Only the Godox Xpro (S) trigger has a button called TCM. The other XT1 and XT2 do not have this functionality. Many photographers use the TTL function because you put your flash on ‘automatic’.

The disadvantage of using TTL is your light is not consistant and when using several flashes (strobes) you cannot use TTL anyway.

Let’s assume you have to photograph 3 different models (medium shot, half body) at the same place wearing different clothes. Model ‘one’ is wearing a white blouse, model ‘two’ a grey blouse and model ‘three’ a black blouse. As a result all blouses will be grey(ish). This is because how camera’s work: Cameras measure 18% reflective light. When you photograph all 3 together the light will be probably correctly balanced as all three combined is 50% grey (18% reflective light).

TTL will work just fine for most of the time and is great when you do not have the time to use manual settings.

How to make the make light consistent?

• Put your camera on manual and adjust your exposure triangle until the light is how you want it. On some cameras you need to switch off the trigger

• Set your trigger in TTL mode(and check your flash is in TTL mode)

• Make the shot.

• Press the TCM button for a second until it says M. Your flash is now put in manual mode and both the flash & trigger will show you the measured value.

There you have it.

I mainly use my light meter to measure the light as it more accurate. A light meter does not measure reflective light but incident light. However when I have very little time I use TTL to TCM.

UPDATE 02/2023:

The new Godox XPro II does not have a TCM button anymore. You just need to fire in TLL mode and when you switch to Manual the value is the measured value from the TTL metering. Nice.