Specifically what sort of subject are you trying to shoot?
Usually the 1/200th shutter speed is adequate for the vast majority of flash photography. I usually slow my shutter down to around 1/60th (it really depends on the light.)
The maximum shutter sync speed is based on the fact that the shutter has two doors or "curtains". One slides open to start the exposure, the other slides shut to finish the exposure. The doors slide from top to bottom to open.. and the second door also slides from top to bottom to close (you would not want to simply have the first door close from bottom to top because that would meant the top row of pixels got a longer exposure than the bottom row of pixels... hence two "doors" or "curtains" are needed to create an even exposure.
When the camera shoots at very fast shutter speeds, the second door starts closing just after the first door starts opening and this creates a "slit" that sweeps across the sensor.
The "problem" with this slit is that if the flash fires, then only the pixels exposed by that slit will get the benefit of the flash. So the flash should not fire unless the entire sensor is exposed.
When you fire with flash, the first door slides open completely, then the flash fires (which is really an incredibly short burst of light... perhaps only 1/1000ths second long) and then the second door closes. Even moving subjets tend to be frozen by flash photography when the camera is using slow shutter speeds if the only light that exposed the subject was from the flash.
See this video which captures the shutter in slow motion:
When you use flash, you actually have two sources of light... the bright momentary light from the "flash" and the dimmer but continuous source of light from whatever ambient lighting happened to be in your setting.
It often helps to produce a more natural looking image to use a SLOWER shutter speed with flash. If I'm shooting a couple dancing slowly at a wedding reception, I'll probably set the shutter speed to 1/60th and use flash. The flash will "freeze" the couple dancing, but the 1/60th sec shutter speed will allow the camera to continue collecting ambient light so that the room looks natural. If I were to use a 1/200th sec. shutter speed then the rest of the room would look very dark and only my subject would appear lit by flash. This is because as the light spreads out farther from the flash, it's intensity dims drastically.
You can shoot faster using "high speed sync" mode (for flashes that support this.) High speed sync causes the flash gun to pulse rapidly while the shutter curtains sweep with that gap moving across the sensor. But since the flash has to pulse rapidly it has to reserve enough power for all the pulses (it can't let go with one big flash or it would have to wait for the flash to recycle and be ready to fire again.) This means the maximum power output of the flash is drastically reduced when using high-speed sync. It's just one reason (among many) why it helps to have a powerful external flash rather than rely on the built-in pop-up flash.
Tim Campbell
5D III, 5D IV, 60Da
