International photographer (Jan-Dec 1934)

Twelve The INTERNATIONAL PHOTOGRAPHER May, 1934 The Steadman Measurements in Motion Picture Exposures By F. Morris Steadman ^ 5i ff t'.-» J *4 ?4 "4 *J 1 5 3) The (1) The Mestizo of Yucatan, Mexico. (2) Flamingoes. Night Blooming Cactus. (A time exposure by electric light every minute or two as it opened in the night. (4) The Kiddies Carnival. (5) Along the Beach. HE measurements here referred to are for measuring: Film Speed. (In seconds of time.) Subject brightness. (In Actinos. ) Working value of lens stops. (In Steads of solid angle.) To review briefly the truths that have been explained carefully in former articles appearing in the International Photographer : The speed of a film is the exposure which it requires to secure a normal chemical effect when the brightness of the subject before the lens is one Actino and when the one Stead stop is used in the lens in taking the picture. This speed is 32, 64, 128 or 256 seconds for films now used, 64 seconds being the speed of the present Plenachrome and Verachrome films. A subject to be photographed has one Actino of photographic brightness or actinicity when the light from its high-light area, shining thru an opening, creates on a small strip of film or standard tinting medium a least visible tint, in 64 seconds, when the tinting medium rests at a distance from the opening equal to one of the diameters of the opening. If the highlight area is bright enough to create that tint in 4 seconds, it proves to be 16 times brighter than if 64 seconds were required, and it, therefore, has 16 Actinos of intensity. Sixty-four divided by the tint time as found by a test, gives the brightness of the area in Actinos. The one Stead lens stop is that now known as F/64. 4096 (64), divided by the square of any F/number, gives the Steads of solid angle dimension in that stop. The Steads of solid angle express the "working value" of a lens stop, just as 4 horses or 16 men express working value in a problem of labor. The F/l light cone has 4096 Steads and the hemisphere has 8 times that value or 32768 Steads. (32M, in practice.) All cones are fractional parts of a hemisphere, just as any angle is a fractional part of a circle and tbe hemisphere has 32M (read 32 thousand) Steads of solid angle just as a circle has 360 degrees of plane angle. The F/ numbers are cone altitude numbers or form numbers while the Steads are pure value numbers. The following table shows the value of the different cones from the form F/64 Please mention The International Photogra to the full hemisphere : The cone forms F/64 F/45 F/32 F/22 F/l 6 F/l 1 F/8 Steads of solid angle 1 2 4 8 16 32 64 F/5.6 F/4 F/2.8 F/2 F/l. 4 F/l The full hemisphere 128 256 512 1M 2M 4M 32M (32,768) Steads I will assume that the M. P. film has the same speed as the Plenachrome and Verachrome films, 64 seconds, which, as stated, is simply the correct exposure for those films when the one Stead stop is used and the subject measures one Actino of brightness. The fact that the motion picture exposures are ordinarily 1/32 of a second makes it evident that, to take a one Actino subject correctly in that time, the lens stop will have to be as many times larger than 1 Stead as 1/32 of a second is shorter than 64 seconds, the speed exposure of the film. This standard motion picture exposure is about 2000 times shorter than the speed exposure, 64 seconds, therefore, the correct stop for taking a one Actino subject must have a working value of 2000 Steads. (F/l. 4). The key to the correct motion picture exposure is, therefore, this 2M Stead stop, since it is the correct stop to Use when taking a one Actino subject with the regular motion picture exposure. (1/32 second). (This is a mathematical key; your lens does not require that stop.) In exposing with the motion picture camera the brighter the subject, the smaller the stop that must be employed. We have found that stop 2M Steads must be used for a one Actino subject. Therefore, if the subject is 4 times brighter the stop must be 4 times smaller than 2M or 512 Steads. The simple rule is as follows: Divide 2M by the brightness of the subject in Actinos, as measured from the highlight area of the subject: The quotient is the Steads of solid angle to use in the lens in making the exposure. Should there be a motion picture film twice as rapid as the 64 second speed films mentioned, then it will require only half as large a key stop to take the one Actino subject properly, or stop 1024 or 1M, which number becomes the exposure key instead of 2M. The speed of such a fast film will be 32 instead of 64 seconds as for the other films mentioned. A practical example : The actinicity of an average sunlit exterior is 128 Actinos, when the sun is well up from the horizon. The stop to use is, therefore, 16 Steads, as found by dividing 2048 by 128. It would be impracticable and also an injustice to the present teachers in our schools to compel them to teach photography as a special extra study and craft. But it is evidently a reproach to our educational system that students must now finish their studies and still remain ignorant of the functioning of light. The right thing is to alter our basic idea of light intensitv and eliminate the POINT SOURCE theory from the physics books so that the study of the basic truths of light would reveal the nature of light variations in photography, in common with all the other light conditions which we experience, thus placing photography in the hands of students in the regular course of study. In other words: Eliminate the error of the POINT SOURCE and let the universal truths of light take their true place in the schools. pher when corresponding with advertisers.