Though the kenko can be use with any conbination of lens cause it does not have protruding element, it cant be use for FF, ie your film camera, since it design to work on dslr (1.5-1.6x crop factor). Not so sure if it works well with eos 1d & eos 1dmkll (1.3x crop factor).
dennis, it's not like that. the Kenko Pro teles (prev version and the current DG ones) will work on FF cameras without vignetting. The new "DG" series just have some coating to improve the image quality on DSLRs. It's just like Tamron's "Di" notation, e.g. 90mm f/2.8 Macro Di, which works equally well on FF/film cameras. (To confuse things further, Tamron's Di-II lenses are for exclusive use on APS-C DSLRs and would vignet on FF/film bodies.).