High Speed Synch Flash

Last night (27th Jan 2015) I gave a talk on the technical issues of using an external flashgun.

I wanted to demonstrate that using a shutter speed higher than 1/200 sec on a typical DSLR will create a black band obscuring the base of the image, the faster the speed the larger the black band.

black-band

Like all good demo’s it failed, the black band did not materialise and I was able to take a complete picture at 1/500 sec. Of course, as I drove home, I realised what had happened.

At the start of the talk I mentioned a feature that most external flashguns have called High Speed Synch (HSS or FP for short) but I didn’t have time to cover this feature.

HSS does allow a faster-than-synch speed shutter to be used. To recap: in normal mode the flash fires when the 1st curtain of the focal-plane (FP) shutter has fully descended. The synch speed is the shortest time that the sensor is entirely uncovered, typically 1/200 or 1/250 sec. Any faster than that the 2nd curtain starts to descend before the first curtain has fully descended, resulting in the top portion of the sensor being obscured by the 2nd curtain when the flash fires. In very short exposures it effectively creates a narrow slit travelling down over the sensor.

 HSS works by making a series (2 or more) of flashes as each part of the sensor is uncovered. For HSS to work the flashgun must know which camera shutter speed is being used so it can fire the flash at the correct intervals.

When the flashgun is connected to the camera it will know what speed the camera is set at and if in E-TTL mode (auto mode) it will engage HSS when the shutter is set above the synch speed.

Last night I had the flashgun mounted on my camera, something I never normally do as it creates a flat image. Also I rarely use E-TTL mode anyway, preferring to use the flashgun manually.

I had set the flashgun to manual and assumed HSS would not turn on. I was wrong. The flashgun was more “helpful” than I had appreciated, it did turn HSS on, even though I am in manual mode, and insisted on giving me a well exposed complete image.

 As a side note, HSS is useful when you need a faster shutter speed as it cuts down on the ambient light. But the flashgun can only create one flash at full power before it needs around 4 seconds to recharge. In HSS mode it needs to create 2 or more flashes, the power has to be shared for each flash and so it can only create each flash at reduced power. The faster the shutter speed, the more flashes are required, the lower the maximum flash power available. Also beware the flashgun will overheat if HSS is used too often.

 Ian Whiting