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A lens is a cylinder. It's round. There's a lot of complicated topics in photography so I thought would 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 optics, the image gets flipped upside down in that process, but because your lens is round it projects a round image out its backside. The 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 have just seen, because sensor size matters, there is no universal focal length that yields a normal field of view.
Normal varies depending on the size of the image sensor in your camera. So to calculate what is normal for your image, you're going have to do a little math. It's easy math though. Just simple arithmetic.
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