Depth of field or the area of acceptable focus within a photograph is influenced by three factors: the aperture of the lens, the subject to camera distance, and the focal length of the lens. Our expectations for photographs are that they transparently represent the subject of the photograph. Each of the three components of the depth of field offers exciting opportunities for experimentation and challenging our expectations for a photograph.
Focal length measures the distance from the sensor or film plane to the tip of the lens, which is the distance from the point where light converges on the lens to the film plane or sensor. Focal lengths also determine the angle of view of the lens. A telephoto lens reduces the angle of view, or by definition an angle of view less than 15 degrees. A wide-angle lens increases the angle of view of a scene or any lens with an angle of view greater than 55 degrees. Changes in the angle of view with both telephoto and wide-angle lens produce apparent distortions: a wide-angle lens causes objects close to the lens to appear larger than surrounding objects; a telephoto lens makes objects within the scene appear closer or compressing space, the opposite effect that we see with a wide-angle lens. Wide-angle and telephoto lens are said to distort perspective, however, this is a misconception. Perspective is influenced by your relative position or distance from a subject. The choice to use a wide-angle or telephoto lens may force you to change your location will influence the perspective of the photograph.
Focal length designations vary across camera formats and have become more complex with the introduction of new formats such as APS and Four Thirds sensors. Focal lengths for a 35mm film camera traditionally have been used to establish the baseline for focal lengths. Focal lengths for each camera format is determined by the diagonal measurement of the camera’s format. In 35mm equivalents, the diagonal for 35mm film or a “full-frame” sensor is 50mm. A wide-angle lens, therefore, is defined as any focal length lower than 50mm, for example, a 35mm, a 28mm, and the popular 24mm are all considered wide-angle lens. A wide-angle lens provides a wider angle of view than a normal 50 lens. A telephoto lens is defined as any lens longer in focal length than a normal lens, and typically a telephoto lens begins with a focal length of 135mm and longer. Lens focal lengths between 50mm and135mm are considered a telephoto lens, but they carry the special designation of a “portrait” lens. Telephoto lenses generate a much smaller area of focus than a normal or wide-angle lens. Wide-angle and normal focal length lens appear to shrink objects in the distance relative to objects in the foreground of the photograph, whereas telephoto lenses, because of the angle of view will appear to normalize the relative size of objects in a scene compared with human vision 
Lumix G3 with Tokina 150-500mm shown here is attached to the Lumix G3 using the LensBaby Tilt Shift Transformer adapter. The Tilt-Shift Transformer adapter accepts all Nikon F mount lenses and works only with Micro Four Thirds camera and is one of the primary reasons I purchased the Lumix G3. With the focal length conversion to the Micro Four Thirds format, the Tokina 150-500mm lens becomes a 300-1000mm telephoto lens. As a tilt-shift lens, the Tilt-Shift Transformer also offers perspective control corrections, which are especially useful with the spatial compression issues associated with the telephoto lens.
Decisions regarding focal length dramatically influence the representation of subjects within a scene. Long telephotos have traditionally only by wildlife photographers and sports photojournalists, but experiments with depth of field, compression, and layering of distant objects and the distortion of scale offer great opportunities for expressive photography. A wide-angle lens has long been the preferred lens choice for landscape photographers, however long lenses, including the “super” telephoto lens present opportunities for highlighting graphic relationships in nature.
DA, Barn and Reflection, Yuba Rice Fields, Yuba County, CA. Barn and Reflection, Yuba Rice Fields was created using the same techniques discussed in the Donner Lake image. Again the Tokina 150-500mm lens was attached to the Lumix G3 using the LensBaby Tilt-Shift Transformer. The Tilt-Shift adapter permitted a slight tilt of the camera body forward to compensate for the tendency of buildings to appear as if they are bending backward.
 Henry Horenstein, Russell Hart, Photography, p. 79.
 David Falk, Dieter Brill, David Story, Seeing the Light, Optics in Nature, Photography, Color, Vision and Holography, p. 115.
 Henry Horenstein, Russell Hart, Photography, p. 83.