People keep asking a simple question: “why do my expensive camera fail to auto-focus when I plug my tele-lens into a 2x converter?”. Unfortunately, the answer is pretty simple. Most auto-focus cameras (digital or not) have a AF sensor limited to f/5.6 detection. If your lens is not opening more than that, the AF is technically impossible (the AF sensor does not see enough to separate details).
So, the f/8 mirror lenses are out of AF. But also, reasonably priced tele-lenses (like a Sigma 400mm/5.6 lens) are just at the limit and if you had a tele-converter (1.4x or 2x) you loose some more and lo! it will no longer focus.
- Canon 1D up to f/8.