You can never go wrong by asking the person that is writing the check what you should be wearing when you show up. In the end you are a hired professional and if you look around at weddings you will note that the wait staff are normally in some sort of pseudo tuxedo (white shirt and black pants). You don't want to be mistaken for a waiter, but you would like to be able to blend into the woodwork when you aren't taking photos.
I would recommend that if you are going to have to wear a jacket it will be worth your wile to get a jacket that is a little big and have it tailored to your needs. Explain to the tailor what you will be using the suit for and explain that you want it to look good, fit well, and have the ability to move with you. You will need the armpits lowered, and an expansion joint in the back, and if it has shoulder pads have them removed. Also have the pockets expanded to hold more film, and replace the pocket liners with heavier cloth, a good tailor may tell you that you shouldn't put things in your pockets, let them know you don't really have a choice, this is a work tool, not a true dress suit.