We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
one with a focal length of 5 inches, the other of 4 inches, and mount them with their centers 3 inches apart. Then the resultant focal length will equal 5x4 divided by (5 + 4) -3, or 20 divided by 6, which equals 3.33 inches. Here the resultant focal length is shorter than the focal length of either alone! If we could mount the same two lenses 2 inches apart, we would have 5x4 divided by (5 + 4) -2 which equals 20 divided by 7, or 2.86 inches. Thus the smaller the separation between the two lenses, the shorter will be the resultant focal length.
Similarly when the distance apart equals the focal length of either lens the effect of the other lens on the resultant is nullified. For instance if in the example the separation were 4 inches, the resultant would be 5 x 4 divided by (5 + 4) -4, or 20 divided by 5, bringing us back to 4 inches for the focal length of the combination. If the separation were 5 inches, the resultant would be 5 inches (5x4 divided by (5 + 4) -5).
This formula is the basis for determining the supplementary lenses often fitted to finders, and to the camera-lenses of some types of color-cameras, to give wide-angle effects where lenses of extremely short focal lengths are actually impossible. Suppose, for instance, we have a 2-inch lens, and fit over it a single supplementary lens (like a still camera's "portrait attachment") with a focal length of 200 inches. In this case let's assume, for convenience, that the fitting is so close the separation is negligible. This would make "F ", the resultant focal length, equal 2 x 200 divided by 2 + 200, or 400 divided by 202. This gives us a resultant focal length of 1.98 inches — a net reduction in effective focal length of 0.02 inches! For really effective results in achieving wide-angle effects, then, the supplementary lens must be of tremendously great focal length.
Hyperfocal Distance
The hyperfocal distance for any lens is that focal setting of the lens at which every object from one-half this distance from the camera to infinity will appear in approximately sharp focus on the film. If the hyperfocal distance is 68 feet, in other words, everything from a point about 34 feet from the camera to infinity will be in reasonably sharp focus if the lens is focused at the hyperfocal setting which in this case is 68 feet.
This point is dependent upon three factors: the focal length of the lens, the aperture used (expressed as an f-value) and the circle of confusion.
If lenses could be made perfect, the circle of confusion, theoretically, would be a point, for the term refers fo the diameter of the image on the film of any given point in the subject. Unfortunately, not even the finest lenses can bring the images of all wave-lengths or colors of light to a focus so perfectly in the same plane that the image of a point will be a point. Instead, some wavelengths come to a focus on the desired plane, while others focus in front of it or behind it. The results is that the point is reproduced as a circle, rather than a point. This is called the circle of confusion. In some of the finest lenses it is microscopically small, but still remains mathematically measurable. The manufacturers of the Robot camera, for example, base their sharpness on a circle
Page Ten