Originally posted by nicolas1970i Just received this TC recently and started playing with it coupled to an A* 300 2.8.
What really strikes me is that you have to pre-focus manually, fairly close to the right point it seems to me, before you can "finish the job" with the TC's AF. What I mean is the range of distances the TC can focus, for any given focusing distance on the lens, is a lot smaller than I thought.
Say your subject is 30ft away: it is easy to be out of the focusing range of the TC, either because you've adjusted the manual focusing ring on the lens too long, or too short.
At first, I thought that setting focus distance to infinity would enable the TC to focus correctly pretty much in every situation, except on the very closest distances.
Is that an attribute of the TC, or due to the fact that it is used in combination with a long, fast lens?
Hi nicolas,
Realize that the original intent of the "Adapter" is to convert MF lenses to use AF, not as a TC per se. If you use it with a 50mm, the most common lens for 35mm film, I'm pretty sure that if you set the lens at infinity, the AFA will focus the full range of distances. I think that magnifying the image 1.7x is due to the fact that this is about the shortest length that could be used to allow the focusing group enough travel to function reasonably. Adding any length between the lens and the body forces some magnification and the directly proportional light loss.
Nikon also made something like this, the TC 16A, which has a similar magnification (1.6x, which is why I made the assumption about this being the shortest practical magnification), but it has a lot of compatibility issues unlike the Pentax unit. AFAIK, the Pentax 1.7x AFA is by far the best implimentation of this concept, so it's unique.
I agree with Dave that once you get used to it, it's not a big deal, and in fact, the benefits it provides (focus limiting and focusing speed, in addition to the optically good magnification) make it a valuable addition to any long tele user (I use it most often with AF lenses).
Scott