[SOldBear]

someone stated that the Pentax-F 1.7 AF Converter "is not a teleconverter"

Because it is not a simple teleconverter. In addition to being a teleconverter, it adds autofocus capability to any lens that you can attach to it.

Pentax never calls it a teleconverter. Rather, it is a "1.7X AF Adapter."

[jatrax]

it is a "1.7X AF Adapter."

So it is an adapter that acts like a teleconverter. But it works. And the things that really are teleconverters often don't work. Like the OP said: confusing.

[PPPPPP42]

I can't say anything personally about AF but things get so dark with most TC's that manual focusing even becomes interesting so I'm not at all surprised it has issues. Someone should see if its just the slow lenses that cause the AF to mess up. I think unless the lens is like a 2.8 or faster expecting AF to work correctly would require very good low light AF function.

I have this one from Sigma its a 2x tele or 1:1 macro and seems to be of very good multi coated quality. It isn't an AF though it is a true working A series. It also has a super awesome bayonet locking guts that come out with a quick twist to make it an A series extension tube though (thus the 1:1 macro).
Never seen more than 2 of these on the net and both were used, one was on ebay, and then using that to search I found one cheaper and nicer on KEH (was only like $30 as I recall which was insane).
Did a link to my album pic of it instead of posting here because the pic is rather big and slightly off topic for this thread. I have many other macro pics in the album of what this can do in macro mode (minimum focusing distance is on the front glass itself with one of my lenses also in macro mode).
https://www.pentaxforums.com/forums/members/46390-pppppp42/albums/4594-pictures/picture48139.jpg

EDIT: I just looked and KEH actually has some decent choices for used AF TC's right now including this which is what you asked for at 1/4 of what Pentax would charge:
http://www.keh.com/camera/Pentax-Autofocus-Lens-Converters/1/sku-AP10999091770K?r=FE

[zdwagner]

EDIT: I just looked and KEH actually has some decent choices for used AF TC's right now including this which is what you asked for at 1/4 of what Pentax would charge: Untitled Page

I could be wrong, and others can speak to this, but I believe the Sigma teleconverters only AF with Sigma lenses, and very specific ones at that...

[jatrax]

I could be wrong, and others can speak to this, but I believe the Sigma teleconverters only AF with Sigma lenses, and very specific ones at that...

I don't think that is strictly true. The issue is that the Sigma TC's have protruding elements which can interfere with some lenses. So the AF is not the problem, it is getting the lens to mount without damage. And yes it is a specific list. I thought I saw a thread somewhere here that listed lenses that would work, but I cannot find it now.

[PPPPPP42]

My sigma TC works normally, the one I linked to appears to be a standard screw drive AF and does not appear to have a protruding element. Even with the Pentax TC's there are some that are meant for only certain telephoto lenses that have long protruding elements (L and S models or long and short).
You can see that here:
https://www.pentaxforums.com/lensreviews/category-Pentax-K-Mount-Teleconverters.html

[Ponosby Britt]

Don't know about the Pentax 300 but I have a KenkoPz-AF 1.5x SHQ. I occasionally use with a Tamron 28-75 for near macro and a Pentax 50-135 for events.

However, I mostly use it for stage performances. With a Sigma 100-300/4 mounted on a loose ballhead & tripod at the back of the theater, I can almost make headshots. Here's an example with the Sigma wide open and iso 6400. Single focus, track with rear AF. When the dancer comes on stage, I establish focus and follow him. There are misses but it works a lot too.

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[zdwagner]

Wow. Great shot.