Through A Glass, Darkly

Optics experts, let's hear from you

johnnyalive (CC Licensed)

Reader Bob Fately asks:

When you 'stop down' the aperture of a camera lens and reduce the size of the hole through which light passes, why does the overall scene darken rather than the edges vignette? I realize the aperture is positioned at the focal point of the lens, but I do not understand why blocking the photons from the outer edges of the cone of light serves to darken everything rather than simply block the portions of the scene at the perimeter. If one ignored diffraction effects, could this mean that if a diaphragm could be stopped down to mere angstroms and only let a few photons through, the entire scene would still be recorded, albeit very slowly?

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