?
We might not be able to tell.
Now if the lenses in our eyes worked like this, our eyes would pretty much have to be
as big as the thing we wanted to look at.
Wanna see what that looks like.
We’re not looking through the eyes of someone for whom, it doesn’t matter how far away
something is, all the proportions are going to be the same.
See how stuff farther back kind of looks like it’s a bit gigantic.
The cars look bigger and the road looks like it's getting wider.
They’re actually all the same size.
Our brains still know what’s farther away based on other queues like: on a flat surface,
how high up do they appear?
Or how fast do they move when you turn your head?
Our brains know that stuff is farther away, and our brains know that far away stuff appears
smaller.
So when we see that far away stuff, not being smaller, we know that that far away stuff
must be larger.
So to use they feel bigger.
If this car drives forward a couple blocks.
Vroom vroom vroom.
We can see that it’s not really changing size.
But it does kind of feel like it’s changing size.
Weird.
Anyway.
That’s a lens that collects like all in one direction.
With our eyes and cameras, we collect light at an angle.
Because it’s an angle, the area that we can see increases with distance.
So this slice will be captured by the lens but also, this whole slice will be captured
by the lens.
The effect ends up being that as an objects gets farther away.
While its size doesn’t change in the real world, that objects total portion of the area
being captured by the lens becomes smaller and smaller.
So its image becomes smaller and smaller.
This is why far away stuff appears smaller.
This is why you can block your vision with the palm of your hand only centimeters across.
Or look up into the sky, and see across the light years in between stars.
Because our eyes collect light at an angle.
OK that’s why the size changes, but why do the proportions change the way they do.
The height of our vision increases at a linear rate with distance.
So if the height of our vision is one meter here let’s say, and back here it’s two
meters.
If we move back by the same distance again it will be three meters high.
Four meters high here.
And shaboopy.
Now it’s the proportion of each of these slices that an object takes up, that’s going
to determine the size of the images.
So if our subject is one meter high, it will take up one one’th of the height there.
The entire height.
Back here a half.
Half the height.
Here one third.
And shaboopashoo.
But with fractions like this, the change in the proportion becomes less and less.
You know, the difference between a half and a third, is greater than the difference between
quarter and a fifth.
What’s happening is that as you move farther back, the size of your vision is increasing
by the same amount but not by the same proportion of your total vision.
OK up close your vision is small.
If it increases in height by one meter, in terms of how big your vision already is, that’s
a lot.
It could double the size of your vision, making objects half the size.
Farther back when your vision is say, 6 meters tall, another meter isn’t that big of a
change.
And any object in the scene isn’t shrinking by as much.
The effect is that the size of an object, with distance, it doesn’t change linearly.
It changes like this.
It goes, it goes, wooshaw.
The proportions changing like this is why the dolly zoom can work.
As we dolly in our subjects enter the range of the cameras view where proportions are
very different.
As we dolly out our subjects enter the range of the cameras view where proportions are
more similar, more accurate.
Up here you’ve got the big proportion changes from one half to one third to one quarter.
And back here you’ve got the relatively smaller proportion changes, for example, from
one tenth to one eleventh to one twelfth.
With no zooming that looks something like this.
When we do zoom all we’re doing is changing the overall size of the image.
As we move forward we’re going to zoom out.
That makes the point we’re focussing on look like it’s not changing size and not
moving.
But because we’re moving in and out of these areas where we get changing proportions, everything
else looks like it’s changing size based on their distance from that point we’re
zooming on.
With a face as you dolly in, nose glasses and mustache get relatively bigger compared
to the size of ears and the head.
The subject and the whole scene appears to stretch.
Move backwards and you get the opposite.
Proportions become more accurate and the scene appears to squish in on itself.
DOOLLLYYY ZOOOOOOOMMM.
Anyway, thanks for watching.
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Also use a bidet it saves toilet paper.
That's my send-off now.
Use a bidet it saves toilet paper.
And it gets you cleaner.
And it saves money.
Whoopiwowooo.