EXPOSURE METERS By Jim Branch & Nasir J. Zaidi, ASC Member (Associate) The usual final adjustment of a motion-picture camera for exposure control is made with the iris diaphragm in the camera lens. While this is a very simple adjustment, a great deal depends upon its accuracy. Much thought has gone into the objectives to be attained by the adjustment of the diaphragm, and the means to obtain a correct adjustment. It is recognized that a prime object of exposure control in motion-picture photography is to obtain consistent and uniform images of the principal subjects. It is very important to obtain flesh tones which will be consistent from one scene to the next. It is undesirable to have flesh tones which will be light in one scene, dark in the next without reason, and again light in the next scene. Correct exposure control will provide negatives which are consistent from scene to scene and can be printed on a very narrow range of printer lights. Modern exposure control is based on the use of a good light meter. The light meter measures the effective intensity of the light, taking into account the sensitivity of the film in the camera and the exposure time. The exposure time is a result of the frames-per-second rate at which the camera operates, and the angle of the shutter opening. Professional cinematographers usually think in terms of 24 frames per second and a 175-degree shutter, which give a basic exposure time of 1/50 second. The light meter combines all of the foregoing factors to give an answer in terms of the appropriate camera lens stop. Never in the history of motion picture photography has the need for accurate exposure control been as great. With the increasing use of digital video postproduction to manipulate original motion picture photography, the cinematographer more than ever needs to monitor how the exposures generated on the original negative as densities will reproduce as digital electronic signals. Maintaining control of creative photographic visions can be greatly enhanced with the aid of a highly accurate exposure meter. The meter should be able to predict how original scene luminance will ultimately reproduce as print densities empowers the cinematographer to confidently explore maximum creative possibilities. Light meters are of two types. Some measure the incident light which illuminates the subject. Others measure the light which is reflected from the scene. The results obtained from the two different types may be quite different. It is important therefore to understand the differences between the two types. Incident Light Meters These meters are normally used at the location of the photographic subject. They measure the light which is effective in illuminating the subject. They give an answer in terms of f-stop or T-stop for the camera lens. The camera lens diaphragm opening is then set to match the effective intensity of the prevailing illumination. When the film is exposed, the various reflectances presented by the subject will then each fall into a given place in the film acceptance range. For example, a face tone of 30% reflectance will fall into the 30% reflectance position in the film acceptance range. This method thus provides consistently uniform face tones from scene to scene. The incident light meter accomplishes its purpose by doing two things. It measures the incident light intensity at the location of the photographic subject. It also takes into account the conditions of illumination geometry; that is, whether the subject has front key light, side key light, or a back key light. The meter combines these factors and gives an answer in terms of the correct setting for the camera lens diaphragm. There are several makes of incident light meters which use a three-dimensional light collector. The hemispherical light collector allows these meters to perform automatically the dual function described above. These incident light meters are normally used at the position of the principal subject, with the hemisphere pointed at the camera lens. The hemisphere then acts as the miniature face of the subject. All illumination which will be effective on the subject, including key light, fill light, line light, hair light, eye lights, etc., will be received, evaluated and integrated by the meter. The meter will then indicate directly the correct f-stop or T-stop for the camera lens. Incident light meters are particularly useful because they may be used on a scene before the principal subject appears. They may also be carried through a scene, with the hemisphere always pointed at the camera lens, to detect uneven illumination, and particularly hot spots, into which the subject may move during the action. This allows the scene illumination to be suitably balanced before the principal subject is at hand. In the case of outdoor photography, it is not always necessary to take the meter to the location of the principal subject. Under such conditions the illumination is usually uniform over considerable areas. If the illumination is the same at subject location and at camera location the meter may be used at camera location. Care should be exercised to point the meter in the proper direction, as though it were at the subject location. Exposure meters, in general, are either analog (with a needle) or digital. The introduction of the analog incident meter with the 3-D light-collecting hemisphere revolutionized the method of determining proper exposure for the cinematographer. Today, a number of companies throughout the world manufacture exposure meters employing the basic incident type principles in their design, but all due credit should be given for the invention to Don Norwood, ASC, who patented it, and Karl Freund, ASC, who was instrumental in the development and design of the original “Classic” Spectra light meter. Most incident meters are provided with suitable adapters so that they may be converted for use as a reflected light meter if the occasion should so indicate. The reflected light adapter can be used in a situation where the cinematographer encounters difficulty in putting the meter into a position to read either the illumination directly on the subject, or illumination similar to that on the subject. Such a situation, for example, might be encountered when taking a picture out of the window of an airliner in flight. The reflected light attachment can also be used in other situations to evaluate the relative brightness of a background. Special Effects When a special effect is desired, the cinematographer may use the incident light meter to first determine normal exposure for the subject. Then he may then deliberately modify that value, up or down, to achieve the desired effect. This can be done with considerable confidence because the incident light meter will give a firm foundation upon which to base the desired modification. Specific Situations There are some situations, occasionally encountered in outdoor photography, which require special attention. 1. Unusually light or dark backgrounds are cause for consideration. When a scene includes an unusually light background, the cinematographer may wish to first use the meter as an incident light meter to determine the basic exposure for the principal subject in the foreground. Then he can convert the meter to a reflected light meter in order to measure the brightness of the unusual background. The second reading is then used to modify somewhat the basic incident light reading. The same procedure could be followed in the case of an unusually dark background. 2. Outdoor scenes that include a subject in the foreground as well as distant objects, such as mountains, in the background, usually also include considerable aerial haze, which may be invisible or only partly visible to the eye, but strongly visible to the camera. A frequent photographic result is a recording of the aerial haze overlaid on the scene background. This would give the appearance of an overexposed background. It is recommended that in such a situation a haze-cutting filter be used to improve the background. In addition, use the procedure previously described for the case of an unusual lighting background. 3. Scenes consisting of a mixture of sunshine and shade areas, with the principal subject in a shade area, can be handled by: (a) using the meter in the sunshine area, or (b) opening up the lens by 1/2 to 2/3 f-stop from the meter indication. Reflected Light Meters Reflected light meters can be classified into two groups, according to function. The meters in each group may give exposure readings which are substantially different from those given by the meters in either of the other two groups. This is due to differences in basic principle of operation. Group 1. These are the meters which are designed to measure the average brightness of an entire scene. Such meters are usually used at camera location and pointed at the scene. For a discriminating observer, this method appears to give acceptable results only in the case of a very limited category of scenes, those which have front-lighting and a foreground subject of medium tone as well as a back- ground of medium tone. In other types of scenes, which include side-lighting or backlighting, or very bright or dark backgrounds, or large areas of sky, the exposure results are questionable. This is because the meter, when used by this method, is affected not only by the unit brightness of each portion of the scene, but also by the relative area of each. Thus a large area of sky would influence the meter to dictate a small lens aperture which might result in an underexposure of the face of the principal subject in the foreground.