I want to set a custom control button, say the Preview button, to toggle thru metering modes on the fly.
That is, toggle thru the matrix, center, spot.
The Preview button seems to only allow one of these.
Can you clarify that you want to press the button once and the camera advances to the next metering mode i.e. the control dial is not pressed
If that's the case I'm pretty sure it can't be done.
Out of interest what's wrong with the metering mode button + command dial?