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

HOME MOVIES & HOME TALKIES 391 FILM $PEED$ AND EXPOSURES By PERCY W. HARRIS, F.A.C.I. Practical Hints on the use of Speed Ratings in Relation to the New Photoelectric and other Exposure Meters IN the Editor's News Reel last month reference was made to the lack of uniformity in methods of calculating cine film speeds, and the purpose of this article is to see whether we cannot arrive at some practical solution to the ever-present problem of film rating. Many Factors In our endeavours to get a properly exposed and well-graded film we are aware there are a number of factors to consider. Fortimately, practically all of these are definite and standardised, otherwise we should be very much at sea. For example, if we run our camera at the normal speed of sixteen frames per second we could rely on the manvifacturer having adjusted the mechanism to exactly this speed, and it is easy to ascertain for what exact period of time the lens is open on each frame. There is, by the way, a slight variation between cameras, but l/32nd of a second can 1)6 taken as the exposure in most cases. The Bell-Howell 70D camera is so constructed that l/27th of a second exposure is given ; the Cine ICodak gives an exposure of l/32nd and a few cameras go as high as l/35th, but, as you will notice, these differences are comparatively slight. Again, there is no lack of agi'eement among lens manufacturers as to what constitutes //3.5, //4.5 and so on — with two different makes of lens at the same stop opening or // number the same amount of light will be admitted to the film so we have no worry on this score. With shutter speed and stop opening known, then, what other factors have we to consider ? Only the subject itself, with its lighting and, most important of all, the sensitivity of the film. Fortunate Folk A few gifted and experienced people can tell at a glance what stop to use on a given subject without consult ing tables or using exposure meters, but these fortunate folk are so rare that we can leave them out of our consideration this month. To get regular and consistent results most people have to use either exposure tables or meters. In the case of tables the month, the time of the day, the clearness or otherwise of the sky and the subject are all brought into the calculation for all these obviously influence the amount of light which is focused by the lens on the film. Having by means of these calculations arrived at the amount of light, we must consider it in conjunction with the film speed, and here we find the makers of tables adopt one of two methods. Either they enable you to calculate your exposure by means of the H. & D. niraibers, or else (knowing as they do the unreliability of such figures) they give their own speed numbers which, as the result of mmierous tests, have proved to give satisfactory and wellexposed negatives with their par-' ticular system of calculation. With the visual meters the principle generally adopted is to turn the scale so as progressively to reduce the amount of light reaching the eye of the user until such a point is reached that certain figures are obscured, or certain signs are just readable. The brighter the exterior light the greater the amoimt of reduction which can else in units which can be used in conjunction with another scale for film speeds. Here once more we have the choice of two methods — the Scheiner or H. & D. speeds and the figures worked out by the manufacturer. We have recently been making a number of investigations with all types of exposure meter and comparing the results given. We have also compared the various exposure meter-makers' film speeds with one another after these have been referred to a common base. In every case the practical working" speed (that is to say the speed which, used in conjimction with a particular system. JUDGING THE "REFEREE" COMPETITION FILMS Tlie final judging of the films in the " Sunday Referee " open competition at Film House. Seated, left to right : Mr. Sinclair Hill, Chairman of the Judging Committee : Lord Lee of Fareham, Lady Lee, Mr. Anthony Asquith, and the Editor of " Home Movies and Home Talkies." Mr. Adrian Brunei (standing) has his back to the camera. For list of winners, see page 383 be made before the requisite dimness is obtained, and by consulting a scale over which the pointer rims we can find the stop which will give the requisite exposure. BUT, this scale must be used in conjimction with another one giving film speeds, and here once more we find two different methods adopted : H. & D. or Scheiner speed numbers and the special speed numbers arrived at by the maker of the exposure meter. In the case of photo-electric meters the amount of light reaching the camera is measured by a special means which eliminates the himian element entirely, for special photoelectric cells are used which generate electric current of an intensity dependent upon the amount of light falling on the cell. This current (or voltage) operates ^a special form of meter which is ^ directly graduated in stop nximbers for one speed of film, or gives a properly exposed film) has been lower than the film makers' claim. Here are examples. One of the oldest and most popular methods of ascertaining exposure for still photography is the Burroughs Wellcome Exposure Calculator. Many still photographers rely entirely on this and it must be said that this calculator has stood the test of time. The speed ratings by this method for some of the most popular films are as follows : — Agfa Rev. Pan . . . . 1/16 „ Ortho . . . . 1/16 ,, ,, Novopan . . 1/24 Kodak Rev. Pan . . . . 1/16 ,, „ Super-sensitive 1/24 „ 8 mm. . . . . 1/8 Gevaert Fine Grain Ortho Neg 1/12 ,, Rev. Super-pan . . 1/24 Pathe Ortho . . . . 1/12 „ Pan 1/24 {Continued on page 390)