Can you tell me exactly what I did wrong in the following photo?
Do you see the problem? If not, let’s quickly recreate the same scenario again in my studio and exaggerate it.
No, the camera is not broke nor will we be encountered by a creature claiming to be the servant of the power behind the Nothing. Rather the cause is a technical limitation. Look at my settings above and pay attention what is changing between them. Yes, the shutter speed. But why?
The answer is sync speed.
The shutter on your typical camera has two curtains, a front and rear one (also known as first and second curtain). When you press the shutter button to take a picture, the first curtain swings up and exposes your film / sensor to the light for the amount of time your shutter speed is set to. Once finished, the rear curtain swings up to close off the exposure. The faster you turn your shutter speed up, the quicker this entire process happens.
There comes a point when your shutter speed is so fast that it’s simply impossible for both curtains to be open at once. The rear curtain has to start closing while the front is still opening, essentially chasing right behind it. (The faster the shutter speed, the closer the rear curtain is to the front.) Instead of exposing the whole scene at once, now the film/sensor is being exposed by a moving “slit,” the space between the curtains. If you’re a little confused, think of it a bit like how a copy machine or scanner works. It doesn’t just copy the document all at once, rather the lamp under the glass moves across your document and puts it all together.
On cameras shooting in ambient light, this process is okay as the scene is constantly illuminated through the shutter actions. It doesn’t matter if our shutter is set at 1/60th or 1/8000th of a second: if the rest of your exposure settings are correct for the scene then you’ll get a nicely exposed picture.
However, the use of flash as your primary light source changes everything considerably. Flash sends out an incredibly brief burst of light that must fire when BOTH shutter curtains are open in order to light the subject/scene. Should one of those shutters still be exposed while the flash fires, it’s going to block light off and subsequently cause the black bar you see above.
Sync speed is simply the fastest shutter speed you can set on your camera where both shutter curtains can be open at once.
My camera’s maximum sync speed is 1/200. In the first example photo, I’m at 1/125 and so there is no issue. As I rise above sync speed, you begin to see that black bar appear. Other camera’s sync speeds can differ, but generally they’re around 1/200 to 1/250. Some cameras are lower, others are higher and some even have a different type of shutter altogether that work differently than the process above.
As camera manufacturer’s speedlites may not allow you to exceed the camera’s maximum sync speed when mounted to the hotshoe, this circumstance most commonly happens when using studio strobes or off-camera flashes with radio triggers. Even at the exact maximum sync speed, you might still get a hint of black as there can be the faintest hint of delay between radio triggers, a circumstance known as propagation delay.
Some triggers may be able to correct this with software settings or a firmware update, but it’s better to stay on the safe side and keep the shutter speed just under the sync speed. This won’t make a difference as the shutter speed has no effect whatsoever on the power of flash exposure when below the maximum sync speed.
The limitation of your sync speed is usually not a problem in a studio shooting environment as the maximum speed is typically more than enough for most model / portrait sessions. Action photos (such as dancers) are trickier but the action can be frozen via other means than shutter speed. I’d say the biggest thing to watch out for is accidentally bumping your shutter up past the sync speed during the shoot, so make sure to keep an eye out for this.
One last note: there are ways to exceed the maximum sync speed such as a feature called High Speed Sync that is available on mid-to-high end flashes, but this feature comes at a penalty and I’ll save this for a future topic.