I bought a new lens recently, a Canon TS-E 45mm f/2.8. It has two unique abilities: being able to rotate (or tilt) the focus plane and being able to shift the view of the lens. For example, look at the following two pictures (ignoring color differences):
Both pictures are with an aperture of f/2.8 which has only a fraction of the picture in focus. In the first picture, the focal plane has been tilted to align with the paper sheet. And in the second picture, with a normal lens, the focal plane is vertical and only intersects partially with the paper sheet. If you have to use a normal lens, you would have to stop the aperture down to get the entire paper sheet into focus. But this requires exponentially more light and also introduces diffraction fuzziness.
But that is not all that this lens can do. It can also shift the lens’ view without moving the camera. I took the following picture with the help of the shift feature.
From where the camera was positioned, the entire spread of cards was not in the frame. How do you get around this? By shifting the lens around.
The camera cannot be raised any higher without hitting the ceiling or fan. But you can shift the lens up to 11 mm towards the top of the camera or towards the bottom of the camera. You can also rotate the lens around in 30 degree increments. This will allow you to get all of the parts of the picture. And you can easily stitch the pictures (there were five in this case) together in photoshop.
The shift aspect of this lens is also used to help remove perspective distortion in buildings. It can also move the lens out of a mirror or reflection in the picture as well.