EXPOSURE METERS Although too often taken for granted today, the exposure meter is a much more recent arrival in the history of photography than the camera. When I began working in Yosemite in about 1920, exposures were arrived at by empirical methods or by reference to a set of tables-and everyone had a different set of tables. I learned from experience how to judge the exposure conditions at Yosemite, where the light is fairly consistent, but when I went on to New Mexico, and later to New England, I found different light situations, and I had to start all over again! My first meter was a Wynne Actinometer, in which a small wedge of sensitive paper was rotated between comparison-patch areas of greenish gray, one for sunlight and a lighter one for lowlight conditions-an early form of the high-low meter scale. The central sensitive paper wedge darkened in value when exposed to light, and the time it required to match either of the patches was set on a dial and converted to an exposure reading. The system was surprisingly accurate, although some found it very difficult to make a precise visual match of the values. Exposure meters have developed rapidly, and meters today are very accurate and dependable. I recommend that you carefully consider the exposure meter at the same time you choose a camera while cost may be a limitation, the meter should be viewed as a long-term investment every bit as important as the camera itself. Many photographers choose small cameras with metering systems built in, primarily because of the convenience. My approach to photography, however, depends on the use of a separate spot meter, tested for accuracy. Only with such a meter can we precisely measure the individual luminance values (i.e., brightnesses) of the subject and use this information to determine exposure and development procedures. In addi-
tion, if the procedures for measuring individual luminances are fully understood, you will find the principles helpful even when using a small camera with automation or built-in meter. Many professional photographers do employ the metering system in their cameras as their primary lightmeasuring instrument, and, properly understood and calibrated, it can provide excellent results. These approaches will be described in Book 2.
Elements of a Meter Exposure meters contain at least the following elements: an electronic cell that responds to light in a predictable way, a power supply, and a dial or other means for calculating camera exposures based on the indicated light intensity. Nearly all exposure meters twenty years ago used the selenium cell to measure light. With such meters, like the old reliable Weston Masters I through V, the selenium cell itself is the power source, since the cell converts the light striking it directly into electric current to give a reading on an ammeter. This reading is then usually transferred to a calculator dial that indicates correct exposure. One limitation of this system, however, is that relatively high minimum light values are required to generate a measurable amount of current. Thus these meters are not useful under very low light conditions. Much greater low-light sensitivity became possible with the cadmium sulfide (CdS) cell. A CdS cell changes its resistance to electric current depending on the amount of light striking it. The cell simply regulates the current from a battery rather than creating electric energy, so it is much more sensitive. However, the CdS cell has the unfortunate property of “drifting/, sometimes taking several seconds to settle into its final reading. This property is related to the cell’s “memory” j exposing a CdS cell to bright light can temporarily