Tiberius47 wrote in post #9350041
The fact is that built in flashes are really weak. They have a guide number of 13 or so at most (roughly, the higher the number, the more powerful the flash). An average flash unit, such as the Canon 430EX has a guide number of 43. You can see that it is much more powerful.
They are, but not that much as you make it look. The internal flash on the 7D has a guide number of 12, but then it is able to illuminate the view the camera sees through a 15 mm lens.
The external Speedlite units 430 EX II and 580 EX II have the ability to zoom, i.e. vary their concentration of the light onto the subject. With longer focal lengths, you see only a small part of the center of the illuminated area, if you use the internal flash. These external flashes can then adjust their light cone, to avoid wasting power on illuminating parts that your camera will not see anyway. It's only at the longest zoom setting (equivalent to a 105 mm focal length on a full frame camera) which the 580 EX II reaches a guide number of 58. At a setting equivalent to what the internal flash illuminates, the guide number is 28. Still a substantial difference, but not quite as much as one could believe.