The Canon EF 75-300mm f/4-5.6 III is incompatible with the Canon EOS R100 without a specific adapter. Here are the details:
- The Canon EF 75-300mm f/4-5.6 III is designed for Canon EF mounts, which are not native to Canon EOS R series cameras, which use the RF mount. - To use this lens on a Canon EOS R100, you would need a Canon EF-EOS R adapter, which allows EF lenses to be mounted on EOS R camera bodies.