Science project: second variable
Look up the data sheets of the batteries you intend to test. These should inform you how lifespan relates to discharge rates and, from this, you can design a load (most probably a constant current load) and then just monitor the falling battery voltage with time.

Because the current taken is constant and the voltage is monitored, the power consumed is simply V x I. Monitoring the lightbulb idea is plain daft because your camera will adjust its light sensitivity as the bulb dims. It's also daft because as the bulb dims it's resistance falls and therefore you have yet another variable to factor in. Simplicity is best.