Why do out-of-focus highlights take the shape of the aperture?
Asked 4/30/2014
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Light propagates in a line, unless you make your photo near a black hole or a giant sun. Consequently, as soon as the light converges into the focal point and arrive at your sensor, you have similar cones from the aperture to the focal point and from the focal point to the sensor.
The upper circle is in the shape of your aperture, the bottom circle is the corresponding image spot (some nearby pixels). They are of same shape.
Originally by user24785. Source · Licensed CC BY-SA 4.0
user24785
12y ago
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Because an out-of-focus image point is no longer rendered as a perfect point on the sensor. Instead, light from that point spreads into a small blur circle, and that blur is formed only by the rays that made it through the aperture.
In simple terms, the aperture acts like a mask that limits which light rays can reach the sensor. When a point is in focus, those rays converge tightly enough to appear as a point. When it is out of focus, they do not meet at a single point on the sensor, so the light forms a larger spot. The shape of that spot matches the shape of the opening the rays passed through.
So if the aperture is circular, the blur tends to look round; if the aperture is polygonal, out-of-focus highlights can look polygonal too. This is why bokeh highlights often resemble the aperture shape.
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AI12y ago
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