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

HOME MOVIES & HOME TALKIES 231 SPEED plus Fine Grain plus Latitude | plus Tone Value minus Halation ^^^ equals ^gtI^ ^mMT ^^^^ 16-mm. Reversible NOVOPAN FILM unequalled for cinematography under artificial light of every description. 40 ft., 50 ft. and 100 ft. spools AGFA LTD. '° «' =«" ^="""- 1-4 Lawrence Street, High Street, London, W.C.2 exhibits appreciably greater speed in the medium densities, may have a much lower Scheiner value than a film of soft gradation, less rapid in the middle tones and superior only as regards the threshold value. At the same time the Scheiner number is no doubt of value in the case of negative sensitive materials, otherwise it would not be so widely used. In the case of material for negatives it may be generally taken for granted that the photographer employs a stojD with which the darkest parts of his subject yield the faintest recognisable deposit in the developer. Thus for an object of small himinosity range he obtains a thin negative showing all details, and by suitable choice of the positive material, as also by appropriate printing and developing, obtains a decent jirint. But the conditions are altogether different in the case of reversible film. Here tlie Scheiner measure of speed will indicate that the user has determined what exposiu-e is just enough in order to give in the finished positive an effect which camiot be distinguished from the maximum density. If it is desired to employ this speed value when taking a subject, what we have to do is to make a jjositive in which the darkest parts are of as heavy density as is possible with the reversible film employed. Thus, in the case of an object of small luminosity range, the lightest parts would thereby be far v^ moved from conaplete transparency. In using a projector of low power or when showing jDictiires in large rooms on a corresponding scale a film of this kind would be quite useless. It is more correct ^to give greater exposure in order to obtain full transparency in those parts of the film representing the brightest parts of the object. With an object of small luminosity range this means that the darkest parts are represented by deposit which is considerably lower than the maximum density. It will thus be seen that the question of threshold speed value is altogether unsuitable for determination of correct expostire of reversible film. In every instance regard must be paid to the Iviminosity range of the object and the probable conditions as regards projection. Actual high speed of a reversal sensitive material is a question, not of high threshold value (Scheiner), but of emulsion with which an object of given brightness may be correctly reproduced by means of the smallest possible stop so as to yield the maximum approximate transparency, that is to say in the lov/est part of the density curve. Exposure on these lines may be determined by sensitometric methods, whereas the Scheiner threshold value of speed is entirely without significance. It will be understood from the foregoing wliat is the value to the cine amateur of any optical exposure meter based on the Scheiner speed numbers. Moreover, such exposiu-e meters almost always determine the mean brightness of the subject as a whole, a method which is satisfactory for still photography but much less so for ctnematogi-aphy. In the case of the latter the point of interest in a subject is always in movement. We are never concerned with getting a correct photograjahic rendering of the whole scene but only with the part which is concerned in the phases of movement. The average brightness of this latter is frecjuently very different from the average brightness of the whole subject. Thus in cine -photography the conditions under which sensitive matei'ial is used are quite different from those in still photography. The reversal emulsions on sub -standard film are made for these special demands and are, in the main, of excellent qualities in this respect. The amateur using modern reversal film can obtain the finest results only if he will take the special qualities of his material into account ; the application of experience gained in the use of roll-film will do more harm than good in this new branch of work. The great majority of cine-amateurs have already used their own intelligence to discover the right method. When one considers the entire result in the shape of films which have been made at such trouble and with exercise of so much patient care since the introduction of reversal emulsion on