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?
Overexpose as much as you can without blowing the highlights -> +1 stop sounds reasonable.
I don't own the 50 1.4 nor the 135 2 but as I hear you should stop down the 50 for IQ reasons but there's no need in stopping down the 135. Sharpening in PP does not add any information to the shot i just makes it appear sharper - stopping down really improves IQ. I'm not telling you what approach to use - you need to find the optimal IQ tradeoff between aperture and ISO for yourself.
When it comes to noise reduction I recommend using a fair amount of color noise reduction but no to very few luminance reduction. CNR will remove any color noise without reducing detail but luminance reduction will reduce detail and therefore sharpness.
I don't really understand why you use spot metering - I wouldn't recommend it especially not if you are trying to overexpose (highlights at the outer parts of the image will not be taken into account during metering and will therefore blow faster).
Additionally try resizing your images to 6MP. You will notice a vast improvement in sharpness and probably in ISO performance and the images are still very large. 21MP is an image size suitable for very large prints - I don't think you need that in this case.
I don't remember you mentioning it but I strongly advise you to shoot RAW and resizing your images after pp and cropping. Try different sizes and use the ones you like best. Except for large posters I always resize my 22MP RAWs to 6MP.