Managed to find the red morph Eastern Screech-owl this past Sunday - had to wait for an hour & a half for him to pop up out of his hole (thought I may have been waiting in in the wrong spot, but lots of car tracks and foot prints in area). Also stopped over today and he was sitting waiting and ready this time.
The first 2 are from today, bright sunshine, fairly harsh light around 1:00 pm. Both shots D300 +600mm f/4 VR + 1.4TC
ISO200, f/8, 1/800sec
ISO200, f/8, 1/640sec
This image was taken this past Sunday at around noon, sun was not as harsh. Also used my 2xTCIII. I haven't used the 2xTC too often as I've found it a bit soft - but I'll let you be the judge, maybe some fine auto-focus tuning, long lens technique improvement (shooting at 1200mm) or PP adjustment??.
I would try f9, f10 & f11 Richard. The highlight on the eye definitely looks diffuse to me (coma from the TC?). In theory f10 is the extinction resolution - the point at which you should treat the system as diffraction limited.