Circles of Confusion1 Comment
It was late afternoon and the sun glistened on the rippled surface of Sleeping Bear Bay. I wanted to capture the simple essence of the peaceful scene, so I chose a low vantage point and framed a row of dune grass against the twinkling lake. The bright reflections are called specular highlights. Since my point of focus was the grasses in the foreground, I knew these out of focus highlights would be rendered as glowing circles of light.
Light passes through a lens in a cone shape. When an object is in focus, the points of the cones of light coming from the object fall on the image sensor. When an object is out of focus, the points of the cones of light fall in front of or behind the sensor, so the light falling on the sensor is a circular cross section of these cones. These “circles of confusion” are usually just rendered as an unfocused object, but specular highlights are rendered as glowing circles.
The size of the circles of confusion are influenced by the aperture of the lens. If you chose a large aperture like f/2.8, the cone of light is wider and the circles of confusion are bigger. If you chose a small aperture, the cone of light is narrower and the circles of confusion are smaller. This is why more of your photograph appears to be in focus (you have greater depth of field) when you choose smaller apertures.
I took a series of photos of this scene, varying the aperture from a large f/2.8 to a very small f/22, and sure enough, the size of the circles caused by the specular highlights varied dramatically depending on the aperture. My favorite, and the image you see above, was shot at a moderate f/8.0.