Shoot the groups with a tripod and a cable release to avoid camera shake.


You should be able to shoo tat ISO 800 with your camera with minimal noise.


Clients can accept indoor shots being a little noisy, just do some post processing noise reduction and be sure to correct white balance before the shot or in photoshop.


Personally the size of the group you have I would try to shoot closer to f9.


Your concerns should be


#1 proper focus (I use single focus on the bride, you can live with any one else slighlty blurred)


#2 avoid camera shake (tripod, remote release, shoot three rapid fire exposures on all shots, second one is usually the best, and lets you pick out closed eyes)


#3 use an F stop to cover the entire group. Watch the depth of the group set up with 12 people this can go three deep (I like F11 for groups)


#4 Push the ISO to get you to at least 1/100 and F9. Don't be afraid to push the ISO it has the least affect on your finished product and that is what you paid for when you bought your Mark II


#5 spend time rehearsing how to position your party before you get there.


#6 watch your backgrounds


#7 Try to have an assistant help with the group arrangement and straightening the groups, you will be too nervous with the camera settings.


#8 IT IS ALL ABOUT THE BRIDE


#9 off camera flash is great but tough to cover a large group. I agree 45 on the key light and if you use on camera direct it to the ceiling ( I use a Gary Fong here) Second off camera flash could be above your head or at the opposite 45. I always set my off camera to manual and do test shots earlier in the day.


Good luck and let me know if I can help. I DO WEDDINGS.