Flash Sync Speed
Ask a pro photographer and he would always insist that you don’t use the on-camera flash. He would give you a number of reasons for not doing that to convince you to buy an external flash and use that off-camera. Simply put, off-camera flash gives much better results than an on-camera flash. However this is also where the problem starts in terms of when to use the camera flash.
A lot of amateur photographers using off-camera flash for the first time complain that they have black bands on their photos and they can’t seem to make any head or tail as to why that’s happening. Keep reading if you also have the same problem.
The culprit is the way your camera shutter works and understanding shutter speed is key. All single lens reflex cameras (both digital and film) have what is known as a focal plane shutter. A focal plane shutter is located on the camera, right in front of the image medium (film or digital sensor). This focal plane shutter has two shutter curtains that open and closes when the shutter release is pressed.
For slower shutter speeds the first curtain opens and travels across the height of the sensor (modern focal plane shutter curtains travel vertically). When it travels to the other end the second curtain starts travelling in order to close the image medium. A flash is fired to sync with this movement and is ideally triggered in between when the first curtain has stopped travelling and before the second curtain has begun to travel.
The problem starts when you increase the shutter speed. The above mechanism is repeated till about 1/200 of a second, only quicker. This means the window of opportunity for firing the flash and making a perfectly exposed image becomes smaller and smaller with faster shutter speeds.
You may ask at this point…
Well, my flash says it can fire at much faster speeds. So, why can’t I beat the camera for speed?
This is because when you start using even faster shutter speeds say, 1/500 of a second and higher, the above mechanism no longer operates the same way. At such higher shutter speeds the second curtain starts moving to close the image medium even before the first curtain has completed its run. This means essentially, you have a narrow slit which travels across the image medium in order to expose it.
A narrow slit travelling across the image medium essentially means you cannot expose the whole image at one go. When the flash is fired (at higher speeds) only a part of the image is exposed (the part that was open when the flash was fired). The part that was already closed by the rear curtain (the top part for modern cameras) will have a black band.
Video: Sync Speed
The solution is to always use a flash speed that is at least the same or slower than the shutter speed used. This way the flash will have more time to expose the whole of the image as the slit travels across the image medium.
Remember, this problem will only crop-up when you are using an off-camera flash. If you are using a pop-up flash or an external flash that is attached to the camera via the hot-shoe or a TTL cable, it will automatically be set to the maximum sync speed allowable (for Nikon cameras it is 1/250 and for Canon 1/200 of a second) for a proper exposure. However, if you are using an incompatible flash mounted on the camera, you will still have to set the flash speed manually.