Over the history of camera design, one area that has undergone profound upheaval is light metering. Photoelectric light meters arose in the 1930s, but for many decades meterless cameras were still common. Finally, in the 1960s CdS cells offered a viable technology for compact in-camera metering, and after this meterless cameras became the exception.
Since that time, cameras have evolved ever more sophisticated types of metering and numerous variations on how exposure is controlled. The clear trend has been towards less manual intervention by the photographer.
Types of Control
Metering options may include any of the following (presented roughly in historical sequence):
With manual exposure, the meter serves purely as an advisory to the photographer; both aperture and shutter speed are selected at will.
- Uncoupled meter
- Semi-coupled meter, either to camera's aperture or shutter control; readout shows suggested setting for the other
- Fully coupled to camera's aperture and shutter; e.g. "match-needle" (or match-LED) readout
- Shutter priority; photographer selects shutter speed, and meter adjusts aperture to match scene brightness
- Aperture priority; photographer selects f/stop, and camera adjusts shutter speed to match scene brightness
- Single program mode; camera varies shutter speed and aperture together to match scene brightness, according to a specific program (e.g. only using speeds below 1/30 when widest aperture is reached)
- Program shift; a program may be overriden temporarily, either offsetting the aperture or the shutter speed from the default program
- Multiple programs; ubiquitous today as "scene" modes—multiple programs optimized for different use cases, such as sports, landscape, portraits.
The options of Program, Aperture priority, Shutter priority and Manual metering on a camera give rise to the acronym PASM.
- Averaging; the meter sensitivity is essentially the same over the entire frame
- Spot; meter sensitivity is concentrated only in one small area, usually marked in the viewfinder
- Centerweighted; a compromise between the first two, with decreased or no meter sensitivity in the corners of the frame
- Multispot; photographer is able make multiple spot readings and hold them in memory for weighting into final exposure (e.g. Olympus OM-4.)
- Beside the lens; saves carrying a separate light meter, but does not compensate for filters, different lens coverage, etc.
- Above the lens; located on the lens barrel within the filter thread; does compensate for filters
- Reading off ground glass from within pentaprism housing, hence Through The Lens (TTL); compensates for all filter, lens, & extension factors (but may be fooled by stray light through eyepiece)
- Reading from behind the lens; meter cell located behind SLR mirror or on a "semaphore" in a rangefinder
- Off the film (OTF); backwards-aiming cells read light falling onto film or a similarly toned shutter curtain (in SLRs, only activates as mirror rises)