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Video transcript

- In the previous video, we saw how lenses can focus parallel light rays to a point on our image plane, resulting in a sharp image. This means that if our image plane is set at the focal length of our lens, then objects that are very far away appear sharp. For example, imagine a scene with Joy standing a mile away from the camera, like this. If we look at the scene from the side, we get this. Notice the light rays that bounce off Joy and enter the camera are approximately parallel. If we place our image plane at the focal length of the lens, Joy will be in focus. But what happens when we try and make an image of nearby objects in our scene? Imagine we move the camera so that Joy is standing just a few feet away. Now pick any point on Joy and look at the light rays that are heading from that point toward the lens of our camera. Notice that these light rays aren't parallel, they fan out. Because they're hitting the lens at an angle, they are redirected to a point a little farther away behind the image plane. Over here. And where the light rays actually hit the image plane, they're spread out in larger circles instead of tiny points. This results in an image of Joy which is out of focus. To see this effect clearly, check out this out of focus shot of lights. These blurry circles are known as Circles of Confusion. They're a result of light rays which haven't been focused to a point on the image plane. And when an image is in focus, the circles of confusion are so small, they appear as points in our image. We can bring Joy into focus by moving the image plane back a little. There. At this distance, Joy will be in sharp focus. Remember with our pinhole camera just the size of the pinhole determined how blurry or sharp everything in our image was. And with a tiny pinhole, the entire scene was in focus. But now, with a lens in there, only a slice of our scene will be in focus. And anything which moves outside of this in-focus region, either too close or too far away, will appear out of focus. This is known as Depth of Field. Filmmakers control the depth of field with their choice of lens and aperture, or F Stop. Let's do an example with multiple Joys at different distances from our camera. If our aperture is very small, that would be with a large F Stop, then the depth of field will be larger and the transition from sharp to blurry is very gradual. To see this, let's start with a subject that's in sharp focus. As I move it farther away, the blurry region doesn't change very much. This is Deep Depth of Field. So more than one of our Joys will be in focus. Now, let's increase the size of our aperture, that is, a smaller F Stop, and start again with our subject in focus. Moving the subject even just a little further away causes the blurry region to get big fast. This is called Shallow Depth of Field. Now, just a tiny slice of our scene will be in focus. So with a small aperture, or large F Stop, the depth of field is deep. The entire scene is in focus. With a larger aperture, or smaller F Stop, the depth of field is shallow. Only a small slice of our scene is in focus. In the next exercise, you'll have the opportunity to explore how both aperture and lens length affect depth of field.