EXPOSURE AND EXPOSURE DEVICES 255 



The acceptance angle of the majority of meters on the market today is entirely too 

 large, so that the meter indications are affected by light from areas which are consider- 

 ably outside the picture area. Large acceptance angles are therefore objectionable 

 from the viewpoint of the photographer since the meter must be more carefully used 

 to prevent erroneous readings. The manufacturing cost of a photoelectric exposure 

 meter is governed quite largely by the acceptance angle, as large angles permit the use 

 of considerably cheaper galvanometers because more current is available from the 

 photoelectric cell resulting from the increased area from which the light is accepted. 



Methods of Limiting the Acceptance Angle. — The acceptance angle of an exposure 

 meter may be controlled by any method which will restrict or prevent oblique light 

 from reaching the photoelectric cell. One method is to locate the photoelectric cell 

 deep into the meter case so that the case itself acts as a baffle. This method is used to 

 a large extent in the meters made in Europe but, owing to lack of space within the 

 meter case, the amount of baffling obtained is so small that the resulting acceptance 

 angle is entirely too large. Some meters such as the Mini-Photoscop exposure meter 

 mount the photoelectric cell on an angle (approximately 45° from the vertical plane) 

 and also utilize a mirror mounted so that the cell and mirror form an angle of 45°. 

 This method quite definitely limits the acceptance in the vertical plane but is not very 

 effective in the horizontal plane, the theory being that the elimination of excessive 

 sky and foreground light is adequate. Other meters such as the Weston, Metrovick, 

 Helios, etc., utilize a mechanical baffle over the photoelectric cell. These mechanical 

 baffles usually consist of either thin plates latticed together or solid or laminated 

 thin plates containing a number of holes, the ratio of the width to the depth of the 

 individual holes being the factor which governs the acceptance angle. The General 

 Electric exposure meter utilizes a mechanical baffle but is different in actual construc- 

 tion from the above meters. Instead of using a relatively thin plate containing a 

 number of holes, this meter has a sliding baffle of appreciable depth (1.4 in.) and 

 contains a single hole (1.53 by 0.74 in.) for the light to enter. This baffle when not in 

 use forms a protective cover for the entire meter. 



Polar diagrams (Fig. 9) show the acceptance angles of a number of exposure 

 meters. It will be seen that while all the meters are more selective to light at normal 

 incidence, many of them are affected too much by oblique light which would be far 

 outside of the view angle of most cameras. 



Scale Calibrations. — Meters having scales calibrated to be direct reading in either 

 /-stops or shutter speeds are quite convenient to use provided the same type of film or 

 plate is always used and also if the same /-stop or shutter speed is used at all times. It 

 should be obvious that, if a scale is calibrated in /-numbers, it can be direct reading 

 only for definite combinations of shutter speeds and film speeds; or if the scale is 

 calibrated in shutter speeds, it can be direct reading only for certain combinations of 

 /-stops and film speeds. Also, while the combination of /-stop and shutter speed indi- 

 cated may be correct as far as exposure is concerned, it may not be a usable combina- 

 tion, since the /-stop may be incorrect in order to obtain sufficient depth of focus, or 

 the shutter speed may be too slow to arrest motion. 



Because of these limitations the trend in exposure meters now is toward using scales 

 with either arbitrary scales, as on the Weston Junior and Cine Meters, Mini-Photo- 

 scop, etc., or with scales calibrated in fundamental units, such as the General Electric 

 meter which is calibrated in foot-candles or the Weston Universal which is calibrated 

 in candles per square foot. These meters are extremely flexible tools as the scale cali- 

 brations are independent of film speeds, /-stops, or shutter speeds. By means of a 

 calculator which is an integral part of these meters the scale readings can easily be 

 translated into a series of exposure values, the choice of any combination depending 

 upon whether the shutter speed must be fast enough to stop motion or whether con- 

 siderable depth of focus is desired. 



