I bought mine in 1991 when I got a PZ-1 and wanted autofocus on my long (ish) zoom a vivitar Series 1 70-200 F3.5. Although the documentation says it only works on F2.8 and faster lenses, I tested it out before I bought it and it worked just fine on the F3.5 vivitar.
Since then, I found a really good condition SMC 300 F4, and again it works very well with this lens, in all but the most extreme low light conditions.
When I got my *istD, I began using this combination (300 F4 and 1.7x AF TC) as an effective 500mm F6.7 lens.
below I have posted a shot taken with this combo, using a flash at 1/150th I think you will agree it is quite sharp.
In the same post you will see a shot taken with my K10D, at 1/30 and a 70-200 F2.8 zoom plus 1.4x TC for comparison.
Scroll down until you get to my posting
https://www.pentaxforums.com/forums/general-pentax-photography/13308-do-you-a...tly-blurr.html
I find the lens with the AF TC works best with long fast lenses at much less than infinity focus. The chickadee for example was shot from about 25 feet.
I find that the cameras all tend to focus short using the TC on distant objects, although this may be a function of hand holding, as a little vertical shake can move the focus point up or down a lot, and with nature shots, (things on the ground) downward motion also means possibly setting the focus on the ground in front of your image.