Ultra wide-angle lenses have unique characteristics. The shorter the focal lengths, the more the lens optically pushes the subject back, to make it appear smaller in the frame. The closer you get to your subject, the more distorted it appears. This is a reason ultra-wides are not used for close-up portraiture. If you've ever seen shots that depict intentional elongation of a subject's nose, it was made with an ultra-wide. The effect can be funny, but not pleasing for serious portraiture. If an ultra-wide in not held perfectly perpendicular to the subject, it will appear to lean backwards. The greater the tilt, the more this keystoning effect is pronounced. This distortion is often found in photographs of architectural subjects if the lens needs to be pointed upwards to include the entire building in the image. It can be corrected with software, tilt shift lenses, or large format cameras.
In the image of the tilting aspens, distortion on the edges of the frame is evident. The aspens on the left and right sides lean into the photo. To get this image, I was literally in and amongst the stand of trees. I used an ultra-wide to include all the elements. Finding just the right location was a slow process as very minor shifts of the camera position translated to major changes in the composition. Just a few inches to the right, left, up or down, meant an entirely new picture. With ultra-wides it's essential you study the entire viewfinder, so you don't include tripod legs, your feet, or head. Also, be aware of flare, if you point the lens upward as it's easy to overlook the fact the sun may be close to the edge of the frame.
In the image of the mountain with its reflection, it was essential I pulled out my ultra-wide to be able to include all that appears. The body of water in which the reflection is shown is very narrow and the actual mountain is close. I had to push the scene back to include the reflection, as it's an important piece of the photo. Landscape photographers often use wides and ultra-wides to place an emphasis on a foreground subject. If the lens is placed close to the foreground element, depth is created, as the foreground looks a lot bigger than it really is—see the above explanation about distortion. The illusion of depth is depicted as the viewer's eye is lead through the foreground, midground and background of the picture. The equivalent full-frame focal length for this image was 17mm.
The final image proves that I practice what I preach. One of the tag lines I tell all my students is to "Exhaust all possibilities." I liked the reflection of the mountain and clouds in the lake, but I wanted to include some foreground color and flowers. I made a number of images of just the mountain and reflection but, in order to include foreground, I had to break out the ultra-wide. Get in the habit of shooting every scene with a wide variety of focal lengths which includes telephoto, normal, and of course, ultra-wide. .