So, there I am in my projector 101 class, and things are progressing quite nicely.
"Let's discuss the mechanics of focus and lens shift," says the esteemed professor. "No problem," says I … easy terms and concepts.
"Let's ponder the mysteries of color space," says the master of PowerPoint. "Gotcha," says I, as we dive deep into the CIE color space chart in all its beauty.
"Let's talk about cooling the DLP chips," says the teacher. "Remarkable technology," says I, knowing that two million microscopic mirrors torched by a 6000 watt xenon bulb get mighty toasty.
"To conclude today's class, let's discuss Scheimpflug," says the brilliant lecturer.
"Whoa doggies!" says I. "Where the hell did this term come from? What's with the name? What's it doing in my projector, and what happened to intuitive nomenclature?"
This was clearly a research paper in the making. Not only was I determined to learn the function's meaning in terms of projection, but I was curious about the derivation of the word itself (and of course, why I was the only one in class looking like a deer in the headlights).
Those Remarkable View Cameras
Round about 1904, an Austrian Army Captain named Theodor Scheimpflug was perfecting a method for improving critical focus in view cameras, and this related directly to his military work with aerial photographs. Scheimpflug discovered a way to correct for perspective distortion, and the result of his research is the patented principle that today bears his name.
View cameras (as Ansel Adams once told me), are those marvelous devices of old – complete with flexible bellows, a lens on a movable axis, a holder for the photographic plate, and typically, a cloth hood to isolate the photographer from outside light. The resulting images had remarkable clarity, and you could perform high-resolution magic that most of today's multi-megapixel wonders can't duplicate.
Three Elements
Three elements are at play in a view camera: the object (the photographic plate), the lens, and the target that's being photographed. (See Fig. 1).
With a standard SLR, both the lens and the digital sensor are parallel to each other, and the "plane of sharp focus" is fixed solid – exactly parallel to the lens plane. But the beauty of a view camera is that the lens can be tilted and even rotated, and thus the plane of sharp focus can be tilted and moved. In very basic terms, Scheimpflug's principle states that if a camera lens is tilted such that the lens plane intersects the film (object) plane, the "plane of sharp focus" must also pass through that intersection. For a more in-depth discussion and all the trigonometry you can eat, search Wikipedia for "Scheimpflug Principle" for the full scoop.
Through proper understanding and application of the principle, the result can be a remarkable photograph in which both near and far subjects are in sharp focus. The key phrase in this discussion is the ability to manipulate the plane of sharp focus by adjusting the lens. If you can't adjust the object or target in order to achieve focus uniformity, you can adjust the lens.
Three Elements at Work
So let's bring the story around full circle. In any movie theatre today, or in any event that deals with large-scale projection, the same three elements are at work:
- The object – the projector's light source, which includes the xenon lamp, the prism and the DMD (Digital Micromirror Device).
- The lens – including all optics and the lens holder.
- The target – the theatre screen.
In theory, all three elements should be perfectly parallel, but there's plenty of room for error. Up in the booth, the projector is tilted on a pedestal. In addition, the screen is often over 30 meters away, and the projector's lens is a highly complex set of optics. Given that these devices are manufactured with high precision, there are still mechanical and optical tolerances that are greatly magnified by the time the image reaches the screen.
Now, Mr. Projectionist, let's throw a focus chart on screen. Hopefully, the modern-day theatre screen is perfectly flat – but if the screen isn't absolutely parallel to the projector's lens, from corner to corner, the result might be razor sharp focus on one edge of the screen, but soft focus on the other (opposite) edge. How can this be corrected?
You certainly can't adjust the screen. Mr. Theatre Manager will not be pleased. Similarly, you can't easily adjust the position and tilt of a 300 pound digital projector after it's been installed. And anyway, by this point, the installing technician has already tuned the projector's output to precisely fit the screen. The solution to achieving focus uniformity at the edges and the center is in fact, the Scheimpflug principle.
A Simple Tweak
To correct for focus uniformity on contemporary digital projectors, technicians can tweak the only remaining adjustable element in the three-part equation – the lens holder itself. Typically, four points of fine adjustment are provided around the lens holder, each equidistant from the center of the lens. Each is essentially a pivot point which when adjusted, moves the axis of the lens in minute increments – horizontally, vertically and diagonally in relation to the light source.
As the lens moves, the plane of sharp focus moves. By using the focus chart as a reference, and by adjusting each Scheimpflug pivot point, the technician (with lots of patience) can bring the planes into true parallel alignment. The result is precise, uniform sharpness from left to right and from top to bottom.
So the mystery (and history) of the function has been solved – but perhaps there might have been a better name for it. Then again, considering the value of the tool, the honor bestowed upon old Theodor is fitting. The Scheimpflug principle actually has numerous applications today – in cinema and event projection, in specialized industrial and creative photography, and even in the field of Lasik eye surgery. Specialized SLR lenses are also available for high-end cameras that tilt, enabling the photographer to adjust the plane of sharp focus as desired.
And next time, when you're at a theatre or an event with a large venue projection system, if the left edge of the projected picture is razor sharp but the right edge is not, you now have permission to shout "Scheimpflug" at the staff. Action definitely will be taken.
