What exactly is a maximum flash sync speed and how can it cause problems

Maybe you're already aware that your camera's shutter speed is limited to (usually) around 1/250th of a second when using flash. But why is that, and what happens if you ignore it, (assuming your camera allows you to ignore it?)

Have you ever noticed, when you pop up your flash or put an external one on top of the camera, the settings sometimes change without you actually doing anything.
You may have a shutter speed of say 1000th of a second dialled in, but then you pop up the flash, and suddenly it changes to 1/250th of a second.

All DSLR cameras and most mirrorless cameras have what’s called focal plane shutters. The reason for the change of shutter speed that I just mentioned, is that these cameras have an inherent maximum shutter speed limit when shooting with electronic flash, and that limit is called the maximum sync speed. It’s usually 250th of a second, sometimes a little slower than that, 180th or 200th of a second. If you have a flashgun attached, many cameras won’t even allow you to set the shutter speed faster than the max sync speed, although some may allow it in the manual exposure mode, my Nikons don’t allow it.

So here's the problem, there are two main parts of the shutter mechanism (called 'curtains'), and with shutter speeds faster than 250th of a second, the first curtain starts its journey to cover the sensor even before the second curtain has time to complete its movement.

Shutter full open Shutter closing when at 500th of a second


This results in a large black bar appearing across the image, as you can see in the awful, unflattering photo of me at the top (I'm actually much better looking than that ;-))

So the moral of the story is that if ever you see a big black bar at the bottom (or at the side if you shot in portrait orientation) of your photo, you'll know that it's because the camera's shutter speed was set too fast, exceeding the maximum sync speed.

There's a terrific flash / focal plane shutter animation in the Max Sync Speed / HSS video on my Part II course, and also more information about flash photography, plus loads of of other photography tips and tricks.