Please can you give me your opinion about this situation,
I have to boom a character, with a 416, Since i need some room tone and ambient sound too, insted of to capture them later, can i fit a mkh 30 in my windshield (rycote stereo properly windshield) and to get an ms recording? Do I risk when I encode the ms, to loose my main goal, (to get properly the mono dialogue source).

Please can you give me your opinion about this situation,
I have to boom a character, with a 416, Since i need some room tone and ambient sound too, insted of to capture them later, can i fit a mkh 30 in my windshield (rycote stereo properly windshield) and to get an ms recording? Do I risk when I encode the ms, to loose my main goal, (to get properly the mono dialogue source).

Room tone and ambience are two different things. Room tone needs to be captured with the same mic that you use for dialog and with all the people and equipment in place that are there when the dialog itself is recorded. For example, if I were to record someone counting slowly 1... 2... 3..., room tone is the sound of the silence filling the gaps between the words. That means, in your case, that the room tone needs to be recorded in mono through the 416, NOT stereo, just like the dialog itself is recorded. The best way to get room tone is to brief your cast and crew that you are going to continue to roll for 30 to 60 seconds at the conclusion of each scene in order to record the room tone and so you want them to be sure to stay in-position, still and silent, until after you call 'cut.'

Ambience can't really be recorded properly at the same time the dialog is recorded since it is the sounds of all the other unseen stuff going on in the background ... wind, traffic on the street, crickets chirping, jack-hammers at the construction site down the block, etc. You need the ambience recording to be completely free of any dialog or other distinct speech. An M/S stereo recording done alongside the dialog recording is not a proper ambience track.

You could record the 416 to one mono track and the MKH30 to a second mono track, matrixing them to stereo in post rather than doing it in the field. That way your recording off of the 416 will be untouched and you can use it by itself for dialog while mixing a copy of it with the side mic for a second stereo recording. However, as noted above, that is NOT what you're looking for in an ambience track since the result is really just a stereo version of the dialog recording.

make sense, thanks alot.