What is Flash Sync Speed?

I’m going to show you what flash sync speed is with a little experiment using my Fuji XT2. You can try an experiment like this with any digital camera and it will give you an idea about what type of manual shutter your camera uses, what the sync speed is, and how far you can push it.

The Fuji XT2 has options for manual and electronic shutter use. The shutter speed on the camera dial goes up to 1/8000s which is the fastest manual shutter speed, the electronic shutter is a lot faster. For the moment we’re just looking at the manual shutter and not at any high speed sync options.

For each shot in this set of images I’m firing the EF-X8 flash supplied with my camera. I’m leaving the aperture constant at f 2.8 and just changing the shutter speed from 1/60s, going down one stop each shot, all the way to 1/8000s. The camera is taking care of ISO and flash power settings and using TTL metering. If you don’t know what that means, please don’t worry as it really doesn’t matter for the purposes of this post. With any modern digital camera these settings should be automatic so pop your flash on your camera, use a tripod if you have one, and get experimenting.

So these first three shots are 1/60s, 1/125s and 1/250s. Perhaps there are some blown-out highlights, but they are mainly even and well-lit.

Keep in mind that 1/250s represents my flash sync speed, and watch what happens….

1/60 s @ f2.8
1/125s @ f2.8
1/250s @ f2.8

From 1/500s onwards you can see the image progressively darkens from the base and that the problem gets worse and worse the faster the shutter moves.

1/500s @ f2.8
1/1000s @ f2.8
1/2000s @ f2.8
1/4000s @ f 2.8
1/8000s @ f2.8

So, what’s happening here?

What you can see in these images at speeds higher than 1/250s is down to shutter movement.

My camera is firing a pre-flash that sends data back to the camera about required TTL (through the lens) flash settings before the shutter is opened and the main flash fires to take the shot.

There are several different types of shutters, but this Fujifilm camera has a focal plane shutter which is like a pair of curtains sitting in front of the sensor. A lot of cameras use this system; all DSLR cameras use it. At speeds of up to 1/250s, the flash fires when the shutter is fully open exposing the entire image sensor. However, at speeds of over 1/250s, the shutter is never fully open. Instead, as one curtain is moving, the other starts moving too, exposing the image sensor in sections. The flash fires, but now you can see the effect of the moving shutter on the image; the flash has been fired when only a small part of the image sensor is exposed. And the faster the shutter speed, the worse this effect is. The camera is exposing the image sensor in smaller and smaller sections or slices. 

You can see from these shots that the Fuji shutter moves down and not from the left or right; it’s a guillotine shutter (the image is upside down on the image sensor so that’s why it looks like it’s running bottom to top). You don’t notice this effect if you’re not using a flash – the sensor being exposed in sections doesn’t make any difference. The camera calculates the exposure and allows for each part of the sensor exposed by the opening of the shutter to receive the same amount of light. But with the flash, the duration of the light is tiny.

This is one reason that photographers who use a lot of flash chose medium format equipment – those super expensive cameras with super expensive lenses. These cameras have a leaf shutter as a part of the lens unit and this increases the costs of the equipment as each lens has its own shutter. This style of shutter can move a lot faster than a focal plane shutter, allowing for better flash results.

I don’t do a lot of flash work and prefer natural light, but it is useful to know the basics of flash photography. On modern DSLRs there’s also the option of using the electronic shutter, which is actually the power to the camera’s sensor being switched on and off for tiny fractions of a second. This can allow you to work with much faster ‘shutter’ speeds than a manual shutter.

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