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).
Instructions for determining camera exposures using a photometric device and its principles of construction.
| Identifier | ExFiles\Box 168a\3\ img522 | |
| Date | 1st May 1950 guessed | |
| one-third of a stop, and vice-versa. The slight drift of the microammeter pointer is due to the unavoidable drop and recovery of the battery voltage and can be minimised by keeping the switch button depressed only for the short time necessary to obtain the required brightness match. 3. Set the colour-of-the-light corrector. If working by daylight set the knurled disc (3) which is found under the eyepiece (4) of the telescope so that the white index mark is immediately under the telescope. If working in ordinary artificial light set this disc so that the yellow index mark is under the telescope. 4. View the subject. From the camera position look through the telescope (4) and (6) at the scene to be photographed. 5. Focus the eyepiece to your individual eyesight by sliding it along the barrel of the telescope. The object lens (6) also may be moved outwards if necessary for close work, but for ordinary use it should not be withdrawn. 6. Orient the spot. Make the black photometric spot in the middle of the view coincide with the image of the darkest part of the scene in which detail is required. 7. Match the spot. Press the switch button again and try to make the spot match the part of the scene selected by turning the broad knurled ring (1). If the spot is always too dark or too light the subject image must be darkened or brightened by turning the knurled disc (7) to the appropriate range so that the photometric spot can be matched. This knurled disc will be found to carry three coloured index marks. One of these is to be set immediately under the middle of the telescope objective as follows :— For bright areas use the blue index mark. For normal areas use the white index mark. For dark areas use the red index mark. When bright areas are viewed and the blue index mark is in use, the shield round the eyepiece of the telescope must be close to the eye. In very bright light additional shielding with the hand above the eyepiece may also be helpful. 8. Read the time of exposure on the appropriate scale (8) in relation to the f/number scale (9), taking care to select the exposure time scale of the same colour as the index mark used for the operation described in the previous paragraph. For '16 frames' cine work read off the stop on scale (9) against the cine index mark (C) on the appropriate scale (8), or against 1/50 sec. for '24 frames' cameras. Note : A small amount of general flare in the photometer objective is unavoidable. Within limits this is actually an advantage since it tends to compensate for the effect of flare in the camera (see “The Exposure Photometer and Modern Exposure Technique”—Photographic Journal, 88A, 230, 1948). Excessive photometer flare should be avoided by matching on a 'darkest object' which is not too near to a very bright area or light source. TO DETERMINE CAMERA EXPOSURES FOR REVERSAL WORK (Colour or Black and White)★ 9. Set the speed scale by bringing the white index mark on the knurled ring (2) opposite the appropriate number on the Film Speed Scale (10). (See paragraph 1 for table of equivalent speed numbers). 10. Make the necessary adjustments described in paragraphs 2 (to standardise the lamp), 3 (to set the colour-of-the-light corrector), and 5 (to focus the telescope). 11. Orient the spot. Select for measurement the most brightly lit white object in the scene which is facing the main light source. If no such object exists an artificial highlight should be made by placing a piece of matt white paper or card so that it is illuminated in the same manner as the subject. The card surface should be at right angles to the direction of light from the main source in order to give the maximum brightness. 12. Match the spot. Make the photometric measurement of the highlight in the same way as described for the shadow area in paragraphs 6 and 7. 13. Read the exposure as described in paragraph 8. For '16 frames' cine work read off the stop on scale (9) against the cine index mark (C) on the appropriate scale (8), or against 1/50 sec. for '24 frames' cameras. Principles of construction The subject is viewed approximately full size, but inverted, through a simple telescope (B) (Fig. 2). By an adaptation of the Lummer-Brodhun cube, a small comparison spot (C) is superimposed on the centre of the image field, this spot subtending at the eye an angle of only ½°. The spot is diffusely illuminated by a small electric lamp via a diffusing screen (I), the lamp being fed from a dry battery (J) through a rheostat (N). The lamp also illuminates a ring-shaped photo-electric cell (H) which ★ More comprehensive 'keytone' and 'incident light' methods of using the photometer for reversal work are described in a paper entitled “Exposure Technique for Reversal Materials” (British Kinematography, 13, 151, Nov. 1948). See also 'Keytone Method' under 'Other applications' (page 17). | ||