Found it. It's in the first paragraph of
the Wikipedia entry on Aperture:
Quote:
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...the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If the admitted rays also pass through a lens, highly collimated rays (narrow aperture) will result in sharpness at the image plane, while uncollimated rays (wide aperture) will result in sharpness for rays with the right focal length only.
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Basically, the wider open the aperture is, the fewer rays of light are going to be hitting the sensor perpendicularly. So, more rays are going to be "defocused" (from travelling a different distance?) and ending up getting blurred on the sensor. Or so I'm reading it. I could be wrong, and hell, it's still just Wikipedia.
