In my opinion you have the following options:
- Use the 50 1.4 instead of 135 2: This will reduce the need of fast shutter speeds due to shorter focal length + you have a 1 stop brighter aperture (if you like the results at f/1.4) -> you will gain about 2 stops of lower ISO
- Use faster shutter speeds and compensate by using higher ISO and OVERexposing (ETTR). Pull back the over exposed shots in pp. I never had the 5DII but I suppose it should be quite fine until ISO 6400 using this technique. Do NOT underexpose like you suggested in your post!!
- Use Flash. An inexpensive 430EXII + diffuser (you are not going to bounce in a church i suppose) will give you all the lighting you need. I understand that you want to preserve the atmosphere so I suggest you use flash for people (candid portraits) and no flash for wide angle interior shots with the 24L.
The 100L should give you some more flexibility due to IS but you need to watch out to keep shutter speeds fast enough to stop your subjects movements. I wouldn't recommend this lens in your situation.
Thanks for this I will try this technique ASAP. How much should I over expose by before it can't be recovered in PP? 1 stop?
Yes I will use my 50 1.4 more next time. I just love the brokeh on my 135l so much i try to use it in every situation I can. Maybe I should have just bouth the 50mm 1.2 haha. I dont usually use my lens completely wide open as they aren't as sharp. Will sharpening in PP recover this?
I am secretly glad you dont recommend the 100L IS. It would hurt me too much to get rid of my 135L at the moment.
Flash has always been completely banned in the churches I have been, which is a shame as a small amount would help so much. If I was ever allowed to use flash, should I gel it to mix with the church lights, or would the ambient light coming through the windows typically be the main source I am blending with?