Shift lenses are often combined with a tilt mechanism, when they are called tilt shift lenses. Tilt mechanisms enable you to change the plane of focus. Shift lenses keep lines straight, horizontal lines horizontal and vertical lines vertical.
But, and here’s the thing, they allow you to look beyond the normal image circle of the lens.
Imagine you’re looking at tall building. You tilt your head up and the building seems to be narrower at the top than down at the ground. You know it is not, and it doesn’t bother you because you know that in reality the building is not leaning in and getting narrower. But that’s what it looks like.
It’s the same when taking a photo of a tall building with a normal lens. If you tilt the camera up to fit the building within the frame, the sides of the building will look like they’re leaning inward.
And if you took a photo like that, it could be perfectly acceptable. But if you were photographing architecture and wanted to show what the building actually looks like, with vertical verticals, then you can’t unless you use a shift lens.
A shift lens is made in to two parts that can move relative to each other. The front part slides upwards. That way it covers more of the scene.Think of it like two overlapping circles that cover more of the scene than just one circle would.
The only downside to shift lenses is that they are expensive to make and to buy. As and when I get a shift lens for photographing architecture in London, I will link to the photos from here.