John,
the other option is that the non-compatibility is a demand from Leica: they did expect the MC to reduce the quality of the 100-400 too much, and therefore, they would not lend their name to the 100-400 if it was compatible with the MC.
Niels
the other option is that the non-compatibility is a demand from Leica: they did expect the MC to reduce the quality of the 100-400 too much, and therefore, they would not lend their name to the 100-400 if it was compatible with the MC.
Niels