I'm still playing with the A-400 and SMC Pentax-F 1.7x AF Adapter. The problem with it is you need perfect light. The AF failed miserably unless I was in direct sun. Maximum aperture is ƒ9.5 wide open, and that is only enough for the AF in bright sun. Without direct sun, it was worse than MF. It would actually move my close MF image to an OOF image. It ruined even chance otherwise sharp MF focus images. It literally gave me nothing.
Cross post to
https://www.pentaxforums.com/forums/122-lens-clubs/55946-300mm-plus-lens-club...nses-1185.html
,
https://www.pentaxforums.com/forums/26-mini-challenges-games-photo-stories/27...thread-21.html
and
https://www.pentaxforums.com/forums/12-post-your-photos/311782-nature-up-clos...ml#post3492263
Another problem was I was after small birds, and I was close enough the noise of the AF scared them. And Couldn't get all of the Jays and squirrels in the frame. 680mm is a lot of reach on APS-c.
But, I got some close ups and head shots.
A-400 and SMC Pentax-F 1.7x AF Adapter
ƒ11, ISO 1600, AV mode
After a bit I got lucky, I got some full sun, and I got a few of the images I came out for, when the Female Purple Finch visited. Screw drive noise scared off her mate before I could lock focus, and I though I took probably 50 images of her taken in less than full sun when the focus wasn't correct, but this last burst, in full sun had every one of a series of 10 in sharp focus.
You can put up with a lot of what you don't want, if in the end, you get what you want. Unlike many of my images there is no brush sharpening on these, just the custom preset. There was no adjustment after that, except to set the limits in Curves.