Hello Arthur!

Thank you for youre informations. Now, some weeks later I have the same question as you, will it be possible to use a 2xTC on a f/4.0 lens like the 500/ 600? If canon not will "build in" this feature, I hope for a c.fn, so it will be the decision of the photagrapher, to use this.

Sorry for my english.

Greetings from germany