Depth of Field

Example

This article is intended as a follow-up to this article.

This is a photograph of my dog, Tigger, with the sun low in the sky, behind him. (He is, thus, backlit.) The photograph was made with a telephoto lens (35mm equivalent of 250mm) at a fairly wide aperture (f/2.8), which combination produces very shallow depth of field. The background is obviously out of focus. The foreground, though there is less of it, is out of focus as well. At the size of this representation, his body looks pretty much in focus.

But look more carefully at what is in focus. To the right is an enlargement of part of the image, showing his left front and right rear paws. In this detail, the foreground and background are both out of focus. Moving your eye up and down, narrow in on the most in-focus spot. It is the horizontal area a bit in front of the front paw. Moving up or down from there, things begin to go out of focus. Looking back to the full image, you see that his face is a little in front of his front paws, so you see that I was focusing on his face.

Although the sharpest focus is between Tigger's nose and eyes, his whole face is in good focus. The focus softens as you move down his back, but there is great contrast between the focus on his back and that of the grass behind him.

If I had wanted his whole body to be in sharp focus, I would have stopped the lens down some (maybe to f/5.6) and focused 1/3 of the way from his nose to tail, which would be about at his shoulder. Stopping down further would produce greater depth of field, and since focus falls off as you move both in front and behind the point of sharpest focus, you want to focus between the two extremes you wish to be in focus. Focus diminishes more rapidly moving toward the camera, so picking a spot 1/3 of the way from nearest to furthest point of desired focus is most successful.

Circles of Confusion

This photograph provides some good examples of circles of confusion. Look at the detail, above, showing Tigger's paws. Notice that in the grass the sunlight creates dots of bright light. These, when in focus, are pretty much "points of light", but as they become out of focus they become circles of light, or "circles of confusion". The more out of focus, the bigger the circle. This is clearly illustrated in the detail to the right, where the bottom is in sharp focus and the top is quite out of focus. The bright points of light at the bottom are just highlights in the grass, but as you move up the image they become circles of increasing diameter.

Bokeh

Circles of confusion are rarely perfect circles of unmodulated light. The shape and quality of the circles of confusion are a function of lens design. The shape of the aperture affects the overall shape of the circles, and the construction of optical elements in the lens affects the texture as well as the shape of the circles. Notice that the circles in this image have brighter edges than center, they have a bright line running through them, and the part above the line is slightly smaller than the part below the line. They look sort of like a jar with a lid. The quality of the circles of confusion is called "bokeh", from a Japanese word meaning "fuzziness". (This lens, by the way, is a Canon EF 70-200mm f/2.8L IS USM lens, which is at the top of Canon's line, but it has pretty peculiar bokeh.)

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