American cinematographer (Jan-Dec 1941)

'Highlight" Exposure -metering With ARTIFICIAL LIGHTING By P. C. SMETHURST THE possibility of measuring lighting contrast and linking it up with both image quality and camera exposure is exceedingly useful in artificial light. Here, there is no sky to lighten up the shadows, and since most reversal films reproduce a scene rather more contrastily than we seem them visually, it becomes important to be able to put sufficient light into the shadows for them to have the required tone values on the screen. Again, the highest reading on the meter as the card is placed in the subject position and slowly turned round to face all the lamps in turn is used for camera exposure, and we may link this up with the shadow light-value on the meter, and find out how much exposure difference there is between the floodlight of the shadows and the accent light which reproduces the sun. A thorough speed test on a chosen film in artificial light will make it possible to draw up a table of the type shown below, in which the relation between the meter-reading on floodlight, accent or key light, and backlight ( all taken from the position of the main subject, of course) can be set down against the effects produced on the screen. After this table has been drawn up, it is in the future possible to duplicate any kind of shot whatever so long as the factors relating to it have been preserved. Accent-light is measured as maximum meter-reading with artificial high-light on the lamps on the camera side of the subject. Shadow-light is simplest measured at right angles to the reading posi tion for accent-light. Backlight is maximum meter reading on the lamps on the far side of the subject from the camera. The foregoing notes have shown that there is nothing particularly difficult in the method outlined to obtain a standard image quality, but it is important that unless processing is consistent it is out of the question to expect that the images will all appear identical in screen quality. For this reason, it is essential to request that the speed test and all subsequent films are given uncompensated time-and-temperature processing, so that no attempt is made by the laboratory to correct special-effects which may seem to them to have been over or under Meter reading above or below standard exposure for matched face tones. Floodlight (giving 'he appearance of the shadows of the scene) exposed. Quite apart from this, the exposure standard chosen by one man will be quite different from that of another, if the two are not using the same power of projector and the same size of screen, and it would be foolish to get the processing people to compensate both their films to give one standard of imag. quality. Color films, in any case, cannot b<compensated during processing for exposure errors, so that in this case it is out of the question to save the image in the laboratory, and unless the exposure has been correct in the first place the results are not perfect. In color films, as with an automatic telephone exchange, any wrong numbers are the fault of the man who does the dialing. Some small practical points may b( mentioned here. If the film used is not backed, there is quite a chance that distant scenes may appear to be overexposed as compared with the foreground, but this is due to light spreading about in the film emulsion, and not to errors in the theory of the present exposure method. With reasonable care, stocks without backing can be used, but if there is any trace of shine on the pressurepad behind the film gate the light passing through the film will be reflected back into the emulsion and the definition and tone values of the scene are bound to suffer. It is always important when taking a reading that the light is not prevented from reaching the artificial high-light either by the body or the hands. This is usually simple in daylight, but not always quite so simple in artificial light. It is possible to use a reflecting surface of neutral grey in place of an artificial high-light, but since few of the greys obtainable are really neutral in tint, errors due to differential colorreflection are not uncommon, and since in any case a speed test must be made there is little advantage in using the grey. A checker-board of black and white squares could also be used, but it cannot usually be cleaned without altering the exposure standard read from it. (Continued on Page 446) Accent light (showing Hacklight (showing the the appearance of the appearance of the lightest face tones) halo around a head) — 2 V2 stops — 2 stops — 1 V2 stops — 1 stop — V2 stop 0 (i.e. standard camera exposure) -f 1 stop + 1% stops -f 2 stops Dark shadows, no details. Full dark shadows, some detail. Medium dark shadows. Lightish shadows. Light shadows for high-key work. Very light shadows for high-key work. Night effects. Accent — Light for low-key image. Medium low-key accent-light. Standard image quality. High-kev accentlight. Very high-key accent-light. Face tones burnt right out. Faint. Noticeable. Pleasant. Strong. Brilliant. Halo burnt out. 430 September, 1911 American Cinematographer