Closing the aperture at a fixed focal length provides

Closing the aperture at a fixed focal length increases the f-number (aperture value), resulting in a longer depth of field. This means that a greater portion of the scene will be in focus, both in the foreground and background. A higher f-number indicates a smaller aperture, which allows less light to enter the lens but enhances the sharpness and clarity of the image over a larger depth. References: Axis Communications camera optics and image quality documentation.

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