# Through A Glass, Darkly

Optics experts, let's hear from you

Vignette

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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Actually, the answer is quite simple, just not apparent at first. You see, light is traveling through the aperture at a number of different angles.

Photons from the sky pass through the aperture at a downward angle, and photons from the ground pass through at an upwards angle. Thus even if the aperture was reduced to the size of a single photon, multiple photons would pass through that aperture, all at various angles. And over time, yes, the entire scene would still be recorded.

This is actually the whole premise of the Aperture in the first place. The aperture is always small and round, while the film behind it is large and square. And yet we end up, not with a tiny round image recorded on the film, but a complete large exposure.

@hood_kauai
So when aperture is smaller it only passes smaller photons, so image is dimmer, right? Your comment still doesn't explain why image is dimmer, not clipped.

Sorry, I neglected to cover that point.

No matter how tiny the aperture, as long as light is allowed to pass through it at various angles, you will still (eventually) end up with a full-size exposure. No clipping will take place.

As to dimness, an aperture only one photon wide will allow fewer photons to pass though (per second) than an aperture two photons wide. Thus the smaller the aperture, the less light is allowed to pass through during the exposure time, and the darker the image becomes.

This is why a small aperture, exposed longer, can produce similar results to a larger aperture that was exposed shorter (less light over a longer period of time, versus more light over a shorter period of time).

Interesting side note: The larger the aperture, the more angles light can come in from any given source in the field of view. This results in "noise" that makes the picture blurry. Hence, smaller apertures result in sharper pictures and more depth of focus, because the "noise" is reduced.

Think of it this way: Say you have an object an apeture and a piece of film the light from the top of the object travels tovard the apeture passes through no matter the size and crisscrosses the light from the bottum of the object the image on the film is upsidedown. Beacuse the light croses itself it all fits through the hole no matter how small it is. Remember that light is hitting all the object and is radiating off the object in evory direction at the same time, the apature allows the light at the correct angles to pass through cross and hit the film (the light path is like an x)light at the wrong angles misses and just hits the camera, appature material, photographer, etc. Also if you stand at a window you can see everything out there, if you back away you can't but all that light is stil coming through the window it's just converging and will eventually cross. You can't see it beacuse you are passed the point that the light crosses and in order to see the inverted image your eye would have to be the size of a building. Try cuting a shape out of paper, take another piece of paper and make a pinhole in it in a dark room shine a flashlight behind the cutout, move the pinhole paper between the shape with the light behind it and the wall and if you can agust it right you can project an upsidown image in the wall.

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