Millimeters, that is. Not counting telescopes or spoting scopes. Just between the crop factor of the camera, teleconverter(s) and the lens? Most I can do is a crop body, 70-300 lens and an old MF 2x converter which puts me at 900mm and effectively f11. Never tried getting a decent pic but the converter does work on the lens. I'll try it in the next few days. Image quality aside, I've never tried stacking converters or if that's even possible and the longest actual lens I've ever used is the Canon 100-400 L and then not with a converter. Watcha got?