1st, I like to use either a Sunpal 120J TTL, in TTL mode, on a Flash Bracket or a Quantum X3Dr on a stand, also in TTL mode.
I prefer the light quality of the Round Reflector, I find it a GREAT MATCH, to the lens quality of the Zeiss Lenses.
Here's how you do this. when shooting prime lenses, you can generally rely on Auto Mode, for Metering ! To Prevent ERROR,
Shift the camera into Manual, using the Seting Noted in Manual. The Flash TTL system works in both Auto & Manual Exposure.
To Do Fill-Flash in TTL, you want to underexpose the flash between 1 1/2 & 1 2/3 of a stop, compared to the camera exposure.
*** IMPORTANT: 1. Set the ISO setting to 1 2/3 of a stop Under Exposure, ( Ex. ISO 100 becomes ISO 320 ) this makes both
the camera & flash under expose 1 2/3 under.
2. Reset Exposure Compensation to 1 2/3 + Over Exposure, ( settings on Auto Knob ). This resets the camera to Normal
Exposure.
This forces the camera to expose normally, while the TTL flash is forced to expose 1 2/3 Under !
IMPORTANT*** REMEMBER TO RESET THESE SETTINGS, WHEN NORMAL EXPOSURE & FLASH ARE DESIRED !