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AEC versus manual technique
AEC stops the exposure once its chambers get enough radiation, so what covers the chamber decides the result.
The rule
With manual technique, you set kVp and mAs from a technique chart, and the machine delivers exactly that. With automatic exposure control, you still choose kVp, mA, the active chambers, and a backup time. The AEC then ends the exposure when the radiation reaching its ionization chambers equals a preset level. AEC controls exposure time, and therefore mAs. It never chooses kVp for you.