The 300TL flash unit was designed for the venerable Canon T90 (pre EOS), and its more advanced features (such as FEL and second-curtain sync) which are not supported by EOS cameras, although it can be used with EOS cameras as a basic TTL flash with A-TTL and Manual modes. Older manual focus Canon bodies (A-1 and earlier) will use the thyristor circuitry in the 300TL to quench the flash when proper exposure is attained.
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