Thank You, Captain Scheimpflug
Even if only to impress your friends with your photographic trivia prowess during a dinner party conversation, you should know who Theodor Scheimpflug is.
No camera is perfect, and one of many inherent limitations is known as depth of field — the range of subject distances from the lens that appear to be sufficiently sharp to the viewer. The truth is, any lens is actually focused at just one single distance, but there exists a range of distances in front of and behind this where tings still look close enough to being in focus that we can't really tell they're not. This range is the depth of field and normally lines up parallel to the film plane (or digital sensor plane these days) as well as the plan that slices perpendicularly through the optical center of the lens (the lens nodal plane).
So long as the main plane of the subject remains parallel to the film plane you can basically point, focus and shoot. Even if your subject is leaning somewhat towards you or away from you, you can expect to capture the whole thing reasonably sharp as long as the depth of field is sufficient to encompass the area you want to photograph. But if the distance span becomes too extreme, at least one end or the other will fall outside the depth of field and appear increasingly soft.
There once was a time when all "serious" photographers shot large format cameras because the film of the day had relatively poor resolving power. They pretty much had to have a large negative if they wanted a reasonable amount of detail in their final images. But large format cameras inherently have shallower depth of field compared to ones with smaller film (or sensor) sizes that most of us shoot with these days. To help work around this problem, their cameras all featured articulating bellows to facilitate a variety of tilt and shift movements to control focus using something known as the Scheimpflug principle. And this is where Theodor enters our tale.
In 1904, Austrian Army Captain Theodor Scheimpflug of Vienna filed a British patent describing the relationship between the film plane, subject focus plane and lens plane to achieve focus. The patent resulted from his work correcting perspective distortions in aerial photography so the Austrian army could drop bombs on things more accurately.
Here's the way it works: Imagine the film or sensor plane extended out in all directions, and likewise with the major plane of your intended subject (the desired focus plane). Now imagine a third plane that lies parallel to the optical center of your lens (somewhere in the middle of the lens, perpendicular to the light path through the lens). If all three planes intersect along a line, the lens will be in focus where you want it. Put another way, think of these there planes as being three consecutive pages of a book. So long as they're all joined along the spine of the book, the one representing the subject plane will be in focus. You can turn any of these three pages varying angles from closed to open and all those configurations will still be in focus. But if you try the same thing with three loose sheets of paper, there are countless arrangements where focus won't be achieved because the pages are no longer hinged together along one edge. The math behind all this is far beyond most of us, but no matter. The principle works even without a calculator and calculus.
Putting this to practice in the field can still be somewhat tricky even if you try to image these three critical planes. To make things easier, start by focusing on the nearest point in the subject plane, whether that's the top, bottom, left or right. Then tilt the lens while watching through the viewfinder as more and more of the subject plane starts to come into focus. As you go, you'll need to occasionally refocus the side you started with slightly using the regular focus adjustment to keep it in focus. But by alternately tilting to get the far side in focus and tweaking the regular focus knob to keep the near side in focus, you can work your way to the optimal overall configuration without dealing with planes at all.
As I mentioned, the larger the sensor or film format, the worse the problem of shallow depth of field is. So it's really no surprise that Scheimpflug's name and his eponymous principle have faded into photographic obscurity. Most of us today use "full frame" digital or smaller formats, and smaller formats automatically get more depth of field to work with. That's just the way the optics and the physics of it work out.
But Captain Scheimpflug is being useful even today. Both Nikon and Canon (and others) make a line of specialized 35mm lenses with built-in shift movements for macro and other work where increased depth of field is needed. Yes, it is digitally possible create a composite image with increased depth of field by stitching together a series of images shot with a normal lens set to varying focus distances, but sometimes it's still easier and better to get the shot with a single press of the shutter. Also, an Oregon company named Lensbaby has popularized a range of optics named the same as the company that produce creative focus effects in large part due to the Scheimpflug principle. This same principle has also proven useful in the corneal photography done before LASIK and other refractive vision surgery.
If you've shopped for a dedicated tilt lens for your camera, you probably found out these things aren't cheap, but they are quite useful if you can afford them. You may also have noticed that these things limit tilt to no more than about ten degrees range of motion. This may seem rather limiting, but in practice it's generally more than enough. If you're shooting a field of wildflowers they're unlikely to be right at your feet. A subject only thirty feet away is sufficient to keep the needed angle under ten degrees even when shooting from five feet above the ground. Cut both distances in half and things work out too. At least here we're only dealing with trigonometry rather than calculus.
Scheimpflug himself claimed not to have independently come up with the principle that today bears his name but rather merely to have devised a practical way of applying it. Indeed, another British patent by Jules Carpentier of Paris from three years before Theodor bears a strong resemblance to Scheimpflug's work. Scheimpflug even mentioned in his patent application that the principal was already well known and referenced Carpentier's writings. So by all rights it probably should be Carpentier's Principle you use to impress your dinner party companions with rather than Scheimpflug's. But the principle itself remains the same, even if you don't really know what to call it.
Oh, and by the way: it's pronounced "Shyme-floog," or thereabouts. If you actually speak Austrian, forgive me.
Update 10/06/2013 - OK, shows how much I know. Reader KH points out that "Austrian" isn't a language. Austrians speak German, another language I don't speak I guess, although I can count in German, if that earns me any points (not).