I enjoy threads where people discuss the need for an ultra-wide angle lens. It is almost always when landscape photography is the subject. Attached are three images taken last year at a lake near where I live to illustrate the difference of the three focal lengths that are most commonly discussed; 24, 17, and 14mm. No they're not masterpieces but they do show how the perspective shifts between focal lengths change the importance of the various elements in the image. At 24mm everything already appears twice as far away as it would with a 50mm normal lens. As you step out to 17mm and then 14mm from the same camera position, the foreground becomes more and more dominate in the image while the background becomes less and less distinct. That foreground domination is the reason to use an ultra-wide for some landscape shots; not to attempt to "get it all in" as many people do. If the perspective looks right to you and you need a little more in the image, you need to step back a little, not switch to a wider lens. If you do switch to a wider angle lens, you need to reassess the shot completely to make sure it is still a strong composition.