Tuesday, August 31, 2010
Control Your Depth of Field
Try the Focus Slice Auto-Align technique to get tack-sharpness from near to far
Shifting The Lens Plane
A conventional lens has the planes of the sensor (or film), the lens and the focused slice of reality all perpendicular to the axis of the lens. Theodor Scheimpflug (1865-1911) stated: “If the lens plane is tilted down, when the extended lines from the lens plane, the object plane and the film plane intersect at the same point, the entire subject plane is in focus.” This is the Scheimpflug principle.
To the landscape photographer, this means if the lens is tilted forward an appropriate distance, you can adjust the plane of focus to run parallel to the ground. What’s created is a wedge of focused reality that’s projected out from the camera, creating an illusion of great depth of field. The limitation is when vertical structures (tree trunks, tall flowers) extend above the wedge of focused reality and can’t be focused by tilting the lens and keeping the ground plane in focus at the same time.
These two photographs are examples of scenes that were sharply focused using the Scheimpflug principle (Figs. 1 and 2). Tilting the lens worked well because there were no vertical objects in the foreground projecting above the Scheimpflug plane of focus.
For lenses that don’t have tilt capability, photographers have relied on the principle of hyperfocal focus. For a conventional lens (without tilt), the hyperfocal distance is a mathematical computation that’s a function of focal length and ƒ-stop based on the “circle of confusion.” Simply put, circle of confusion describes the smallest image element that retains identifiable details.
Hyperfocal distance is the distance at which, when the lens is focused at that distance, everything will be in “acceptable” focus, based on circle of confusion, from half the hyperfocal distance to the horizon. What’s acceptable focus? That’s a subjective judgment made by the print industry and has been based on viewing an 8x10 print at a distance of approximately one foot.
For example, if you’re using a Canon 24mm lens at ƒ/16 that’s not a full sensor, the hyperfocal distance is 12 feet. Focused at 12 feet, everything from six feet to the horizon will be “acceptably sharp,” based on standards set by the printing industry. The technique produces results satisfactory for most photographic situations, with the only limitation being that the overall image can be made sharper using different techniques.
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