Ran into a odd issue with the E-M1X CA-F focus today with the 300mm + 1.4 TC. Was having some issues with the single small focus point as it seemed to be front focusing as bird feather detail was missing or blurry. Been that way for a couple of days too. Decided I'd re-run the Micheal Tapes slant AF tuning chart and its software and I had to resort to a +10 in the setting for the 420mm lens (300mm+TC) which seemed a lot. I put it at the shooting distance of 35 feet too which I was using for the birds. I put the 300mm combo on set to +10 for the AF Tuning (CA-F) and it is behaving normally now and feather detail came back. It was warm at 85 degrees F today in the sun, and the body got up to around 120 on the black top and the 300mm lens hood got up to 157 degrees with an IR heat gun. Don't know if the heat of the body or lens is altering the AF Tuning either. Edited: Found my old PDF lens tests using the Focus Tune (whew!). All my prior lenses were either +1 or -1 for tuning as below: Olympus 7-14mm f/2.8 Pro At 7mm +1 At 14mm +1 Olympus 12-100mm f/4 At 12mm AF Tune +1 At 100mm AF Tune -1 Olympus 45mm f/1.2 Pro -1 Olympus 75mm f/1.8 -1 Olympus 300mm f/4 -1 Olympus 300mm+1.4TC +1 Must be the heat or distance that is playing with the 300mm and TC to cause it to front focus to where it takes a +10 from the indoor test where it was +1. That or maybe the polarizer is causing the AF shift, but I'd guess it is the heat of the lens outdoors.