The work of the commercial photographer is of the most varied character, but it may be divided into three sections according to whether the photographs are intended for use as a record, often necessary in engineering and construction work, to replace samples for the salesman to carry in soliciting orders, or for use in adver tising. In the second field the photographs are often coloured by hand, a process which is almost obsolete in other branches of photography but which is rendered necessary by the absence of any cheap and satisfactory process of natural colour photography upon paper. Photographs for use in advertising may be designed either to represent the article, as in those employed for catalogue illustrations, to show the actual use of the article in the hands of the consumer and thereby to create a desire for it, or merely to attract attention. This field of commercial photography gives great scope for artistic composition and has developed very greatly of recent years.
Two characteristics of commercial photography are its use of suitably trained models, who are chosen with the greatest care, and the employment of special means for the photography of coloured objects.
Panchromatic materials and light filters were adopted by com mercial photographers as soon as they were available, and a thor ough understanding of their use is essential to satisfactory work at the present time. The apparatus used in commercial photog raphy is remarkable chiefly for its adaptability and portability: it is necessary to photograph all kinds of objects in all sorts of places, and only a small portion of the work can be done in the studio.
Commercial photography is growing rapidly, a fact evident from the extensive use of photographs in advertisements.
the copying of documents, etc., photographic methods having the advantage of accuracy and speed. For this work a so-called "Photostat" camera is used in which a roll of paper is employed on which the document is recorded. (See Plate III., fig. 2.) The result is in the form of a negative and for many purposes these negatives are satisfactory; if positives are required, the nega tives are re-photographed in the same camera.
In addition to its use in libraries, and so forth, for circulating extracts from books or copies of letters and other documents, photographic recording is now used very largely for the prepara tion of legal records, especially in connection with real estate. For such purposes a special "ledger" paper is used coated with emulsion on both sides, so that the photographs can be bound into books in the minimum space.
For the photography of ancient or falsified documents use is made of ultra-violet light, by means of which it is possible to decipher palimpsests, forgeries and so on.
Another application of photography to recording is that of the so-called "Factograph" camera. (See Plate III., fig. 3.) This camera includes a spool of sensitive paper on which the record can be made, a lens and lighting box being placed in front of it so that when the camera is pressed down on an object such as a metre dial to be photographed lamps supplied by dry batteries will illuminate it. The object will then be in focus on the paper and many records can be made in quick succession.
Another special device carrying out the same idea is that due to G. L. McCarthy, who has designed a camera intended to photograph upon a strip of motion picture film all the checks passing through a bank. This camera, which is known as the "Recordak," provides a permanent record which greatly dimin ishes the risk of fraud. A more recent form of this camera, known as the "News Recordak" is employed to photograph newspapers on 35 mm. film, so providing an enormous reduction in storage space.
Special cameras have also been designed for photographing the inside of rifle barrels in order to study the erosion of the bore, for lowering into oil wells in order to photograph the shaft, and for use with cystoscopes to photograph the interior of the body. All these cameras involve the optical principles of the periscope (q.v.).