In part 1 of this mini series it hopefully became clear that flash photography usually involves two light sources, the ambient light and the flash and how to match the brightness of the two light sources to achieve the desired effect. In part 2 we’ll look at some rather technical aspects of flash photography you need to understand to really make your flash unit a creative asset.
Flash Sync Speed
Since the flash duration of a flash unit is extremely short (from a few milliseconds at full power to less than 1/50.000 sec. at minimum power), the shutter of the camera and the firing of the flash must be precisely coordinated, i.e. the opening of the shutter and the flash must be “synchronized”.
To grasp the concept of flash synchronization speed it is first necessary to understand how a focal plane shutter works. A focal plane shutter consists of two independant curtains, that prevent light from reaching the sensor. When the exposure begins the first curtain starts moving allowing the light to hit the sensor. After a certain time the shutter is fully open and the sensor is completely exposed to the light. Then the second curtain starts moving and closes the shutter. However, this works only for relatively slow shutter speeds. Depending on the camera model this may be up to 1/200 s or 1/250 s. Once the exposure time gets shorter than that, the shutter never fully opens. Instead the second curtain starts closing the shutter even bevor the first curtain has fully exposed the sensor. This essentially means that the picture is exposed via a slit that moves over the sensor. The shorter the exposure time, the narrower the slit. The following illustration shows how a focal plane shutter works.
As long as you don’t use a flash you don’t need to worry about this somewhat peculiar behaviour of a focal plane shutter. In flash photography, however it becomes very important as you can’t use a flash with fast shutter speeds. The reason is the very short flash duration. The next illustration shows what would happen if you used a very short exposure time with flash.
As long as you use comparatively long shutter speeds (the left column in the illustration above), the flash is triggered as soon as the shutter is fully open. Due to the short flash duration the shutter may well stay open for some time after the flash fired.
Once the exposure time becomes so short that the shutter no longer fully opens, the flash cannot illuminate the whole sensor area. The resulting picture would be mostly black with only a (narrow) slit beeing exposed correctly. Modern cameras therefore don’t allow you to use such fast shutter speeds when using flash. But in the olden days when cameras where fully mechanical this happened regularly.
The fastest shutter speed at which the shutter is completely open is called the flash sync speed. Refer to the camera user manual to find out which shutter speed that is for your camera.
If you want to use a shorter exposure time than the flash sync speed with flash, you can use the so-called short high speed synchronization. More about this in part 3 of this series.
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