Some questions about shifting axis lens.

In an ordinary lens, the optical axis and the focal plane are perpendicular, right in the center of the focal plane.

The shift lens can change the position of the optical axis or the included angle with the focal plane. That is, the main optical axis of the whole camera lens is translated, tilted or rotated on the premise that the plane position of the camera body and the film remains unchanged.

You can experience it with a magnifying glass.

The head doesn't move, the distance between the magnifying glass and the head doesn't change, just move the magnifying glass in parallel, and the range seen through the magnifying glass also moves with the magnifying glass, but the image doesn't tilt. Tilting the magnifying glass can only keep a narrow strip clear.

Therefore, the lens of the translation mirror can be translated in parallel, so that the subject is parallel to the focal plane, and the perspective distortion under the building can be eliminated when shooting buildings, etc. Tilting can change the angle between the lens and the focal plane and produce an inclined depth of field.

Canon's shift lens is very famous. TS-E24/3.5L can completely eliminate the distortion when shooting wide-angle buildings and landscapes, and the imaging is excellent. TS-E90/2.8 can create a special depth of field effect when shooting advertisements or portraits. TS-E45/2.8 can also take stunning photos that are clearly tilted far and near, so the shift lens is deeply loved by many commercial photographers.