I'm trying to create an engagement photo for a client with them holding a sparkler with writing to the the right of the photo. I've done my homework! I've practiced with the settings on "bulb" to keep the shutter open during the writing of the words. I've figured out that it must be very dark to get the right effect, and that my subjects need to stay perfectly still while my assistant does the writing. Problem is I get a flash at the start of the shoot and at the end. I feel like we only need the flash to go at the end of the exposure. Is there a way to change that setting?