Photography News Issue 30 absolutephoto.com
65
Technique
High-speed flash sync explained
1/1000sec at f/4, ISO 100
1/4000sec at f/4, ISO 100
Flash sync problems are caused because of the way the shutter in your DSLR or CSC works. The design that’s common to the vast majority of cameras is called a focal plane shutter and it has two curtains, one of which controls the start of the exposure (called the ‘first curtain’), and one at the other the end (called the ‘second’ or ‘rear curtain’). Modern cameras have vertical-travel shutters; older film cameras have horizontal-travel shutters. For an even exposure (see below), the entire sensor needs to be revealed by the curtains at the point the flash is fired; ie. the first curtain must be completely open before the second begins to close. The fastest shutter speed where this happens is called the flash sync speed or X-sync. At shutter speeds beyond the maximum flash sync speed this isn’t possible because, to create the short exposure, the second curtain is closing over the top of the sensor before the first curtain has revealed the bottom. Therefore only a small part of the sensor is exposed at any time. Imagine the flash going off at that moment and you can see how a dark band is caused – that part of the sensor wasn’t uncovered at the point the flash was fired. High sync modes get around this in several ways. The most common is high- speed sync (HSS), and it’s found in many speedlights and some studio-style heads, like the Interfit S1 used here. In HSS mode, the flash fires a series of very short pulses throughout the exposure, rather than the normal single flash. This creates something that’s very similar to a continuous light source, so each part of the sensor is lit throughout and no shadow is created. The issues of HSS mode are in terms of power and range; the flash often needs to be closer to the subject as output is less and, just like available light, brightness drops as the shutter speed increases. Additionally, lots of battery power is used to generated the thousands of pulses.
1/250sec at f/4, ISO 100
Above It’s important to get the general exposure balanced with the light from the flash. Fortunately, the Interfit S1’s high-sync speed mode works the same as continuous light so it can be calculated along with the ambient light. Here, 1/1000sec was fine, but 1/4000sec was too fast, darkening the backdrop in an unrealistic way. At the camera’s regular sync speed (1/250sec) it was very overexposed.
Freezing the water With the flash firing, and the water running, another test shot was required to confirm that the ‘raindrops’ were sufficiently frozen. At the 1/1000sec shutter speed there was still a little too much movement for my liking, so I increased the shutter speed to 1/2000sec. Not wanting to change the aperture setting, this meant raising the ISO to 200 to get the same ambient exposure level. Due to the method of high-speed flash used by the Interfit S1 – a series of rapid pulses which works like continuous light so not visible to the naked eye – no adjustment is required in the flash power, because it responds just like the ambient light. However, the range is affected and the faster the shutter speed you use, the closer the flash will need to be to the subject to get the desired effect. Magnification and shutter speeds Shutter speed is very important in freezing themovement of thewater in this shot, but the speed you need is governed by the rate of the
water and the magnification of the lens you’re using. You can’t do much about the former, but try shooting at different focal lengths and you will notice some changes in the shutter speed it takes to stop the droplets. This is simply down to how far the water moves across the frame in the time the shutter is open; at a wide-angle and low magnification, it won’t be far – possibly just a few pixels – so you might get away with a slower shutter speed; but longer focal lengths and higher magnifications will show more movement and therefore faster speeds are required. In the second set of images – those with the orchid – I shot indoors, against a window to get the backlighting, while using the Interfit S1 as the main light. Here, a macro lens was used along with a water spray to generate the droplets, and because of the hugely enlarged subject, the shutter speed needed to be faster; 1/4000sec. Going back to the power of the flash being reduced as the shutter speed rises, this also meant that the S1 needed to be closer, at around 2ft.
1/1000sec
1/500sec in HSS mode
1/2000sec
1/125sec
1/500sec
1/500sec in normal mode
1/2000sec
1/4000sec
Above The flash sync speed is the highest shutter speed usable with flash without problems; the latter being illustrated as a dark bar on the image, a shadow caused by the shutter curtain. You can see the effect for yourself by connecting a flash to your camera using a PC sync cable and shooting at a manual shutter speed faster than the maximum sync. Such problems are avoided when a dedicated flashgun is mounted in the camera’s hotshoe. The two ‘talk’ to each other so that the maximum flash sync speed is not exceeded – high-speed flash sync has to be deliberately selected.
Above At a lower magnifications, or with slower moving water, longer shutter speeds can be used. Try some test shots to make sure what works. Here, 1/1000sec was blurring the water too much, but a speed of 1/2000sec fixed the issue.
Above The need to stop the water’s movement means picking the correct shutter speed. However, the more magnified the view, the faster the water will appear to cross the frame, so the higher the shutter speed will need to be. Even at 1/2000sec on a 105mmmacro lens, the drops are still blurred, but 1/4000sec stops them dead.
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