[BigMackCam]

Just an FYI - even Sigma's own 1.4x TC shows as MF-only on their compatibility chart. Whether that means it won't ever work in AF, or that it needs a brightly-lit, high contrast subject for AF to work, I don't know - but if this is the case with Sigma's own model, I wouldn't hold out a lot of hope with the Pentax TC...

[bigfoxkiB]

This is what I am thinking too. If it was a slower-than-F4 problem then it would lock on in liveview but the AF just drives from one end to another, only locks on when I pull back the focus by hand at an appropriate moment (kind of manually altering the PID loop of the AF drive). In OVF phase detect the focus is just too quick to step in like that.
My theory is the TC doesn't slow down the AF which it should and does on all my other lenses. All my other lenses are screwdrive though. Again, natively it works just fine, quick and snappy both in OVF and liveview.

I bet if the lens communication was okay the TC would step in and slow the AF accordingly.

It sounds pretty much like a delay between communication cause by the TC.
Assuming there will always be a delay between lens and body when the TC is put on. Some lens can work with the delay and AF properly. Some are not or too fast.
But I really have no idea why the delay doesn't apply on lens doesn't work. Maybe a bypass?

[normhead]

I wouldn't dismiss it so easily, today's AF sensors can work in -3EV, f/11 shouldn't be a problem (though reduced to only center point) let alone near f/8. Also the Sigma 17-50 was reported above having the same problem and that is an f/2.8 lens.

I have never seen a modern AF sensor work on a lens with a wide open aperture of ƒ11, have you? I not a big fan of what "should" be true.

I'm going with the "some lenses just aren't designed to support a TC theory,

Just leave it at "the TC doesn't work with some lenses" and "TCs are not really designed to be use with wide angle lenses." Many manufacturers even tell you which lenses their TCs can be used with, I know Sigma does. The Sigma 1.5 and 2x TCs work with a very specific list.

What I'm saying is, if a specific TC doesn't work with a specific lens, accept it and get on with your life. There most likely isn't a fix.

[clackers]

I wouldn't dismiss it so easily, today's AF sensors can work in -3EV, f/11 shouldn't be a problem (though reduced to only center point) let alone near f/8.

I'm not sure you understand the issue, Sterby.


Light level is nothing to do with it, geometry is.


The light has to be split wide enough to register on two different sensor rows in the AF module. f5.6 is the normal rating for this capability, so this is why even the slowest consumer lenses are capable of this.


When you shoot at f16, remember that to focus first of all the lens opens up to wide open, then closes down again, otherwise the beams are so parallel the module will struggle.

[rick pafford]

i have non 50-500 lens. i have tamron and kenko teleconver. i manual focus to infinite. there is a thread
where a person uses a teleconver on non os 50-500

[normhead]

I can MF any TC on any lens I own, I think this discussion is about getting the camera's AF system to work with it.