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Chapter 5:
Depth of Field
It is, of course, well known that if we focus a camera on an object at some definite distance from the camera lens, there will be a finite range of distances in front of and beyond the focused object in which everything appears acceptably in focus, while outside that range everything becomes progressively more blurred at increasing distances from the plane of best focus. The actual extent of this range of acceptably sharp definition depends mainly on the distance of the subject from the lens, the aperture of the lens, and on the manner in which we look at the final print, but it also depends to some extent on the type of subject being photographed, the resolving power of the film and paper emulsions, and the aberrations of the camera lens. However, for the sake of simplicity, the following discussion will be based on the assumption that we are using grainless film and aberration-free lenses, and we shall end with a few remarks on the effects of these neglected factors in actual practice. For ordinary fairly distant objects, the depth of field of most cameras is generally adequate, provided that we take care to focus the lens on the subject of principal interest. In a portrait this is generally the front of the face; failure to focus correctly in such a case may be artistically disastrous. However, when we have occasion to focus a lens down to one or two feet, we are surprised to find how little depth there is, even at small apertures. For example, a 2-inch lens at f∕8 when focused on objects at 12 inches distance has a depth of field of about half an inch in front of and beyond the focused distance. A small depth of field is sometimes very useful, for instance, as a means of eliminating an unwanted background (Fig. 5.1).
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