What is flash sync speed? How does it work and what are its limitations? The video below takes you through a quick and simple explanation of why you get the black band on your image when using flash and how to avoid it.
When You Click The Shutter Button
Most DSLRs have a two curtain mechanism in front of the sensor. When you press the button, the sensor is exposed. First, one curtain goes up exposing the sensor, and the second curtain follows to hide the sensor after which they both reset their position. The amount of time that the sensor is exposed is based on your shutter speed setting. A slower shutter speed will leave your sensor fully exposed, while a faster shutter speed could mean that only a sliver of a window will be open between the curtains.
When Your Flash Fires
Typically, when you press the shutter button and the first curtain goes up, the flash will fire a single burst of light and the second curtain closes. If your shutter speed is set too high when the flash fires, only the uncovered section of the sensor in that moment will be exposed. The areas of the sensor that are obstructed by the curtains during the flash fire will remain unexposed and you will have black bands or shadows across your image.
Maximum Shutter Speed Sync
Cameras have what is called maximum shutter speed sync. It is the fastest shutter speed you can set on your camera where both curtains will fully expose the sensor when using flash and avoid the black bands. Most cameras have this setting at 1/180th of a second or 1/200th of a second.
High Speed Sync
Some higher end cameras and flashes have a setting that allows the user to override the maximum shutter speed limitation without sacrificing the image. In Canon, this feature is called High Sync Speed (HSS) and in Nikon it’s called Auto FP. When this feature is turned on, the flash fires more than one burst of light like a strobe effect. It begins before the first curtain goes up and ends after the second curtain closes. This allows all the little sections to be exposed resulting in an image with no bands.
Note: When a Canon flash is attached to a Canon camera (or Nikon flash to Nikon camera), the camera will set the limit on the maximum shutter speed which protects the user from accidentally getting black bands. It won’t allow you to go past the maximum shutter speed unless you turn on the HSS feature.
Drawbacks to High Speed Sync
1. Heavy battery use – because the flash fires more than one burst of light in rapid fire it consumes batteries quickly.
2. Expensive Equipment – combinations of third party triggers like Phottix and YongNuo with third party flashes give you the power to do off camera flash affordably. But in order to make use of the HSS and Auto FP features, you will have to invest in triggers and flashes that can accommodate that function like the newly announced YongNuo 622N-TX and the Canon 600EX-RT Speedlight Flash and they are generally a higher investment.
Keep your shutter speed at 1/200 or slower when using flash unless you have the HSS (Canon) or Auto FP (Nikon) feature turned on which allows you to raise your shutter speed when using flash.