Lenses Classifications from camerapedia
A normal lens has an angle of view that approximates how the human eye sees a scene. A lens is considered normal when its focal length is approximately equal to the diagonal of the film format. Lenses shorter than normal are called wide-angle, while those longer are called telephoto. In 35mm photography, 50mm is considered to be the normal focal length, even though the actual diagonal of the frame (24mm x 36mm) is 43mm. For medium format photography (frame size 2-1/4″ square, or 6x6cm), normal is generally 80mm.
A wide-angle lens is a lens with a focal length shorter than normal. A wide-angle lens, as opposed to a fisheye, is normally well-corrected for geometrical distortion, i.e., straight lines appear straight. However, as the angle of view is wider than what is seen by the human eye, they can create an impression of distortion; this perceived distortion increases as the focal length of the lens decreases. A fisheye lens (see below) is an extreme example of a wide-angle lens.
A telephoto lens is any lens with a focal length longer than normal. A telephoto lens brings far subjects closer, like a telescope, hence the name. They therefore have a smaller angle of view than a normal or wide-angle lens. The longer the lens, the more likely that camera shake will blur the image; for this reason, longer telephoto lenses are usually used with a tripod to steady the camera.
Fisheye lenses have the widest field of view of any lens group. The geometrical distortion of the image is purposefully maintained high, and straight lines appear curved if they are near the edge of the image. This creates distortion of the resulting image in a dramatic way. Fisheye lens fall into two categories:
- Circular fisheyes: have a 180 degree field of view when measured along the smallest dimension of the image, resulting in a circular image with black corners. In 35mm format, they usually have a focal length around 8mm.
- Full frame fisheyes: have a 180 degree field of view when measured along the diagonal, so the image extends on the full film plane. In 35mm format, they usually have a focal length around 16mm.
In general, fisheye lenses are expensive and little used in everyday photography.
Fisheye adaptors: auxiliary lenses are available that simulate a fisheye field of view. This is cheap way to play with the fisheye effect without investing in a dedicated fisheye lens, although, as with most auxiliary lenses, the quality of your images will not be the same as those taken using a ‘real’ fisheye.