... ¹

David Joyner, wdjoyner@gmail.com, CC, Attribution + share Alike, http://creativecommons.org/licenses/by/3.0/us/ or GNU Free Documentation License, for all text and images. GIMP was used to create Figure 1(a) and to edit Figure 2 (which was taken by the author). The other images come from Wikipedia, although Figure 4 was modified slightly from the image at [DoF] by adding some labels using GIMP. Last revised 2008-08-15.

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... infinity²

We say an object is ``at infinity'' if light from it enters the lens along a ray perpendicular, or nearly perpendicular, to the lens plane.

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... f-stops³

The terminology ``stop'' seems to arise historially from the fact that for older cameras, aperture was achieved by means of a sliding aperture card, as in Figure 3. When the correct exposure was reached, you stopped sliding the card across the frame of the lens.

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... photograph⁴

This example is from the Wikipedia article [H] on hyperfocal distance.

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