Keep all your lines parallel. That's the main point of a shift lens. Shift, and it's vertical counterpart: rise, allow a lens to move along an axis and redirect the camera's point of view without actually tilting the camera. In the case of rise (the more commonly used of these two movements) the lens is able to allow the camera to see higher without the camera tilting up. This is important because if your camera remains parallel to what you are photographing, then the subject's lines will remain parallel in the rendered image. Think of it this way: If you stand on the street looking straight at a building its sides go straight up and are parallel. It is not until you tilt your head back to look up the building that its sides seem to fall in. This is because your head and the building are no longer parallel. If you could climb up a really tall ladder and keep looking straight across at the building, its sides would remain parallel all the way up.⠀
So, as a photographer, you could carry around a tall ladder, climbing it with your camera so that you could photograph buildings straight on, keeping them square. Or you could use a lens that shifts, allowing the camera to look up while still pointing straight ahead. That is why rise is such a popular feature in architectural photography. You can more easily get the tops of buildings in without them converging or leaning.⠀
Usually a lens like this Pentax 75mm f4.5 for the 6x7 system is called a shift lens, not a rise lens. Shift lenses generally do both though. This 75mm has a rotating collar allowing you to re-position the shift's direction of movement from horizontal to vertical or even angles in between.⠀
P.S. While this post is specifically about the 75mm shift lens, we do have a full Pentax 6x7 kit getting ready to hit inventory in about a week. Stay tuned! Meanwhile, the link in our bio will take you to this lens in our inventory. ...