What is flash sync speed?

Even though a burst of flash lasts a fraction of a second, you can’t shoot with very fast shutter speeds

Flash isn’t just useful for lighting up the dark, it’s great for getting rid of shadows on sunny days. One of the challenges of shooting with flash in daylight is that you may not be able to set a fast enough shutter speed to expose the rest of the scene correctly. This is because you’re limited to the flash sync speed – also known as the x-sync – which is the fastest shutter speed that can be used with flash. It’s typically 1/200 sec, but you should check your camera manual to be sure.

If the scene requires a shutter speed that’s faster than the flash sync speed in order to expose it correctly then the picture may end up looking too bright. This is because with the flash switched on, the camera has to use the longer exposure time required by the flash. So if the sync speed is 1/200 sec, but the scene requires a shutter speed of 1/800 sec to expose it correctly, then the image will be overexposed by two stops (1/800 sec to 1/400 sec is one stop, and 1/400 sec to 1/200 sec is another stop).

There are several ways to get around this. You may be able to reduce the brightness of the scene so that the shutter speed required to expose it correctly is equivalent to the flash sync speed or slower. To do this, set a lower ISO or a smaller aperture, or fit a Neutral Density (ND) filter to the lens. As an alternative, you may be able to set the flashgun to High Speed Sync. This allows you to use flash with shutter speeds faster than the flash sync speed. The drawback is that flash power is reduced, so you may need to position the flash unit closer to the subject.

Normal This image needed a shutter speed of 1/800 sec. But with the flash switched on, the camera set the shutter speed to 1/200 sec – the resulting image was too bright!

With High Speed Sync enabled on the flashgun, the camera’s shutter and burst of flash were able to synchronise at 1/800 sec – the resulting image was correct.

You can use an ND filter to reduce the brightness of a scene so that a slower shutter speed can be used.