Why do cameras go in and out of focus? Why can’t everything just be in focus?
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With a typical still or movie camera, you have a lens system that focus light from the field onto a flat sensor.
https://www.physicsclassroom.com/class/refrn/Lesson-5/Converging-Lenses-Ray-Diagrams
The diagrams here show light passing through opposite sides of a lens. If you narrow that lens opening further and further you get a pinhole camera: https://en.wikipedia.org/wiki/Pinhole_camera
But pinhole cameras are pretty dim because the hole that light passes through is so small. In photography this is the **aperture** component of exposure. The smaller the aperture the more that appears sharp. This is called **depth of field**. There is a tradeoff of much light passes through for a given amount of time. The other components are shutter speed (how long the film or sensor is exposed) and how sensitive said film or sensor is.
But why *should* everything be in focus with a camera? Shallow or deep depth of field can be an artistic choice. Portrait mode on smartphone cameras simulates the background blurring that’s otherwise only possible with larger lenses.
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