Yes, with a 1.4x teleconverter, your 50-135mm f/2.8 would become a 70-198mm f/4. With a 2x teleconverter, it would become a 100-270mm f/5.6 and suffer significantly more degradation of image quality. A sharp lens can stand up to a 1.4x TC reasonably well, but it takes a very sharp lens to give good results with a 2x TC (e.g. the Canon supertelephoto primes that cost several thousand dollars). Even with a 1.4x, the results will not be as good as a native focal length (i.e., a Canon 70-200mm f/4L would be better than your Tokina with a 1.4x tc) - but teleconverters cost a lot less than full lenses!