Photography's beginning was based on photographic systems, like the Camera Obsura, which recorded images without a lens. The Camera Obscura did record recognizable image, however, the extremely small diameter of the hole opening required very long exposures. Photographers soon realized that a lens was necessary to make quality photographs possible with reduced exposure times. The search for a solution began.

All photographic lenses perform essentially the same function. The primary job of the lens is to collect all of the light rays coming from the scene in front of the camera and project them as sharp images onto a piece of film in the back of the camera. The addition of the photographic lens, increased images sharpness and reduced exposure times from several seconds down to a fraction of a second.

Daniele Barbaro, a Venetian nobleman, was credited with installing the first lens into a Camera Obsura. He increased the size of the hole opening and inserted a lens from a pair of gentleman's reading glasses. Several hundred years later, the convex lens was considered an essential component of camera design.

      Original Scene          Convex Lens     Focal Plane/Film Plane

The convex lens is thicker in the middle than on the outside edge. The shape of the lens enables it to collect a large number of light rays from a single point on the subject and then refract them back into a single point on the on the camera's focal plane. This point of convergence is called the focal point. The problem with the convex lens, on it's own, is that it bends the different colors of light in different degrees. When the lens refracts the light rays back together on the focal plane, they do not meet at exactly the same focal point. This defect is called a Chromatric Aberration. This aberration causes larger circles of confusion which are the building blocks 
of the recorded image. The final result is that the overall sharpness and quality of the image is compromised.

To correct this problem, a series of lens elements were used in conjunction to refocus the light rays and enable them to reach one exact focal point. This image depicts a cross-section of a lens with several elements. Photographic lens design continues to improve, however, even today's most sophisticated lenses are based upon this basic multi-lens element design.

This basic lens design has been modified over the years and has produced photographic lenses of varying focal lengths. The focal length is the distance between the lens and the focal plane when the lens is focused at infinity. These varying lenses require different numbers and sizes of lens elements in order to maintain a high degree of quality.

Photographic lenses have come to be known by their focal length. A standard lens is referred to as a 50 mm lens. Lenses are also referred to in relative terms. They can be considered short (wide-angle), normal, and long (telephoto). The focal length of the lens dictates it's level of image magnification and also the angle of view that it covers. As a general rule, longer focal length lenses have the highest level of image magnification and the most narrow angle of view. The pictures below represent a 6mm, 50mm, and 600mm lens. They are shown in relative proportions.

6mm F/2.8 Fisheye-Nikkor

50mm F/1.2 Nikkor

600mm F/4N ED-IF Nikkor

In addition to magnification and angle of view, the lens also controls focus, perspective, and depth of field. Cameras with interchangeable lenses offer the photographer a wide selection of tools for image composition.


by J.D. Brasesco