History of The Single-lens Reflex Camera - The Introduction of Light Metering

The Introduction of Light Metering

Professional Photographers of the 1940s and 1950s time-period preferred to use hand-held meters such as the Sekonic selenium cell light meters, and others which were common during these periods. These hand-held meters did not require any batteries and provided good analog readouts of shutter speeds, apertures, ASA (now referred to as 'ISO') and EV (exposure value). Selenium cells, however, could easily be judged for their light sensitivity by simply looking at the size of the cell's metering surface. A small surface meant it lacked low-light sensitivity. These would prove to be useless for in-camera light metering.

Built-in light metering with SLRs started with clip-on selenium cells meters. One such meter was made for the Nikon F which coupled to the shutter speed dial and the aperture ring. While the selenium cell area was big, the add-on made the camera look clumsy and unattractive. In order for built-in light metering to be successful in SLR cameras, the use of Cadmium Sulfide Cells (CdS) was imperative.

Some early SLRs featured a built-in CdS meter usually on the front left side of the top plate, as in the Minolta SR-7. Other manufacturers, such as Miranda and Nikon introduced a CdS prism which fitted to their interchangeable prism SLR cameras. Nikon's early Photomic finder utilized a cover in front of the cell which was raised and a reading was taken and the photographer would either turn the coupled shutter speed dial and/or the coupled aperture ring to center a galvanometer-based meter needle shown in the viewfinder. The disadvantage of this early Photomic prism finder was that the meter had no ON/OFF switch so the meter was constantly 'ON', thus draining battery power. A later Photomic housing had an ON/OFF switch on the Pentaprism. CdS light meters proved more sensitive to light and thus metering in available light situations was becoming more prominent and useful. Further advances in CdS sensitivity, however, were needed as, CdS cells suffered from a 'memory effect'. That is, if exposed to bright sunlight, the cell would require many minutes to return to normal operation and sensitivity.