Took the K5,
Rogue Safari and newly acquired Vivitar TX 400mm to the RSPB for some test pics. Winter is lingering, it was a dull grey day so the flash extender was just the job. "M" mode, shutter limited to 1/180th- too slow for fast moving tits! Tripod mounted, IR remote. I had to prefocus on the feeder and then trust to luck that some of the birds would be in more or less in the plane of focus with the swinging of the feeder and movement of the bird.
Couple of the better efforts....
The flash extender is a good piece of kit IMO. The Viv is a bit average.