Right then, well it seems you mostly did pretty well in the quiz from last week. The question was “What is the maximum sync speed of the EOS 50D?” The answer, as 73% of you got right, is 1/250sec.

So what is the maximum sync speed and why is it useful to know? There are two main reasons for using High Speed Sync – the first one being to make the background darker – in other words to achieve a shutter speed fast enough that the background (ambient) light forms less of the exposure, with the flash lighting the subject. We’ll deal with this more in a future post, as today we’re going to look at the second reason – the use of flash with fast lenses. The following two images were taken a couple of seconds apart. The lighting is exactly the same in both of them – a flash off camera to the left of the subject. They were taken on an EOS 5D Mark II with an EF 50mm f/1.2L USM lens. If you don’t use high speed sync flash, you end up with a situation where your shutter speed is locked at the maximum sync speed. This varies by camera, so here’s a quick list for the current models:
EOS 500D – 1/200sec
EOS 40D/50D – 1/250sec
EOS 5D/5D Mark II – 1/200sec
EOS 1Ds Mark III – 1/250sec
EOS 1D Mark III – 1/300sec

The problem with not going higher than the sync speed is that to get the right amount of light from the ambient light sources (ie to avoid overexposing the image) you need to stop down the aperture. This leads to image number 1 – not a bad shot, but too much of a distracting background.

1/200sec, f/11

If you enable high speed sync flash, then you can turn the shutter speed up – all the way to the maximum shutter speed  (up to 1/8000sec on most EOS models). That gives you picture 2: - a better image due to a less distracting background. Notice how the exposure and lighting are the same in both.

1/6400sec, f/2

The idea of high speed sync is that the flash fires several times rather than just at the start or end of the exposure. It needs to do this as the shutter only uncovers part of the sensor at a time during the faster than sync speed shutter speeds. Think of it as a moving slit passing over the image, since that is actually what's happening.

It’s not free though – you’ll lose some flash power as the flash has to work harder over a longer duration, so you may need to move your lights around to get them closer to the subject, or use more than one Speedlite to increase the amount of light available to you.

High speed sync is available on the Speedlite 550EX, 580EX, 580EX II, 420Ex, 430EX, 430EX II, Macro Ringlite MR-14EX and Macro Twinlite MT-24-EX.