Many ultra-wide-angle, or short-focal-length, lenses are known as fisheye lenses. These lenses have a focal length of less than about 17mm (for a 35mm camera). The ultra-wide-angle view of these lenses results in image distortion. Straight lines appear curved and curved Lines may appear straight.

The use of fisheye lenses makes objects appear to diminish in size rapidly as the distance from the camera increases and objects which are close to the camera appear far apart.

Depth of field with a fisheye lens is very great. They often give depth of field that extends from only inches in front of the lens to infinity so that focusing is not necessary.

Rectilinear Lenses

A rectilinear lens, despite its wide angle, has normal rectilinear corrections so straight subject lines are straight in the image and there is no obvious distortion. The width of objects close to the camera appears emphasized because of the steep perspective produced by a rectilinear lens.

Macro Lenses

A macro lens is used for closeup photography and is a valuable lens for any imaging facility to have. These lenses come in various focal lengths and are capable of producing up to one half or even life-size 1:1 images. For example, a 100mm macro lens produces a 1:1 image just as a 50mm macro does. With a 100mm lens, you do not have to get as close to the subject. This is especially useful when you are taking pictures of live creatures or doing closeup medical photography.

Normal Focal-Length Lenses

The standard or normal focal-length lens for a 35mm SLR camera is from about 40mm to 58mm (the most common being 50mm). This focal length gives a field of view roughly the same as that over which the eye gives satisfactory sharpness-thus the name normal

Figure 1-35. Arrangement of lens elements in a telephoto lens.

focal length. With a normal lens, the angle of view and the image size you see in the viewfinder are normal. That is, you get much the same impression as you would get if you look at the subject with one eye. Many Navy photographers claim, however, that the wider angle of view of a 35mm lens (for a 35mm camera) is preferable. Others maintain that an 85mm focal length is better for general use because it enables the picture space to be filled more easily with subject matter.

A normal lens can be used for making pictures of people if you do not get too close. When you fill the frame with the face of the subject, you get image distortion. It is better to stand farther back and include the shoulders of the subject in the picture. This eliminates distortion.