connanschilling:
in regard to robins comment, i couldn't understand the part of turning off the Arduino during the timelapse.... the motor might be needing some strength wile its stopped, so i think its going to be all the time on!
If the motor's internal friction is not enough to hold position and if it is essential to keep power on the stepper motor to prevent the camera moving then you will need a large power source. The motor will be drawing a lot of power all the time. I don't think you have told us what motor you have. Nema 17 just defines the size of the face plate and fixing holes.
Have you considered my suggestion for a brake?
If you need a low power option then I think you will need to use a DC motor with a worm-gear reduction. The worm-gear prevents the load from moving the motor. The problem is that you will have to find some other method for speed and position monitoring.
...R