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 for two big reasons:
1.) The first and obvious reason is that fast sync helps stop motion. 1/250 is not what we want and 1/125 is out of the question. We need to shoot at 1/500 or faster to stop action.
2.) The second reason, which is rather abstract and often misunderstood, is that faster sync speeds help get enough flash power to balance with direct sunlight. 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.
The faster sync lets us shoot at larger apertures, which requires less flash power, which lets our flash recycle faster, which lets us shoot more frames per second.
Direct sunlight is harsh and needs fill.
Changing ISO does nothing to help since it's the ratio of ambient to flash light that's important. Here's why:
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. Only the distance, aperture and ISO affect it.
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! See also Shooting Sequences with Flash.
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. Faster rated sync speeds simply give you more options. You always can use slower speeds with flash.
At low light levels slow shutter speeds are needed to let in enough ambient light. Sync speed is not a limitation. We don't need to hold back the ambient light to keep it from overpowering the flash. In dim light we always wish we had more ambient light!
To sum up, sync speed is critical for using fill in daylight, and fill is critical to getting good images in direct sunlight.That's what makes sync speed critical, in addition to stopping motion.
Sync speed isn't an issue in dimmer light.
Marketing departments like to push the "FP" trick flash modes that seem to let you sync at any speed, even 1/8,000. This doesn't count as far as I explain later here. Canon calls this "High Speed Sync" and I forget what Sigma and others call it. It is not a substitute for genuine fast sync speeds.
Stopping Motion back to top
At slower sync speeds like 1/180 you're far more likely to get subject or camera motion and blur than you are at 1/500. You knew this.
Depth of Field Flexibility back to top
With faster sync speeds you can use larger apertures while retaining the correct ambient light exposure for fill flash. This is the key to all the other related benefits. 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.
Maximum Flash Range 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.
Flash Battery Life 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.
Flash Recycle Time 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).
Maximum Frame Rate 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.
Buffer Depth 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.
Allows Use of Smaller Flashes 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!
What Limits Sync Speed 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.
Nikon has done the world a great favor in the D1, D1X, D1H and D70 cameras by adding an electronic shutter to take care of the faster speeds. By doing this the sync speed becomes unlimited. 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 omit the electronic shutter and are limited by the mechanical focal plane shutter.
Point-and-shoot digital cameras usually sync at high speeds like 1/500 because they also have electronic shutters.
Leaf shutters as used in professional cameras like the Hasselblad and large format cameras can sync at any speed. This is because their leaves open completely at all speeds, at which point the flash is fired. There is no slit or partial opening at the fastest speeds.
FP Modes (also called High Speed/FP flash synch on Canon) back to top
Now that you know that the shutter needs to get all the way open to let the instantaneous pop of an electronic flash expose all of the film or sensor, what if instead we made the flash stay on long enough to let the curtains of a focal plane (FP) shutter complete their travel from one side to the other? We could let the flash expose the film or CCD at any shutter setting during the time it takes the curtains to travel from one side to the other.
You can do that as a trick mode with many high-end flashes and cameras. It's called the FP mode.
There are many disadvantages, which is why I don't ever use these modes.
1.) FP mode often reverts to totally manual exposure calculation. If so, it's only useful for shooting things that stay at the same distance so you can calculate it. I saw this feature illustrated in a Nikon brochure, and lo and behold, the two examples were 1.) a water skier shot from the boat (the rope stays the same length) and 2.) a shot of a race car made from a camera bolted to the car and fired remotely. Some newer cameras like the D2H have automated this, finally.
2.) The flash always pops at full power on the flashes I've seen. Thus as above you lose battery life, have long recycle times, no high frame rates and all the other disadvantages above.
3.) Since only a fraction of the light at any time is exposing the film or CCD you lose a lot of light, again getting you back to the problems of limited flash range. The loss of light also depends on the shutter speed you use, thus the flash guide number used in manual calculations in 1.) above changes with shutter speed! You lose most of the light at the faster speeds and lose less at slower speeds.
