Some photographers ask that no one else take pictures during the formal photography. Is that reasonable?

Yes. If the photographer is trying to work quickly through a series of formal pictures, a snap-shooter might slow the process. A problem arises when there are several cameras aimed at a formal group – the subjects will be looking at different lenses. The diverted attention will ruin the professional’s picture. Or, if the professional is using a light-sensing trigger device (“slave”) on a second flash, other flashes will trigger this flash, ruining the pictures [that you are paying for] and wasting battery power. There are many other reasons why a professional might ask others to refrain from picture taking during formals. On the other hand, many guests find snapping a fewpictures a pleasant part of attending weddings, so it should not be a big deal for your professional to be relaxed and let people snap away when formals are over. Ideally, guests should show respect for the bridal party and photographer and put down their cameras during the formal photos.