Tilt-shift lenses are used to control perspective. They are great for landscape and architectural photography or in any kind of photography where you need unusual control over field of focus and depth of field.
Not all Tilt-shift lenses are created equal. The newer, more expensive lenses such as the Canon TS-E 24mm f/3.5 L II I used to make the images in this post allows independent rotation of the tilt and shift axis. Most lenses won’t do this.
But all TS lenses do the basics which is they allow perfect focus along any plane. You aren’t limited to the plane of focus that you have on your camera – i.e. the straight up and down orientation of your sensor. You can keep things from key-stoning for instance. You can play with perspective to create unusual blur and bokeh.
These lenses are based on the simple movements of the original view camera – though not quite as robust.
TS lenses are also tough on beginners because 1) they cost a bloody fortune 2) They don’t usually offer autofocus.
This is one case where if you have no experience with TS lenses – you absolutely MUST read the manual that ships with the lens. There are controls on TS lenses that you will not be familiar with and that will cause much frustration if you don’t take time to learn them.
TS lenses are not for everyone. But if you want to do something unique in terms of control over perspective, focus and bokeh and can afford it, a TS lens can’t be beat if you’re using a 35mm format camera.