21 August 2011

Scheimpflug theory

Susan Brannon
21 August 2011
You read in my previous article on Ansel Adams, that one of the two techniques he used was the Scheimpflug theory.  I will try to make this theory as simple as possible.

The Scheimpflug principle is a geometric rule that describes the orientation of the plane of focus of an optical system (such as a camera) when the lens plane is not parallel to the image plane.

Normally, the lens and image (film or sensor) planes of a camera are parallel, and the plane of focus (PoF) is parallel to the lens and image planes. If a planar subject (such as the side of a building) is also parallel to the image plane, it can coincide with the PoF, and the entire subject can be rendered sharply. If the subject plane is not parallel to the image plane, it will be in focus only along a line where it intersects the PoF. As in the image below.
With a normal camera, when the subject is not parallel to the image plane, only a small region is in focus.Image from Wikipedia.
 When an oblique tangent is extended from the image plane, and another is extended from the lens plane, they meet at a line through which the PoF also passes.
Rotation of the plane of focus

With this condition, a planar subject that is not parallel to the image plane can be completely in focus.

When the lens and image planes are not parallel, adjusting focus[1] rotates the PoF rather than displacing it along the lens axis. The axis of rotation is the intersection of the lens’s front focal plane and a plane through the center of the lens parallel to the image plane, as shown in Figure 3. As the image plane is moved from IP1 to IP2, the PoF rotates about the axis G from position PoF1 to position PoF2; the “Scheimpflug line” moves from position S1 to position S2. The axis of rotation has been given many different names: “counter axis” (Scheimpflug 1904), “hinge line” (Merklinger 1996), and “pivot point” (Wheeler).

There are a lot more of mathematical equations to this principle, too difficult for me to understand, which means, if I cannot understand the mathematics of the principle, how can I try to explain the equations to you?

The bottom line? 
A)   When the lens and the image planes are parallel, the depth of field extends between parallel planes on either side of the plane of focus.
B)   When the lens plane is not parallel to the image plane, the blur spots are ellipses rather than circles. 
So if the planes are not parallel, then you will get blur spots down the horizon.  This is the concept that Ansel Adams understood, that therefore he tilted his lens down, to parallel the image planes.  You can buy tilting lens for this occasion, or use your tripod.

To get into this further here are some great links: Checklist for view cameras, by Howard Bond
How to Focus the View Camera by, Q. Tuan Loung
 These articles are for large format photography.
 Here is a nice page titled the Scheimflug Principle photo stream

Okay, so normally this principle is for large format cameras. What about Digital?
Good news!  You can purchase a tilt lens for your digital camera.

This is what Nikon says, "Tilt/shift photography (specifically tilt/shift miniature faking) is a technique for making regular scenes look like miniatures. By artificially making the depth of field of a picture extremely shallow using a special lens, you can duplicate the effect of a macro lens, making the scene seem much smaller than it actually is, resulting in a really cool effect. You can also use the adjustable distortion to draw attention to specific parts of a picture, which is compelling for portrait photography." 
Here is a great example of the differences:
For a lesson on how to focus your tilt lens see this article.

Basic Information taken from Wikipedia