the other to maintain a constant exposure. On some models, the coupling of shutter speed and aperture setting was optional, so that photographers could choose their preferred method of working depending on the situation. Use of the scale on such cameras is discussed briefly by Adams (1981, 39). Modern cameras no longer display exposure values as such, but continue to offer exposure modes, which support users in employing the concept of counter-adjusting shutter speed and aperture at a fixed point of exposure. This can be found in features such as Manual Shift on some Minolta, Konica Minolta and Sony Alpha or Hyper Manual on some Pentax (D)SLRs since 1991, where the photographer can change one of the parameters, and the camera will adjust the other accordingly for as long as the Auto-Exposure Lock (AEL) function is activated. In a wider sense, functions like , Pa / Ps Creative Program Control (by Minolta, Konica Minolta and Sony) or Hyper Program (by Pentax) belong to this group of features as well. The additive (logarithmic) system Although some photographers (Adams 1981, 66) routinely determined camera settings using the exposure equation, it generally was assumed that doing so would prove too daunting for the casual photographer. The 1942 ASA exposure guide, ASA Z38.2.2-1942, featured a dial calculator, and revisions in 1949 and 1955 used a similar approach. An alternative simplification also was possible: ASA PH2.5-1960 proposed extending the concept of exposure value to all exposure parameters. Taking base-2 logarithms of both sides of the exposure equation and separating numerators and denominators reduces exposure calculation to a matter of addition: where is the aperture value: is the time value: is the exposure value: . is the speed value (aka sensitivity value): is the luminance value (aka brightness value): is a constant that establishes the relationship between the ASA arithmetic film speed and the ASA speed value . The value of is approximately 0.30 (precisely, ). is the reflected-light meter calibration constant ASA standards covered incident-light meters as well as reflected-light meters; the incident-light exposure equation is where is the scene illuminance is the incident-light meter calibration constant The use of for illuminance reflects photographic industry practice at the time of the 1961 ASA standard for exposure meters, ASA PH2.12-1961; current SI practice prefers the symbol . ASA PH2.12-1961 included incident-light metering in the APEX concept: where is the incident-light value: (German sources typically use (for