www.kenrockwell.com/tech/syncspeed.htm
About these reviews INTRODUCTION Sync speed is the fastest shutter speed you can use with flash, period. You cannot use a faster shutter speed than the sync speed with flash. If you try on a camera more than about 20 years old you'll get a partial blackout of the image, and modern cameras override you electronically when the flash is on. A shutter's rated top speed is irrelevant because it doesn't work any faster than the rated sync speed with flash. Sync speed is important to pros for two reasons: 1) The first and obvious reason is that fast sync helps stop motion. Fill flash in direct sunlight is critical to getting good shots in sunlight. This is why you'll see a flash atop every pro newspaper photographers' camera. Nature photographers have a heck of a problem with this, because we shoot at long distances from little animals. It's tough getting enough fill on a bird 50 feet away, even with those stupid concentrator lenses in front of our flashes. Ditto for using fill for shooting sports through a long lens. Changing ISO does nothing to help since it's the ratio of ambient to flash light that's important. Here's why: Flash Sync Speed Exposure equals the area under each curve. The total area for the ambient light increases as you consider more time horizontally, due to a longer shutter opening. The area (exposure) for the flash doesn't change so long as the exposure is at least 1/1,000 second to capture it all. The exposure from ambient light increases as exposure time (flash sync speed) gets longer, but the contribution from the flash stays the same. One uses smaller apertures with longer exposures, which keeps the ambient exposure constant but requires more power from the flash to keep up. It is difficult for battery powered flashes to compete and balance with direct sunlight as flash sync speeds get slower. I often run out of range in daylight with slower syncing cameras, or my flash has to fire at full power and take a long time to recycle. a) Flash is an instantaneous burst which lasts only about 1/1,000 of a second. You get the same contribution to exposure from the flash regardless of shutter speed. b) The contribution to exposure from ambient light varies with shutter speed, but not distance. Different ISOs or apertures don't change the ratio between flash and ambient. c) Changing aperture or ISO does nothing to affect the ratio between flash and ambient light. The factors which do change the ratio are distance, shutter speed or flash power. d) If you have the freedom to go to a higher sync (shutter) speed you can use a higher ISO or larger aperture to keep the same ambient exposure while increasing the sensitivity to the flash. Most battery powered flashes have to work very hard to compete with direct sunlight unless they are very close. Faster sync lets you get further away or use less flash power. Using less flash power increases battery life and reduces recycling time which increases frame rate!
e) Here's a subtle distinction many people miss: simply using a lower ISO or bigger aperture setting doesn't help unless you also have access to faster shutter sync speeds to keep the ambient exposure unchanged. Example one: You can't reduce the required flash output by increasing the ISO. You will need to use a smaller aperture to keep the ambient exposure constant because you can't increase the shutter speed above the sync speed. Because of the smaller aperture at the higher ISO you still need the same flash output. A higher ISO can't get you more range or better battery life. Example two: A lower ISO can let you use larger apertures for portraits, but it won't speed up recycle time (and thus frame rate) or increase range. If you set a lower ISO and use a larger f/stop you still need the same flash power output. A faster sync speed would give you more range or require less power from the flash when using the larger aperture. There are no disadvantages to the availability of faster sync speeds. At low light levels slow shutter speeds are needed to let in enough ambient light. We don't need to hold back the ambient light to keep it from overpowering the flash. That's what makes sync speed critical, in addition to stopping motion. Trick Modes Marketing departments like to push the "FP" trick flash modes that seem to let you sync at any speed, even 1/8,000.
back to top With faster sync speeds you can use larger apertures while retaining the correct ambient light exposure for fill flash. Simply setting a slower ISO does nothing to help all the below issues since you have to pump out more flash power at the slower ISO.
back to top Larger apertures let in more light from the flash which allows you to get further away with any given flash. For a full stop increase in sync speed you get a 40% increase in range since you can open the aperture up a stop. For a two stop increase (going from 1/125 to 1/500 for example, which lets you open your aperture two stops) you double your maximum flash range.
back to top Larger apertures let in more light from the flash and thus less power needs to come from the flash for each shot at a given distance. Less battery power is used to recycle the flash for each shot, since modern flashes only use the power needed for each shot and conserve the rest for the next shot.
back to top Since less power is used for each shot it takes less time for the flash to recycle since it only needs to replace the power that was used. If you haven't noticed this, time how long your flash takes to recycle when set to a manual power setting of 1/8 (almost immediate) and then see how long it takes when set to full (about 5 to 10 seconds).
back to top You can't shoot your next shot until the flash is ready. When the flash recycles faster you can shoot faster and at a higher frame rate. Most flashes are still ready to go when used at lower powers at the larger apertures you can use with higher sync speeds.
back to top Most flashes can shoot continuous bursts at lower powers. The lower the power, the more flashes they can fire off at a time. If more flash power has to be used for each shot due to smaller apertures caused by slow sync speeds you cannot get as many continuous flashes without breaking cadence and having to wait a few seconds for the flash to recuperate. When you loose your flash oomph you can't continue shooting your sequence.
back to top For every stop of increased sync speed you only need half the flash power. Thus a smaller, less expensive flash may be all you need. Even better, the built-in flash might be all you need for fill!
back to top Electronic flash is an instantaneous blast of light. At full power your flash may only last for a thousandth of a second. At typical power levels and with automatic settings it's probably be closer to 1/10,000 of a second. The mechanical focal-plane shutter of film 35mm SLR cameras and Leicas are two curtains of metal or cloth that zip across the front of the film. At slow speeds like a full second they zip fast enough to appear to open and close immediately. What's not obvious to the naked eye at fast shutter speeds is that the second curtain has to start zipping across the film right behind the first curtain. It has to do this because the curtain speed is not instantaneous. At fast shutter speeds the film is effectively exposed through a slit that zips across the film. If you pop a flash at one of these faster speeds then only the part of the film behind the open part of the slit would be exposed to the flash. The sync speed is the fastest speed at which the entire film or CCD can be open to light. This is determined by how fast the shutter curtains move. At speeds faster than the sync speed the slit that travels across the film or CCD narrows. If you used flash at faster than the sync speed (you can't do this on modern cameras) you would only expose the part of the film behind the slit to the flash.
D70 cameras by adding an electronic shutter to take care of the faster speeds. The only reason the D1, D1X and D1H (and for all I know the D70) limit the speed to 1/500 is because of the loss of efficiency above that speed for shoe mounted flash. Amateur digital SLR cameras om...
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