When photographers are learning to work with flash, be it an off-camera studio strobe or an on-camera flash, they inevitably run into trouble. The biggest culprit is a mysterious black bar or stripe that appears across the frame, which only happens with flash. What on earth could be causing that black bar across your flash photos? The answer lies in the mechanics of how a focal plane shutter works and how it’s synchronized with the firing of the flash.
First of all, it’s important to understand that the flash is fast—really fast. It’s much faster than the camera’s shutter opening and closing. So the flash light is “on” for only a fraction of the time the shutter is open. Herein lies the rub.
Think of a focal plane shutter as a mechanical version of the curtains on a window but with both curtains starting on the same side of the window. With the shutter closed, only one curtain covers the window. When the shutter opens, the first curtain (the one that’s already covering the window or camera sensor) opens up by moving away from the second curtain. When it’s time for the exposure to end, the second curtain closes by following the first curtain and spreading itself across the window/sensor. At this point, the shades are drawn, but now they’re on the other side of the frame from whence they started.
To recap, there are two curtains, which work by the first one opening and then the second one closing. It’s the interval between the first curtain opening and the second one closing that represents the duration that the shutter is open. It’s the shutter speed.
For slow shutter speeds (slower than 1/125th of a second for sure) there’s enough time between the first curtain opening and the second curtain closing that the entirety of the sensor is being exposed simultaneously. It’s during this interval that the flash fires, providing even exposure across the entirety of the frame.
But when the photographer dials in a faster shutter speed—say, 1/250th of a second or faster—the entirety of the sensor is never exposed at the same instant. Because with fast shutter speeds, the first curtain begins to open, moving across the frame. But before it gets all the way to the other side, the second curtain starts to close. The faster the shutter speed, the shorter the duration between the first curtain opening and the second curtain closing. This means in practice, the flash is going to fire when only a portion of the frame isn’t covered by the shutter curtain. And the part that’s covered by the shutter will appear as a black bar.
This is why cameras have a sync speed—the maximum shutter speed at which the sensor will be fully exposed by a flash. While many cameras claim a safe sync speed of 1/250th of a second, photographers may find evidence of a black bar across the top of their pictures at this speed. Even at 1/200th, you might find a hint of shadow at the top edge of the frame—evident in the examples shown here. Notice that as shutter speed shortens, the black bar increases. At 1/800th of a second, only a tiny slit is exposed at the bottom edge of the frame when the flash fires.
If you find a black bar across your flash photos, you’re using too fast a shutter speed. Try slowing it to 1/125th, which is almost always safe. And while some cameras might work at 1/250th, more often than not you’ll start to see the nefarious black stripe creeping into the edge of the frame.