Taped Contacts - 1.4 X

This is interesting! As Brian says, youre not really getting more aperture, just fooling the camera. My cameras, being Sonys and Minoltas (same thing - Sony bought Minolta), don't need any such trickery - my 100-200 f4.5 zoom retains autofocus on a 2x convertor where it is an f9, but I'm told it's centre-spot only which is the only mode I ever use anyway. So I don't need to employ such tricks. However, I have a slightly faulty lens - a 24-85mm - where the fault is a poor contact in the encoder that tells the camera what the focal length is, and as the actual FL is adjusted away from the 24 that the sensor defaults to the AF starts hunting a lot, though it get there eventually. This leads me to believe that the servo motor is tuned to the lens and needs to know the FL for best results. My teleconvertors - 1.4x and 2x, both Teleplus, seem to do the conversion on chip which makes matters simple.

Incidentally, I understand the reason Minolta/Sony made the AF work that way was to accomodate the 500mm f8 AF mirror lens, the only one of its type ever sold.