Pacific Northwest Nature Photographers Tips, Tricks, Techniques & Tutorials
Plane of Focus –vs– Depth of Field
by Gary DeWitt
The photo above is meant to illustrate a concept. Two thirds of the image is the actual photo. I shot this at Fern Spring in Yosemite. Mostly I was practicing technique, rather than trying to produce a great picture, though I was intriqued by the colors and textures. The result illustrates what a view camera can do that a normal camera cannot: alter the plane of focus. Note that there are three boxes drawn in the picture. The areas roughly covered by those boxes are reproduced on the right at larger magnification. The lower box was about 4 feet from the camera, the middle about 6 feet and the upper about 8 feet. Note that the top and bottom box's contents are sharp but the middle one is out of focus. Also note the geometry of the shot: the pool of the spring, on the left, spills over a low rock damn to a lower level on the right. The water drops a foot or so.
How is it possible to have objects at 4 and 8 feet in focus and one at 6 feet be out of focus? Wouldn't the depth of field stretch from nearest sharp object to farthest sharp object and include everything in between? With a normal camera, that's true. But a view camera allows the front and back of the camera to be tilted relative to one another. This creates a new plane of focus that is not parallel to the film plane. In this case I focussed on the plane of the pool. The depth of field now extends above and below the plane of focus (actually the FOD is wedge shaped, with the narrowest part of the wedge towards the camera). The two in-focus sections lie on that plane, while the out-of-focus leaf in the center section is below the plane of focus and outside of the wedge of the depth of field.
There is a theoretical explanation of this called the Scheimpflug Principle, which states that if the film plane and lens plane intersect then they will also intersect with the plane of focus. This is just one of the things that attracts me to working with a view camera. Tilt-Shift lenses, like Canon's, work in the same manner. I thought this illustrated it well with a real-life example.
Gary