So, here's something cool that's related to the mathematics of photography which we'll do more of. And it's that zooming and moving are not the same thing, right. So some people think oh if I zoom in, it's just like getting closer to something. Well, it's not true. Here are two pictures of the same car. Okay? And you can tell already that they look somehow different even though it's the same car, and the car is in the same place. And the reason that they're different, is that on the left, the car was taken with a very large field of view. So that's a very small focal length. So that means the photographer had to get close to the car. Okay. The image on the right was taken with a small field of view, a larger focal length, so the photographer had to back away from the car. So even though that the car is the same size in the image, you'll notice right away, that the shape seems quite different, all right? The one on the left seems to have what,

a lot of what's called perspective distortion, and it's not really distortion at all. It's just the effect of perspective. And that's why I said, we're going to do the mathematics of that, really not today. We'll, we're going to do that in the next one. This has dramatic effect on taking pictures of people as well. So here is a example again taken from the,

the web, the website in there says StevenEastwood.com where it has tutorials on lens perspective. What we have here is pictures of the same woman. But over here, it's a 350 millimeter lens. Standing very far away zoomed in, here is a 19 millimeter lens standing much closer in order to be able to get the same size face. And you'll notice that you get this incredible apparent change of the shape of her face. And over here that's referred to, again, as perspective distortion. But that's not really a correct word.

It's just the effect of perspective. This is why photographers use particular lenses for portrait work. And they tend to pick a lens that lets them stand, what they call a normal distance away. It turns out when you and I think about what a picture of somebody should look like, we tend to think of what it looks like standing somewhere around six, seven, eight feet away. Or maybe a little bit further, but certainly not closer. And that's why they pick the particular lenses that they do. Here's another example of what's called the effect of focal length on perspective. And here it's being called correct perspective effect. And this is a image out of the Zisserman and Hartley book. They're actually talking about the effect of perspective and weak perspective. We'll talk about that actually next lecture. But here two pictures of the same building. Again one taken with a much wider field of view lens. So the one on the left, so you see this effect as the object gets smaller away. And with a telephoto lens, where the image on the right, you don't see any converging of the lines. And we'll talk about the mathematics of that next time.