Home Movies and Home Talkies (Jun 1933-May 1934)

176 HOME MOVIES & HOME TALKIES THE PROBLEM OF REVERSAL FILM By Dr. H. LUMMERZHEIM EDITOR'S NOTE.— This article gives a very clear explanation of the special problems connected with reversal film, and many helpful hints on how to get the best from this kind of stock. It is translated from the German and has also appeared in the 1933 "Jahrbuch des Kino-Amateurs" SUB-STANDARD film is characterised by three features, which fundamentally distinguish its treatment from that of standard film. These features are the smaller size, exclusive use of safety base and the reversible emulsion. The size represents the correspondingly reduced bulk of apparatus and all the advantages accruing therefrom. Thus the use of a lens of short focus and the possibility of clockwork drive confer a facility in the use of the camera which places sub-standard film at a great advantage over standard film even for professional work. The safety base does more than exempt the use of sub -standard film from statutory safety regulations ; it permits a fundamental simplification of the construction of the projector and thus gives it the universal applicability which the amateur requires. Finally the reversible emulsion pro *^S 1 — — 1 \ ^N \ ^ n\ "^ ^\ s •*^s^. ^i . . . . 1 Fig. 1. Various times of development in the first developer vides a new jDhotographic technique. At the i^resent time we may put aside as of no importance the fact that reversible film was originally adopted for the sake of cheapening amateur cinematography. It was assumed that the amateur as a rule was content with one film only of a subject and thus needed only one length of the expensive perforated base. Substandard technique has developed new methods. The reversible emulsion has made possible film pictures of a beauty and "plasticity" which can be attained by the old printing process. Such remarkable success naturally implies not only that the process has been carefully worked out and continuously perfected by the makers, but that the amateiu has made himself reasonably familiar with its jaroblems. What is the reversal process ? In ordinary photography, when developing an exjjosure, the process, as is well known, consists in a darkening of the parts where light falls by reduc Y 2' 1 ^ A . ' s ,\ v JSSL. ^-:\>i\ s^ xV \ \ «!»!: — ' u\\ \ \ ^\M^ \ " 1 2 J A 1 1 Fig. 2. Developed as in Fig. 1, but with addition to first developer of a solvent of silver bromide. Negative curves plotted for development time of 8 minutes in same developer ing the yellow silver bromide to black silver. In the negative process the residual silver bromide is dissolved out in the fixing bath, with the result that an image is obtained which transmits more light the less the action of light on the particular part when making the exposure. Hence the brightest parts of the subject appear the lightest in the image, and viceversa. But an image may be produced in almost as simple a way to .show the parts affected by light as the lightest, and the unexposed parts as dark. For this purjjose all that is necessary is to dissolve the reduced silver developed on the parts exjDosed to light, leaving the silver bromide in situ in the film. This may be done, for example, with an acid solution of bichromate or permanganate. If the plate or film so treated be exposed to daylight and then placed for the second time in a developer, the resulting image shows the heaviest darkening in the parts where the action of light when taking the pictvu-e was least, that is to say, it is a positive. This second process, yielding a correct image of the subject directly. became known not long after the negative process. On account of the opposite character of the results, it has received the name " reversal " to distinguish it from the original process. The term is not altogether apt and may mislead the uninitiated somewhat. It might be thought that this process would have been able to displace the negative process completely immediately on its discovery. It may be objected that the process yields only one copy, and that, as a rule, a considerable number of positives are required. But this argument does not hold water, since the original positive obtained by the reversal process may readily be reproduced. The reversal process in photography has certainly been most u.sefuUy ajjplied to colour plates and films, reproduction of docimients, rapid jjliotography of the Photomaton type, and for substandard cine films. But the true reason for its playing a part sub Exposed with stop 3.5 5.6 9 8 8 8 mins. Fig. 3. Developed in first developer Exposed with stop 3.5 5.6 9 ^ 8 10 mins. Fig. 4. Developed in first developer ordinate to that of the negative process is very different. The aim of all photography is an image or picture which produces a pleasing effect by its gradation of tones. Putting colour photography aside, we endeavour to obtain, by means of differences of tone, as perfect a rei)resentation of an object as is ])ossible without the aid of bright colours. In technical language, this aim involves two problems. The first is the nature of the range of brightnesses ("gradation," as it is called) corresponding as closely as possible with the object. The second is a suitable density, which, in a positive transparency, is generally obtained if the brightest part of the object is rejjresented by a transparent area. In the negative-positive process it is relatively easy to achieve this aim, for tlie properties of the positive