Tilt Shift in Portrait Photography
Tilt shift is a photographic technique that can be used for altering the focus plane of an image, as well as correcting perspective problems that result from the laws of optics. Tilt shift can only be accomplished using tilt shift lenses. These can be especially useful to have when working in portrait photography.
Let's say you're shooting your subject in front of a building or a fence. The image plane needs to be parallel to the background object in order for parallel lines on the object to remain parallel in the photograph. If that is not the case, then these lines will appear tilted and will create the look that they are going to converge with with other.
Although the subject is the most dominant object in the frame and the background is out of focus, this issue is noticeable and can be bothersome. A tilt shift lens can correct the perspective problem by shifting the angle of the lens without moving the camera. This new perspective can solve the perspective issue and that will insure that straight lines remain straight.
Altering the Depth of Field
Portrait photography is all about creating attention to the subject. This is achieved by both the framing of the image and the employment of depth of field to create focus layers that direct attention in the image. For example, when using an ordinary lens, you position the subject so that it is the most dominant thing in the frame. You then use depth of field to make it sharp in focus, while the background is blurry. Thus, putting all of the attention on them.
With an ordinary lens, the area of sharp focus runs on an imaginary line that is parallel to the lens. This is called the focus plane. Two objects that are ten feet away from each other can both be in sharp focus, as long as they are both the same distance from the lens.
With a tilt shift lens, you can use the tilt feature to change the angle of the focus plane so that it is no longer parallel to the lens. Therefore, you can have your subject in sharp focus at ten feet away from the lens, while a background object that is thirty feet away is also in sharp focus because they are both on the same focus plane.
The tilt can also be used to create extreme shallow focus with your subject that allows his face to be sharp, while the rest of his body is soft. With a tilt shift lens, the creative possibilities in portrait photography are increased, allowing you to create photos that are different from the rest of the pack.