To primarily use the Zoom Lens is the better choice of the two lenses; but you will need a flash unit also. To take the 50mm lens would be advisable.
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Thanks for your comment about the choice of lens William (call me Bill). Poor choice or not & contractual bit is accepted but this 'professional photographer' has failed to turn up on 2 occasions - so we will see.
To take on then role to "back up" the Professional at the behest of the B&G, who have an existing contract with a professional photographer is probably a poor choice on your part.
You (and the couple) should be aware of all the possible outcomes of this choice: including those possibilities if the Professional has an exclusivity clause in the contract and acts upon it.
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This is the
"Flash in Church Thread" to which jcuknz was referring
WW