At least FP mode gives you the flexibility to use any shutter speed and also because you lose a lot of light you can get to the larger apertures.
This can be a handy feature in limited applications, however there are so many limitations you see why I don't consider it anywhere as useful as a healthy true sync speed.
Some of the very latest cameras like the Nikon digital SLRs and Minolta Maxxum/Dynax 7 have FP modes that are TTL and do vary the power down from full. This way you have only the disadvantage of 3.) above. I have not researched these; believe it or not I'm too lazy.
Survey of Sync Speeds back to top
1/2,000 and above: Nikon D1, D1X and D1H cameras when used with an external PC cord. The Nikon D70 when not used with a dedicated flash, for instance, an old 1980s non-dedicated flash or probably also when used with the shoe-mount sync adapter for external flash. You can fool the D70 by undedicating the flash by covering all but the large central hot shoe contact.
These camera's electronic shutters are efficient enough to sync clear up to the full 1/16,000 or 1/8,000 speed, although of course with most strobes you'll lose a lot of light. You won't get a partially black frame as you will when using the wrong sync speed with a conventional focal plane shutter camera.
1/1,000: Rollei PQS series leaf shutter lenses.
1/500: Most professional DSLR and film leaf shutter cameras including Hasselblad, 4x5" view camera lenses, the Plaubel 67, and the Nikon D1, D1H, D1x and D70 when used with shoe-mounted flash. Also most point-and-shoot digitals, since they often incorporate electronic shutters.
1/60: Amateur SLRs from the 1970 and 1980s, like the Olympus OM-1, Minolta SRT series and X-700.
1/50: Leica rangefinder cameras, even today (pretty poor!)
1/30: focal plane shutter SLRs before the 1970s, X-sync leaf shutters used with M type flashbulbs, Russian medium format SLRs like the Kneb/Kiev.
Flashbulbs back to top
Flashbulbs use magnesium foil that burns when triggered by an internal electrically fired squib. They usually burn for about 1/30 of a second and take a little while to reach full brightness. Thus shutters intended for flashbulbs have a position marked "M" which triggers the flash an instant before the shutter actually opens.
You can use all speeds on a leaf shutter with flashbulbs and M sync, although of course at the faster speeds you'll not capture all the light of the flashbulb. The guide number tables that come with your flashbulbs reflect this by having different values for different shutter speeds. Flashbulbs, unlike electronic flash, are not very good for stopping action. That's why Nikon named their electronic flashes "speedlights" when they came out in the 1960s.
For focal plane shutters you need special "FP" (Focal Plane) flashbulbs if you want to use them at shutter speeds faster than 1/30. Just like the screwy FP modes of some fancy electronic flashes, use at higher speeds wastes a lot of light but it works. FP flashbulbs attempt to have a constant light output across about 1/30 of a second so as the focal plane's slit moves across the film you get even exposure. Regular flashbulbs have a peak of light and are best used with leaf shutters if you need to shoot at faster than 1/30.
Older cameras and shutters intended for use with both electronic flash and flashbulbs have either a two position switch or two separate sync terminals. "M" is for flashbulbs and "X" is for electronic flash. "M" fires the flash an instant before the shutter opens to allow the flashbulb to be at the best brightness. "X" sync triggers the flash the instant the shutter is completely open, since electronic flash fires instantly.
You can use most flashbulbs on modern cameras at 1/30 at either sync setting.
If you use an electronic flash at the M setting you'll see no flash on the film, since the flash would have been triggered an instant before the shutter opened. This used to mess a lot of people up in the 1970s since the flash would go off, but not show up on film!
Glow-in-the-Dark Trick back to top
As you've read above, "M" sync on older shutters fires the flash before the shutter opens, usually ruining your photo with electronic flash.
On the other hand,what if you photographed something that glows in the dark? Yeah, the flash will pump it and your exposure will be made at the maximum glowing with no contribution from the flash. This is a cool trick for photographing phosphorescence.
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