Photography artistic and scientific Author:Robert Johnson Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER II. APPARATUS—LENSES, CAMERAS AND TRIPODS. Although it is quite possible to take photographs without any lens whatever, by the use of what is known... more » as a " pinhole," the lens is yet the most important part of a photographer's apparatus. The "pinhole " possesses all the qualifications sought for in an ideal lens except that of rapidity. It produces images that are absolutely rectilinear, that are equally sharp all over the plate, it has unlimited " depth of focus " (that is to say, that objects at varying distances from the camera are brought to a focus in one plane), it has no particular focus (that is to say, that the plate may be moved nearer to or farther from it without destroying the sharpness of the image, and so a greater or smaller angle of view may be included) ; but exposures require to be so prolonged that it has no practical value, and pinhole photographs are simply interesting curiosities. Photographic lenses may be divided into three main classes, portrait, rectilinear, and landscape, according as some one or more qualities necessary to the particular purpose have been gained by the sacrifice of others, desirable, perhaps, in themselves, but less essential. The object aimed at in portrait lenses has beenrapidity, since the dull light of the studio entails comparatively long exposures, which mean, in very many cases, the movement of the sitter. The rapidity of a lens depends almost entirely upon the ratio which its effective aperture bears to its focal length ; the increase of this ratio has prevented the portrait lens being entirely corrected for spherical aberration. It also reduces the depth of focus; so that in some cases if the sitter's eye be focussed, the tip of the nose is visibly out of focus; this is an extreme case and denotes a poor lens...« less