I think this is a great title trick I’ve learnt about the Leica SL. I never paid much attention to the front button until I tried it one day and discovered the GREYCARD mode. It is an extremely handy feature to quickly set the custom white balance without guessing. I remember I used to do it on my Canon DSLR bodies but it was a bit awkward and time consuming. SL implements it in a simple and elegant way.
I use ExpoDisc to set my camera’s white balance in any tricky light situation.
I put the ExpoDisc in front of the lens (against the filter rim) and press the front button on the SL body for 1 sec to enter the GREYCARD mode for setting the white balance. The SL will set the white balance automatically, and preview the new setting in the view finder. Then it’s just a press away on the LRB (lower right button) on the back to confirm the new setting. Voila.
The results are obvious. See the difference this makes in a simple example of the two images below. The light is pretty tricky at night with LED bulbs and any digital camera can be easily fooled into some weird white balance settings if used in the automatic mode. If you shoot RAW (DNG on SL) it can be manually corrected in post, however it’s easier and faster to set the white balance in-camera and not worry about it later with a series of images waiting to be processed.