第一節(jié)A little larger meaning that what you're looking at appears bigger.

Other types of lenses might be curved on one side and flat on the other some are curved inwards those are concave while others are curved outwards those are convex.

If you're farsighted and you were reading glasses then your glasses have biconvex lenses like this magnifying glass that is they're convex on both sides.

They can make things up close appear in focus.

These types of spectacles were invented way back in the middle ages.

If you're nearsighted like i am then your glasses are concave on both sides these are more complicated to engineer and those didn't come along until later.

Eyeglasses though or magnifying glass use a single lens one piece of glass.

Things get far more interesting when you start combining lenses.

On october second sixteen hundred eight hanns lipperty from the flemish province of zealand applied to the hague for a patent on a certain instrument for seeing.

Far and soon after that there were gobs of people making spyglasses.

A typical spyglass has a weak convex lens at the far end and a strong concave lens near the eye.

And these devices were constructed largely by trial and error it was galileo who worked out the math behind the optics of multiple lenses and improved the simple spyglass to a practical.

Now similarly your camera lens is not a single piece of glass instead it's lots of lenses lots of pieces of glass sometimes a dozen or more.

Each individual lens is called an element.

Each element serves a particular optical function and sometimes elements are cemented together into groups throughout the lens a group serves as single opt.

Lens design is a process of tradeoff and ballot so for example you might set out to design a lens with a particular magnification power and so you choose some elements that give you that.

Amount of magnification.

But maybe those elements yield an image with bad distortion that is lines on the edge of the frame are bent and curved so you add another element behind that to correct a distortion but then.

Maybe that element introduces a bad lens flare of bright reflection that washes out part of the image so you add another element to correct that and so on and so on.

When your lens shopping you might see that a particular lens has twelve elements arranged in three groups or something like that if a salesman offers that to you as a selling point you should definitely.

Knowing nod to indicate that you've already worked out the maths of the optics in your head and that you deeply understand the advantage of this construction but in the end the number of elements in group should not.

We are buying concern unless you're an optics nerd who has a technical curiosity to satisfy.

Later we're going to look at how to choose a lens and internal construction is not going to be one of our big concerns.

What does matter in your use of a lens is an understanding of focal length aperture and focus and we're going to study all of those in this course.

第二節(jié)，Your eye has a particular field of view that is you can see a particular amount of width with an naked eye.

It's a little hard to quantify exactly what normal human field of view is because our peripheral vision is very wide.

But that gets very unfocused at the edges so usable field of view is a difficult thing to gauge.

Things outside of my field of view i can't see.

A lens also has a field of view which is determined by the lenses focal length.

Technically focal length is the measure in millimeters from the middle of the lens to the principal point of focus of the lens now you don't actually need to know where these points are.

All you have to know is what happens when you've changed focal length and we'll be looking into that extensively later in this course.

Any lens that offers the same field of view as the human eye is said to be a normal lens.

Here's an image shot with a normal lens and it looks pretty much like the field of view that you would expect to see if you were standing in this location.

Again if you were actually there you would have a larger peripheral sense but that part of your field of view would be very out of focus.

Any lens that is longer than normal is considered a telephoto lens.

As you go more telephoto your field of view gets narrower and magnification increases things that are farther away if you're closer.

Any lens that is shorter than normal is considered a wide angle lens your field of view gets really wide which means you can take in a wider vista.

But most things in your shot appear small because of this wider angle so.

Your first important field of view concept is this.

As focal length increases field of view narrows and magnification increases.

Trust me you know all this stuff already when you zoom in things get bigger and appear closer.

What you may never have thought of before is that this zooming function results in a lessening of field of view when you zoom out things get small and you have this nice big wide field of view.

Now it might seem like we're just.

Paling with putting words on things that you can intuit and feel your way through but it's very important that we have this terminology worked out for most of what's going to follow in this course so.

For example.

I say go shoot with a telephoto focal length you now know that i mean a focal length that's longer than a normal lens.

When i say interiors are best shot with a wide field of view than you know that i mean a shorter focal length.

So you want to know what focal length is normal for your camera what is telephoto and what is wide angle.

What does a normal focal length.

That depends on your camera's image sensor.

第三節(jié)，The lens is a cylinder.

It's round.

A lot of complicated topics in photography so i thought we just really revel in this simple one for a moment anyway light comes in the front of your lens it gets focused by the lens and is projected out the back.

Thanks to the physics of objects the image gets flipped upside down in that process but because your lens is round it projects a round image out its backside.

But sensor in your camera though is rectangular so it crops a rectangular image from that circle that your lens projects now think about what happens if you change the size of that.

Rectangle.

A smaller rectangle is going to crop a narrower image meaning one with a narrower field of view in other words the same focal length will deliver a different field of.

View depending on the size of the image sensor in your camera.

Because that image sensor is going to crop a narrower or wider part of the image.

Earlier we explained that a lens with the same field of view as the human eye is considered a normal lens.

However as you've just seen because sensor size matters there's no universal focal length that yields a normal field of view normal varies depending on the size of the.

The image sensor in your camera.

So to calculate what is normal for your image you're going to have to do a little math.

It's easy math though just simple arithmetic.