[Noisychip]

I was wondering whether a TC should work with all (tele) lenses.

I read that the Sigma TC's have a protuding element that goes into the lens, so this one won't ... But how about the Tamron TC?

Can I use it with any lens, at least mechanically? Will it work with M or screwmount lenses? Can I put a portrait lens on it and have a medium tele with reasonable quality?
And how about SDM?

E.g. a Tamron 90mm f/2.8 macro with a 1.4x TC = 126mm f/4
Or a Sigma 24-60 f/2.8 +1.4x = 84mm f/4
Or a Sigma 105 f/2.8 + 1.4x = 147mm f/4
Or a Sigma 180 f/3.5 + 1.4x = 252mm f/4,9
Does this make sense?

PS: I'm not going to use a TC with the kit lens @ 18mm wide open

[Bramela]

I have found that the Tamron (1.4X) takes all K fitting lens. I f you wanted to use with a screw fitting lens you would have to add an adapter. I have never done this so cant recommend or otherwise.
Have never had any problem with lens fitting to the 1.4X, having used Pentax, Tamron andSigma lens with it no problem.

[axl]

I have found that the Tamron (1.4X) takes all K fitting lens. I f you wanted to use with a screw fitting lens you would have to add an adapter. I have never done this so cant recommend or otherwise.
Have never had any problem with lens fitting to the 1.4X, having used Pentax, Tamron andSigma lens with it no problem.

Do they work as AF? or if it's under certain F stop it's MF only?

[Noisychip]

I have found that the Tamron (1.4X) takes all K fitting lens. I f you wanted to use with a screw fitting lens you would have to add an adapter. I have never done this so cant recommend or otherwise.
Have never had any problem with lens fitting to the 1.4X, having used Pentax, Tamron andSigma lens with it no problem.

Is the Tamron converter still available new? The only converter for Pentax I could find new is the Sigma.

[Noisychip]

Do they work as AF? or if it's under certain F stop it's MF only?

I would expect AF to start hunting when (resulting) max. aperture is below ~6. I've read that most AF systems are specified for f/5.6 and faster, to work reliably. If you take a f/5.6 lens + 1.4 TC, we get f/8.

[Mechan1k]

AF hunting can occur in poor light ... although I have use my FAJ 75-300 at 300mm (f/6.3) and it was fine most of the time ... only once or twice it hunted.

On a bright day though ... I doubt you'd have an issue.

I have the Tamron x1.4 TC ... and it have mounted all my lenses (just as a test) ... and they all worked perfectly.

The Tamron TC cannot be purchased new anymore ... although there is a link on the last page of the TAMRON TC thread of who does still have it though.

[Lowell Goudge]

What you need to consider, with the use of any TC is the possibility of mechanical interference.

I have several TC's, and while the Pentax 1.7x AF TC has recessed front elements, my two Sigma C's have front elements that are flush with or slightly proud with respect to the lens mount, and therefore only work with specific lenses. (mostly SIGMA)

Additionally, as others have indicated, AF systems will hunt depending upon the maximum apature.

I find that the Pentax 1.7x AF-TC will work and autofocus properly on lenses faster than F4 (resulting maximum apature is F6.8) It will not focus AT ALL on F5.6 lenses and I have seen reports from some that it will function (but I can't verify this) on F4.5 lenses.

The limit really is a resulting maximum apature with the TC installed of F6.8 for good AF performance.