The camera has a mechanical shutter with two "curtains" (or doors). The first slides open, the second slides shut (following the first curtain in the same direction). This causes all pixels to be exposed for the same amount of time.
But since the shutter is mechanical, it takes an amount of time for the first curtain to slide completely open, and also for the second curtain to slide completely closed. For very fast exposure times (for your camera that's any speed faster than 1/250th sec) the second curtain actually starts to close before the first curtain has completely opened. This results in a "slit" that sweeps across the sensor.
The flash itself has a very brief duration (often around 1/1000th sec but it really depends on the flash unit, the power, etc.).
When using fast shutter speeds (faster than 1/250) there is no time when the whole sensor is exposed... only part of the sensor is exposed. ONLY the part that was exposed will get the benefit of the flash and this results in a wonky looking image with a bright band and the rest is dark.
To combat this problem, many external speedlite flash models support "high speed sync" (HSS). In this mode, the flash actually pulses very rapidly (you don't notice it) as the shutter sweeps across the sensor. Each "pulse" provides light for that portion of the sensor so that the completed shot is evenly lit. Basically you get to use very fast shutter speeds and still have flash.
But there is a trade-off...
Suppose the flash has to pulse at least 8 times during the exposure. That would meant that no single pulse is allowed to deliver the full-power of the flash. It has to reserve enough power for all the pulses. No single pulse can use more than 1/8th of the power. In some cases this may not be adequate (but in many cases it will be fine).
You have a few options.
1. Since the flash actually illuminates your subject only very briefly, often the flash itself will "freeze" the action even though the shutter speed was slower (1/250 or slower) because there wasn't enough light to notice subject motion. You actually end up with a good looking shot.
But you mentioned the flash is just "fill" - which implies there would be enough light for a faster shutter speed even without the flash and if that's true, then you might notice the blur.
2. Get a speedlite flash that supports high-speed sync... and you want a powerful flash (no point in getting something that can't deliver the power). E.g. The Speedlite 600EX II RT or the Speedlite 430EX III RT. The 600 is more powerful.
3. If the speedlite doesn't have enough power for your shots when using HSS, you can "cluster" them. You can group multiple flashes and they all fire simultaneously and pump out just a portion of the light. The combined effect results in what appears to be one very powerful single light (it's really a cluster of lights.)
The built-in pop-up flash isn't nearly powerful enough to do this.
Tim Campbell
5D III, 5D IV, 60Da
