Through-the-lens metering
1963 (Topcon RE Super)
Putting the light meter inside the SLR, behind the lens, so it measured exactly the light reaching the film, whatever the lens or filter.
How it began
External meters did not know which lens or filter was in front of the film. In 1963 the Japanese Topcon RE Super placed a cadmium-sulphide cell in the SLR’s mirror. The Pentax Spotmatic followed in 1964 and made the idea popular.
How it works
Light passing through the lens reaches a cell inside the camera, through the mirror or near the viewfinder prism. The reading accounts for the lens’s focal length, filters, extension tubes and close focusing.
Why it mattered
It made metering accurate in difficult cases such as close-ups and long lenses, and made automatic exposure on SLRs practical.
Limits and what replaced it
A single reading still averages the scene. Multi-zone metering, first in Nikon’s FA of 1983, compares areas against stored patterns; digital cameras meter directly from the sensor.
Today
Every interchangeable-lens camera meters through the lens; mirrorless cameras meter from the sensor itself.