and, oh, you probably can't really see me right now let's throw a lens on that camera.

Ah, there we go.

Look at that.

Let's just lock it into place.

ooh.

Much better.

Now look deeply into my eyes, we're seeing each other through

3 lenses, 4 if you wear glasses.

Now, light is coming off of my eye and going through

my glasses and into the camera lens, and then

out of your screen into the lens in your eye.

In fact, let me grab one, most camera lenses have many pieces

of glass stacked inside of them, so while

we call this single piece of equipment a lens, there's actually many different “lenses”

bending light between the two of us.

And we're gonna take a look with our eyeball lenses, at some optical DONGs, things you

can do online now guys.

Ricktu's Ray Optics Simulator is a great way to visualize how light moves around when

distorted by various surfaces.

You can make beams, rays and other light phenomena, interact with various reflective and translucent

surfaces.

You can change a whole bunch of factors, like the density of the beams and the shape of

the reflective or transparent surfaces.

So why does light bend like that?

We'll get to that in a second, but first let's talk about why light can go through

things at all.

You can't see through me, or at least I hope you can't

So what makes glass so special??

When a photon hits something opaque, like you or me, it either scatters away in a random

direction or it's absorbed by the material.

Transparent materials on the other hand don't absorb light, but instead let it pass right

through, and that's what a simple sheet of glass does.

The science of why this happens is very cool and very complicated, 60 symbols has a wonderful

video on the topic, you can check it out right here

but basically when light hits an object, the

energy in the photon is absorbed by the electrons in the object's atoms.

With glass, however, the price of entry is a bit too high, and the atoms in the glass

want more energy than visible light can provide, so the light just keeps on truckin.

This applies to visible light, but not so much for UV light.

Check out my video from last year where we use predator vision.

It's all about UV cameras.

and ultraviolet and infrared.

Anyway it's really cool.

See this?

This is a lentil.

The word lentil is where the word Lens comes from.

Lentils were never used to make lenses or anything, but they look the same a basic lens

shape.

Legumes are a tasty treat.

Anyway, lenses, or really any curved glass, bends the light that passes through it.

Jake!

How does this work??

Well my inquisitive little turtle:

Great Question

it takes advantage of two fundamental properties of light.

The first is that light slows down in certain types of materials, like air, water, and glass.

Now you might be saying “ i thought the speed of light was constant” well that's

just the speed of light in a vacuum.

Again, there's a lot of weird quantum stuff going on, but the important thing is that

once light enters other mediums, it's forced to interact with the other atoms hanging around,

and those interactions slow everything down.

The second property of light is that it is not just a particle, but also a wave, which

means it fluctuates in three-dimensional space.

Imagine a group of runners all running next to each other, tied together with ropes.

If the group starts running through mud at an angle, one of them will be slowed down

faster, and the others will have to change angles to stay parallel.

That's essentially what's happening when light curves through a cup of water or a camera

lens.

This allows us to do all sorts of crazy things to incoming light.

You probably have zoomed in to something with a camera before.

If you have, you may have also seen these levels of magnification measured in millimeters,

for example, this video is being shot on a 50mm lens.

That number measures the distance between the front lens and the back lens.

This is called the focal length.

On this site you can get an idea of what focal lengths look like in real life.

You can mess with the actual zoom of the camera as well as the camera body that captures the

image.

Seems pretty straightforward right?

Well it's actually a very complex process to make sure that the image stays sharp and

consistent while changing magnification, and it involves many layers of glass of different

shapes.

when you twist a zoom lens like this you're actually physically moving the piece of glass

around inside.

Some lenses actually stick out when you do this.

What this does is move what's known as an “afocal zoom system,” which involves a

few concave lenses (lenses that bend light inward) with a convex lens between them (which

bends light outward).

So as the light from the object you want to magnify comes in it gets focused and then

re-spread out to make the image appear bigger on the image sensor.

Keep in mind that there are some types of lenses that don't zoom, and they're called

prime lenses, oh and if you zoom in on your phone it's not the same process.

that's just the camera enlarging the digital image rather than actually bending the light

to make an object appear closer.

It's the same thing we do in our videos where we punch in to make it look like it's a closer

shot

when it's really like this

Ok, so you can zoom in all you want, but going back to the beginning of this video,

what about when everything is out of focus??

Michael just did a DONG episode on how to use a pinhole

to create a 2 dimensional image,

and a perfectlysmall pinhole will create a perfectly sharp image.

As we make the opening bigger, we're going to end up with more blur.

On a camera lens the system that measures the amount of blur is determined by the size

of the opening, also called the aperture or f-stop.

This website lets you get an idea of how changing the fstop changes the level of blur around

the object that's in focus.

Now anyone who uses cameras regularly has noticed something strange when changing the

aperture, as the fstop increases, the aperture gets smaller.

This seems backward, but that's because fstop is a fraction.

This fraction to be exact.

But wait, why go through all this math and glass when I just said a pinhole keeps everything

perfectly sharp.

Well the simple fact is that pinholes are very small, and they don't let in a lot

of light.

Going back to Michael's video for a second, you can see how an out of focus image comes

from receiving light from multiple points on the image sensor from the same point on

an object, and that's very not good.

What lenses allow us to do is take much more light in, but bend it so that there is still

a one-to-one relationship between a point on the object and a point on the sensor.

Of course that sensor can be anything, like your eyeball.

Your eye has a very similar structure to a prime lens, so it can't zoom, but it's

very good at focusing light onto the retina.

Your cornea, lens and retina mimic a camera lens almost exactly, but instead of moving

the lens back and forth like in a camera, it stretches and squeezes.

Those with near or far sightedness have a problem with one or more of the mechanisms

that should allow their eyes to focus, like the lens unable to change shape correctly

or the cornea being warped in some way.

So there's some info for you on lenses.

I think lenses are so incredibly cool.

Because, little side note, this may not be that interesting to you but it is to me

When you buy a set of lenses, a lot of the time they have to color match

the glass, because the glass can actually change the look or the color that you see

on the image

So most times when you buy really nice lenses

The entire set of lenses is color matched so if you switch from like a 14mm to a 25mm

So on and so forth

they all still look the same even though the distance that it's showing is different

but the color will match.

and that's important because again there are so many lenses in just one of these lenses

Anyway I could go on forever about lenses.

I love them.

I think they're so so neat, and just so wonderful, like what a crazy invention.

But, that's for another time maybe.

You know what's for now.

More DONGs

Right down there!

Ones that we talked about.

Playlist of dongs right over here!

And As Always, Thanks For Watching