From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Guide on photographic exposure corrections, with sections on copying and projection printing techniques.
| Identifier | ExFiles\Box 168a\3\ img524 | |
| Date | 1st May 1950 guessed | |
| as indicated in the following table, depending upon the particular keytone to be matched. The corrections given are for keytones directly illuminated by the major light source. Keytone corrections (Applicable to the white film speed index mark only). | Reflectance of keytone matched% | Typical keytones | Keytone correction to basic film speed | |----------------------------------|-----------------------------------------------------------------------------------------------|-----------------------------------------------------| | 100 | Magnesium carbonate block ('Standard White') Fresh snow Sunset cloud fringes (not too near to sun)* | Nominal B.S. logarithmic exposure index number | | 80 | White blotting paper Matt white card Clean white paint | +1° | | 65 | Slightly weathered white paint | +2° | | 50 | Old weathered white paint | +3° | | 40 | | +4° | | 30 | Normal face tone (diffuse highlight) | +5° | | 25 | | +6° | | 20 | Bronzed face tone (diffuse highlight) | +7° | * For sunsets with reversal colour film give double the indicated exposure (colour sensitivity correction). For example, to place a normal facial highlight at its nominal relative brightness level of 30% when using a film having a B.S. exposure index of 29°, the white index should be set at 29+5=34°, the diffuse facial highlight being accordingly matched with the photometer (See Fig. 13). Should the density level resulting from the above technique be found to be too high or too low for the particular purpose required, the setting for future exposures can, of course, be raised or lowered accordingly. It may be of interest to note that this is the essence of the more comprehensive method recommended for reversal (monochrome and colour) work and referred to earlier in the footnote on page 9. 18 COPYING The difficulties inherent in copying are due to the small latitude in exposure and to the inability of the eye to judge whether the copyboard is evenly illuminated. These difficulties are especially noticeable when process plates are used, but are easily countered with the S.E.I. Photometer. It is essential that the emulsion speed should be accurately known and usually this entails an experimental determination. For line work, the highlight method is used and the exposure must give adequate background density in the negative. Over exposure must be avoided however, because this leads to loss of detail in the finer lines. As a rough guide to the order of exposure index numbers applicable to line work, 24° has been found about correct for Ilford Process Plates in ordinary 100 watt tungsten lighting when using the white film speed index mark and matching the white background of a black printed original. The usual allowance for camera extension must, of course, be made (i.e. the indicated exposure must be increased as the square of the ratio:— 'negative-to-lens-distance' ÷ 'lens-focal-length'). It is probable that a slightly different exposure index number may be necessary for each emulsion batch. For continuous-tone work, normal emulsions are used and the exposures are based on the shadow method. (See Fig. 15). PROJECTION PRINTING The application of the photometer to the determination of exposures for enlarging (or reducing) saves much time and material once the principles are understood. The exposure is assessed by matching a given part of the image on the easel, and to obtain consistent results it is necessary always to use a constant diffuse white surface, say a piece of clean white blotting paper, on the easel. The reading is then, in effect, a relative measure of the light which will fall on the bromide paper when this is substituted for the blotting paper. Because of this no account has to be taken of the lens aperture, degree of enlargement, size or type of illuminant or type of negative in use—except that any variation of negative image or light source colour from those used initially to determine the paper speed may cause an error. If such variations cannot be avoided an allowance must be made. The yellower the negative or light source the longer will be the exposure required. Assuming constant colour and development conditions, the use of the photometer will, within wide limits, result in the measured image area always being reproduced in the print at a constant tone level. There are three general methods of using a photometer for this work, (a) matching the brightest significant image tone, (b) matching the darkest significant image tone and (c) matching some specific image tone. 19 | ||