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Lens diaphragms moving in an out.
Close-up shot of lens F-stop or aperture blade moving from wide open to smallest aperture.
Khan Academy
Khan Academy: Depth of Field
In this video [3:25] we'll explore why regions in our scenes can go 'out of focus.' The region of our scene which is in focus call depth of field.
Khan Academy
Khan Academy: Camera Lenses
Now we'll add a lens to our camera and explore the idea of an f-stop.