Hi David As Bruce says there are some good quality TC's on the market.
The Kenko 1.5x TC works very well with the FA 135 f/2.8 but I've never used it with the FA 50mm f/1.4, AF speed and accuracy don't seem to suffer and image quality is respectable especially stopped down 1-2 stops. The Sigma 1.4x has a limited number of lenses that it will work with, the proximity of the lenses rear element is the most critical factor, if it protrudes it will come into contact with the front element of the converter John
