British Kinematography (1950)

94 BRITISH K1NLMATOGRAPHY Vol. 17, No. 3 LIGHT MEASUREMENT FOR EXPOSURE CONTROL Arising out of a paper by Captain Don Norwood under the above title in the May, 1950, issue of the Journal of the Society of Motion Picture and Television Engineers, Mr. J. F. Dunn has addressed a communication to the Editor of that Journal, of which the following is an abridgement : — " I wish briefly to take exception to Mr. Norwood's statement on p. 595, in which he claims that ' his concept of Effective Illumination, which takes into account illumination intensity and relative positions of observer, subject and light source has not heretofore been crystallized or formulated ' (the latter italics are mine). In contradiction to this statement, reference may be made to the November, 1948, British Kinematography containing my article ' Exposure Technique for Reversal Materials,' from which it may be noted that I described this effect (which I named the * Duplex ' technique) over two years ago before a meeting of the British Kinematograph Society in London. " Since reading my paper in 1948, a slight improvement over my originally described ' Horizontal Duplex ' method has been developed. The new recommended ' Direct Duplex ' method, which has been worked out by my colleague L. C. Walshe and myself, still consists of taking two readings with an incident light meter (of the flat window type), namely a * camera direction ' reading (as previously recommended) and a ' major source direction ' reading (as originally recommended for use alone by P. C. Smethurst, who first introduced the incident light meter technique in England in 1936 — see his paper in Journal of the British Kinematograph Society, Vol. 1, No. 1, December, 1937). " The required exposure for average work is then given by simply taking the geometric mean of the two Duplex readings, i.e., the mid point on the stop scale between the two readings. For clear-cut conditions this technique will be found to line up almost exactly with the exposure levels recommended by the principal reversal colour film manufacturers, and it has proved to be highly successful in practice. It has, incidentally, already been fully described in a book on this subject which is in preparation and will be published in due course." Captain Norwood, in a reply to the Editor of the Journal of the S.M.P. & T.E. (a copy of which was kindly addressed to the Editor of British Kinematography) mentions that his experience with incident light meters dates from 1933, and cites the following U.S. patents : Nos. 2,214,283, 2,489,664 and 2,444,464. He continues : " It has been recognised that some workers in this field have had a more or less hazy realization that more was involved in incident light measurement for exposure control than a simple measurement of light intensity. Various corre:tive expedients have been proposed by some of these workers, such as pointing a meter with a plane surface light collector toward the camera from subject's position ; pointing said meter toward principal light source ; aiming said meter towards a point half way between said light source and said camera ; pointing meter at camera and at light source in turn and using a mean reading as suggested by Mr. Dunn. However, none of these makeshift methods appears to indicate a full and clear-cut realization of the basic principles involved in the matter. None of the experimenters has, to my knowledge, brought forth precise and comprehensive formulas such as those shown in (15) and (16) on p. 595 of the May Journal. " 1 do not agree that Mr. Dunn in describing his Duplex Method in British Kinematography has given a clear-cut, well crystallized comprehension of all the factors involved, as well as a formula for accurate solution of the problem. For instance, his formula for calibration of incident light meters was preceded by a quite similar formula on p. 14-16 of the I. E.S. Lighting Handbook, published in 1947. Neither formula takes into account the vital factor of geometrical relationship of subject, camera and light source. If this relationship were understood it would seem that it would have been put into Mr. Dunn's formula. " It is of interest to examine under specific conditions Mr. Dunn's recommended method of operation. As an example, we find that Mr. Dunn's Direct Duplex Method would give identical exposure control settings for a cross-lighted arrangement (90° keylight angle), and a back-lighted arrangement (135 keylight angle), where other factors remain constant. Reference to instructions issued by leading colour film manufacturers (see Note 5, my paper) will show that a considerable difference in exposure control setting is recommended for cross-lighted and back-lighted conditions. The difference is usually about one /-stop. It is generally believed that reversal colour film will not tolerate an error of one /-stop. Therefore Mr. Dunn's * tworeadings ' method when used with colour film under the common conditions described above will give errors which lie outside the acceptance latitude of the film. This also negates Mr. Dunn's statement (next to last paragraph in his letter) that his method lines up with recommendations of film manufacturers." In a rejoinder Mr. Dunn emphasises that he had no intention of depreciating the work of Captain Norwood ; the calibration equation given in his original paper was not claimed as novel. He continues : M Though a full range subject illuminated by front or flat lighting just about lines up