Focal Length and Image Size of Digital Camera

The focal length defines the “size” of the lens. As it happens with circles and spheres, were the radius is enough to define a group of similar object, the same can be said of lenses and focal length. A lens with a large focal length does in a larger scale exactly what one with a shorter focal length does; the only thing that changes are the absolute values, the dimensions of the images measured in millimeters: The size of the images is greater, the greater the focal length is. The eye has practically only one focal length, and therefore the size of images depends only on the distance it you are from the object being observed. On the other hand, the image of an object produced by a lens of, for example, 100 mm of focal length will be double the size of an image formed by a 50 mm at the same distance from the object. The focal length and the size of the images are directly proportional . In the previous example, a 100 mm lens produces a linear increase of twice (all dimensions of the image are multiplied by 2) as much as a 50 mm lens. (Note that the linear increase refers to all the dimensions of the image; if we compare the surfaces of the images formed by the lenses of the example, we will note that the area of the larger will be four times that of the smaller.)

Digital Camera Lens Coverage

The image projected by a lens is circular, with maximum presentation in the central zone, and decreasing quality close to the borders. The useful area is called coverage , and has a diameter much larger than the diagonal of the format designed for the lens. Depending on the size of the sensor, a lens designed for a compact digital camera has a coverage of a diameter no larger than 25 mm; for a 35 mm camera the minimal coverage necessary of the lenses is of a 45 mm in diameter; in 6 x 6 format it has to be of 85 mm and in 9 x 12, of 150 mm (or even more if the camera accepts off centering. A lens can be used with any format that can fit in it's coverage; for this reason, large and medium formats offer the possibility of interchanging film holders of different formats. If we maintain an focal fixed, the absolute size (in millimeters) of the images is invariable, and the different formats only determine the frame of a larger or smaller portion of the total image.