Wednesday 2 July 2008

X-sync (flash sync) speed

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The shutter in your camera consists of two metal plates (known as the first- and second-curtains) which travel sequentially (the time lag being determined by the shutter speed you set) to expose the digital sensor to light. They travel vertically, as the distance is shorter and allows faster shutter speeds. The sequence is illustrated in the above pictures. Once the shutter speed reaches a certain point (known as the X-sync speed), the second curtain is on the move before the first curtain has come to rest. This means that the sensor is only ever exposed to a narrow band of the scene at any moment. Have a good think about it - once you've got to grips with the concept we can explain flash sync and second-curtain sync.

A burst of flash lasts only milliseconds, and since you want all of the area in your shot to be illuminated equally, the sensor must be fully exposed to the scene when the flash fires. It's no good if only a narrow band is exposed as only this area will be lit by flash - the rest will be dark. Therefore the camera needs to synchronise the firing of the flash with the movement of the curtains. The fastest shutter speed at which this can occur is known as the X-sync, or flash sync, speed. The faster the curtains can physically move, the faster the X-sync on the camera will be. On a Canon EOS-5D it's 1/200 sec - not that impressive.

So what, you might ask. Unfortunately, this does create limitations (particularly for the long-suffering wedding photographer). Imagine you're outside on a very bright sunny day - it's midday and there's no shade available. You want to take a picture of the bride and you're aware that the lighting will create high contrast shadows under her brow (panda eyes) and nose. Since you're a competent photographer you decide to add a touch of fill-flash (maybe with some positive flash exposure compensation). You've stopped your lens down to f/22 and set your ISO speed as low as it will go (to get the slowest shutter speed you can) but the camera still only needs 1/1000 sec to expose the scene correctly. You switch the flash on to take the shot and you see the X-sync shutter speed flashing in your viewfinder. This is the camera telling you that if you use the flash you're going to get this shutter speed and consequently over-expose the shot. You could add a neutral density filter and polariser to reduce the amount of light reaching the sensor, but at a wedding you're unlikely to have the time. If you have an EX-series Speedlite there is an alternative escape route - I'll tell you about it in the next post.

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