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Video transcript

- Earlier, we found that to make a very bright image with our pinhole camera, we had to use a large aperture. But when we did this, our resulting image was blurry because light rays from each point in our scene spread out into a larger area on our image plane. Here's an example. The light rays are coming in from the right and they're just about parallel, like they're coming from an object like the sun that's really far away. With a tiny aperture, just one tiny point on the image plane would be illuminated by these rays. But to make a bigger aperture work, we need to aim all of these light rays onto a tiny point without discarding any of them. We want to bend them. And what do we use when we want to bend light? A lens. If we add a lens to our camera, it will cause our light rays to converge like this. The point where these parallel light rays converge is known as the Focal Point. It turns out that the shape or curvature of the lens determines how sharply the light rays are bent. If we increase the curvature of our lens, the rays focus very close to the lens. And if we decrease the curvature, they focus farther away. What we want is to focus the light rays exactly on our image plane. That will give us a sharp image. And for any particular distance between lens and image plane, there's a lens design, or curvature, that will focus the light rays on the image plane. We call the distance at which the lens focuses these parallel rays the Focal Length of the lens. You can have a lens that focuses close to the aperture and you'll get a wide field of view. We call that a Wide Angle Lens. For example, a 28 millimeter lens, like the one we're using here, has a focal length of 28 millimeters and produces an image with a wide field of view. Or you can have a lens that focuses light further away from the aperture. We call this a Long Lens. For example, a 120 millimeter lens, like the one we're using here, has a focal length of 120 millimeters. So we'll capture less of the scene, like this. While a 50 millimeter lens, like the one we've switched to here, gives us this natural-looking perspective. For any given lens, the film or light sensor needs a certain amount of light to record an image properly. Too little, and it just sees darkness, Too much, and it just sees white. We control how much light it receives by adjusting the size, or diameter, of the aperture, which we also measure in millimeters. Some cameras, like this one, allow you to set the aperture using this ring. But the numbers on the ring aren't measured in millimeters. The numbers here are called F Stops and is defined to be the ratio of the focal length of the lens to the diameter of the aperture. So if we want a 25 millimeter aperture on a 50 millimeter lens, we'll set the F Stop to two because 50 millimeters divided by 25 millimeters equals two. F Stops are handy because they tell us exactly how much light gets through, no matter the focal length of the lens. For example, if we set the F Stop on a 100 millimeter lens to two, we get the same amount of light as we do with a 50 millimeter lens with the F Stop set to two. The F Stop is there for an important idea. Let's stop here so you can get some practice with it in the next exercise. And just to make it clear, I did take that with an F Stop of two.