International photographer (Feb-Dec 1929)

September, 1929 The INTERNATIONAL PHOTOGRAPHER Fifteen On the other hand, if the width of the object space and the distance of the camera from the subject are known, the focal length of the lens to be used is easily determined by: 24 D F= -yT for the full standard aperture __ 20 D F ^W for the sound aperture. For example: A width of 32 feet is to be covered and the maximum distance available is 56 feet. The focal length of the lens to be used will be found thus: F= 24 X 56 32 for the full standard aperture 20 X 56 ■= 42 m.m. 32 = 35 m.m. for the sound aperture. A 40 millimeters will be sufficiently near the focal length required for the full standard size aperture while a 35 millimeters will respond to the requirements of the sound aperture. To explain how the above equations may be arrived at, we shall reason as follows: A composite objective may be considered as a single lens of a certain thickness. In order to geometrically trace the path of the light rays which pass through it and concur to form an image of an object, it is only necessary to know the position of the two Gaussian points within the lens, which are known as the Nodal point of entrance and the Nodal point of emergence. The focal length of a lens is measured from the Nodal point of emergence N1 to the point where the axis of the lens crosses the image plane I which is to be found where an object at an infinite distance from the lens forms a perfectly sharp image. In motion picture photography the angle I1 N1 I2 is determined by the focal length of the lens and the width of the camera aperture (24 m.m. for the full standard aperture and 20 m.m. for the sound aperture). The angle in the object space O2 N O1 is equal to the angle I1 N1 I2 in the image space because, according to the Gaussian law, any ray emanated from an object and entering a lens in a direction O N, emerges from it as if proceeding from the Nodal point of emergence N1 and in a direction parallel to the incident ray. The geometrical tracing of the path of the extreme rays concurring to form an image in a motion picture camera may be simplified by disregarding the internodal distance, as shown in the figure. Since the two angles encompassing the image and the object space are equal it is evident that the two angles shown by the shaded portion of the figure, and which for brevity we shall call a and al, are also equal. It is a truth that the angles a and al being equal, the ratio between the length of a perpendicular traced from any point of O N and the distance from this point to N is equal to the ratio between the length of a perpendicular from any point of N I and the distance from this point to N. In trigonometrical terms these ratios are called the tangents of the angles a and al respectively. Since the perpendicular I I1 is constant (12 m.m. when the standard full camera aperture is considered) and N I represents the focal length of the lens, we have tan al 12 and tan al = focal length of the lens supposing the focal length of the lens to be 50 m.m., we have 12 50 Now since the two angles al and a are equal, their tangents are also equal and, therefore, if we call X the length of a perpendicular O O2 and we arbitrarily fix the distance N O at 32 feet, we will have 12 X 50 32 and, therefore, X = 7.68 feet. This represents evidently half the total width of the object plane O1 O2 and, consequently, according to the data given above a 50 m.m. lens facing, say, a wall 32 feet from it, will cover a width of the wall equal to twice 7. 68 feet or 15.36 feet, roughly 15 ft. 4 in. If we represent this width by the symbol W we may, from the above, write the equation 12 X 32 W: 50 -=2 For convenience, since the factors 12 and 2 are constant we may simplify the equation to the form W= 24 x 32 w 50 or more generically if we represent the distance of the lens from the object by the symbol D, and the focal length of the lens by the symbol F 24 D W = — W F which is the equation given at the beginning of this article. In the case of the sound aperture, and following the same reasonings the factor 24 becomes 20 and, therefore, 20 D W = w f It is to be clearly understood that the results thus obtained are only approximate, since they are based upon the assumption that the lens is focused at infinity irrespective of the distance of the object from it. It is well known that to focus an object at a finite distance from a lens the latter has to be racked forward and, therefore, the distance N I becomes greater than the focal length of the lens. This racking forward of the lens is, however, very slight, especially for lenses used in motion picture work, and since the distance from object to camera is usually relatively great, the difference involved by the racking forward of the lens and the resultant errors may be disregarded in consideration of the convenience and rapidity with which the camera angle can be determined with sufficiently close approximation. Motion Picture Kngineers. the constant t m.m. for full aperture dimensions may he changed to L'.'i and the equation could therefore read w= 2i D w F o * Since the projecting' machine aperture has a width of 23.01 millimeters, according to the standards adopted by the Society of THE DARK ANGEL The wing of the Dark Angel once again touched Local 659, when Abbie L. Hoke, beloved mother of Brother Ira Hoke, went to her long home after a severe illness during which she was almost constantly attended by her devoted son. | Abby Furbush, a gentlewoman born, first saw the light April 29, 1860, at Clam Falls, Wisconsin, a town founded by her grandfather, native New Yorker, who came "west" to engage in the lumber business and whose holdings covered a large section of the Badger state. Abby Furbush sprang from real old pioneer stock, people prominent in early Colonial times, veterans of the French and Indian wars and also of the Revolution, one of her forbears being a member of George Washington's staff. After finishing school at Cordova, III., where her parents moved when she was a girl, Miss Furbush went to Minneapolis, Minn., where she became so intrigued with the printing trade that she learned it and became a member of the Minneapolis Local of the International Typographical Union. Later, feeling the lure of the west she transferred her affiliations to Local No. 95, I. T. U., Helena, Mont. Later, as Mrs. Hoke, the subject of our sketch returned to Cordova which was her home until 1910 when she came to Hollywood, where she lived until her passing with her only child, our Brother Ira Hoke. Mrs. Hoke maintained her interest in Union affairs until the end and was greatly interested in the welfare and growth of Local 659 and its magazine. To Brother Ira goes the heartfelt sympathy of every member — a noble son and his mother's greatest comfort. o OUR FRONT COVER You can't read it, but the name of the artist who shot our front cover for September is none other than Brother Frederic Colburn Clarke, photographer, poet, writer, painter, etc. Now turn back to the front cover and see if that desert shot could possibly be improved. o A SUGGESTION A suggestion that may relieve a lot of unnecessary keen competition and heartbreaking disappointment: When a first cameraman gets a job or is after one, and a second cameraman calls him about a job as his second, let the first man tell him frankly if he prefers some one else (whether he himself has the job or not). If they have worked together before and the first does not like the second personally or otherwise he should tell him what his faults are and give him a chance to improve rather than stall him off with a fake excuse, thus keeping him in suspense. Being very sensitive myself I was more heart broken about the attitude than about the loss of the job. I thank you. A Brother Member.