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What exactly is flash sync speed and should it be a factor in a buying decision?

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See also photo.stackexchange.com/questions/1615 for some good answers explaining how this works. –  mattdm Jan 13 '11 at 13:20

5 Answers 5

up vote 20 down vote accepted

Flash sync speed is the maximum shutter speed possible when using a flash. For most flashes, the flash sync speed, sometimes also referred to X-Sync speed due to the use of Xenon in the flash bulb itself, is around 1/200th to 1/250th of a second. When using flash, your maximum shutter speed is limited to the flash sync speed. In many cases, this is perfectly adequate, as the flash pulse itself is sufficiently short enough (around 1/1000th of a second), and brightly lights up the scene beyond the normal ambient lighting for a fraction of the time the shutter is actually open. Flash sync speed can sometimes be a limiting factor, such as for action photography, as 1/200th or 1/250th of a second may not be enough to stop some kinds of action being photographed when fill flash is not able to overcome ambient lighting.

Some higher-end camera gear is capable of higher flash sync speeds. Some models support up to 1/500th of a second, which is better for photographing action. There are also alternative flash sync modes for better camera gear and flash gear. Normally, flash is synced with the "forward" shutter edge, and fires when the forward shutter curtain edge has opened and is moving. An alternative sync mode that syncs flash with the "back" shutter edge. When shooting with back curtain flash, you can produce action ghosting, and freeze your subject at the very end of the exposure, which is sometimes a desirable effect for sports photography. Finally, there is "high speed sync." With this alternative sync mode, cameras may sync to flash at any shutter speed, even up to 1/8000th of a second on top-end models.

High speed sync does have some limitations. With normal flash sync, there is a single pulse of the flash. In high speed sync mode, the flash pulses continuously thousands of times a second. This ensures that the scene is illuminated for the duration that the shutter is open and accommodates the behavior of a camera shutter at such speeds. The drawback here is that to provide enough power for continuous flash pulses, the power of each flash is less, by around 1 stop per stop of higher shutter speed. Additionally, since the scene is illuminated continuously for the duration the shutter is open, the flash itself is not as useful for "stopping" action. This is often not that big of an issue, however, as the higher shutter speed itself is capable of freezing action (particularly at 1/4000th or higher.)

If you don't need high-speed flash sync, any flash supporting a standard sync speed will suffice. If you need to sync flash at extremely high shutter speeds, then you will need both a camera body and a flash that supports high speed sync. You won't be quite as limited with high speed sync, but keep in mind that the power of your flash will be a little less than normal. (Generally, this is not a problem at all, and you can usually open your aperture to compensate...but it is a factor to be aware of.)

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+1: well written! –  David Schmitt Jul 19 '10 at 7:10
    
Indeed! Accepted –  jfoucher Jul 20 '10 at 9:42
    
And another reason why the Nikon D-40 is still wildly popular. I keep wondering why Nikon did away with that .. perhaps just too much of an unappreciated feature for the intended users of that camera. –  Tim Post Jul 21 '10 at 13:50
2  
I think this could be improved w.r.t. action; it's leaving out that the duration of the flash is at least as good at stopping action as the shutter. Sync speed only needs to stop action in fill-flash situations. –  ex-ms Jul 25 '10 at 19:46
1  
Looks good to me! –  ex-ms Jul 25 '10 at 21:39

Short Answer: Probably not a Factor in Buying Decision

definition:

The sync speed is the fastest shutter speed you can use when using the flash.
It depends mostly on the camera body.

typical behaviour:

Most flashes will sync with most cameras at around 1/250.
Unless you have specialist requirements, they will all perform in a very similar way.
Other factors are more likely to help you decide which flash to buy.
(e.g. price, manufacturer, recharge time, length of flash, controls, remote control, swivel, ...)

freeze the action:

This does not typically affect your ability to freeze the action, because (as @che points out) the flash only lasts for 1/1000 of a second (typically), so for images where the subject is being illuminated mostly by the flash, it will be frozen as though you were using a shutter speed of 1/1000.

Confusing? I find it a bit confusing, to be honest, but it does all work out logically once you get the hang of it!

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X-sync is the lowest shutter speed during which the shutter is entirely open at some time, and thus allows use of flash. (You don't want it to light just top half of frame.)

X-sync differences don't play that much role in stopping action (flash pulse duration is around 1/1000 anyway), but rather in eliminating ambient (or overpowering sun, if you want) with flash in photos when you have both kinds of light.

If you have a flash-lit portrait outside, you usually want to have the surrounding landscape a bit darker than the person. Now, if you use a full power of your flash, and get proper exposure of foreground at f/8 and ISO 100, out might even get background overexposed at 1/200 sec if it's sunny day. Being able to go to 1/500 might make the shot possible, or allow you to raise ISO to 200 and save some flash power to get faster recycle times.

Sometimes you can kind of "cheat" by using high speed sync which fires the flash multiple times to cover all parts of the frame. This doesn't really help in this situation as it eats flash power you need to have foreground exposed properly.

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Thanks for the exemplified answer, che. But should I take the sync speed into account when shopping around ? –  jfoucher Jul 19 '10 at 5:19
    
I sometimes mix flash and daylight, and I can still live with camera that has X-sync at 1/200 sec. I'd think twice about buying something even slower. 1/500 X-sync would be nice, but there are things that matter a lot more to me (such as sensor size, price and weight). –  che Jul 19 '10 at 6:44

IMO, it can be a factor in a buying decision, but when/if it is, you'll generally know it ahead of time. That is to say, if you've been running into problems (e.g., when using flash under daylight) and wishing you could get a faster flash sync, then it can certainly be worthwhile to get a body that syncs at higher speed. On the other hand, if you haven't run into a problem, then chances are pretty good that you really don't care.

At one time, X-sync speed was a serious consideration. When most cameras only synced at up to 1/60th or 1/90th, there were quite a few situations where it caused of a problem. The obvious problem arose when you had decent (but not really bright) ambient light. If you wanted, say, 1/500th to stop action, but only had ambient light to support (say) 1/125th, you ended up with problems either way -- if you didn't use a flash, you only get 1/125th, and with it blur. If you did use flash, you could only use 1/60th, so you had to stop down quite a bit and use the flash at (close to) maximum power to overpower the ambient light enough to keep it from leading to "ghosts". Shooting at full power, however, lead to longer flash cycle times, so you were more likely to miss shots as the flash recycled.

Unless you collect relatively old cameras, however, you'll probably get an X-sync of at least 1/200th, which is enough to prevent problems under most conditions. Being able to go higher is sometimes handy, but not all that crucial.

A few additional bits and pieces, though it's probably more in the range of trivia than useful information for most people:

  1. You can get X-sync at up to 1/1600th of a second on a few cameras (some of the PhaseOne medium format bodies). Flash selection gets tricky though, because to work right, you need a flash with a duration of less than 1/1600th of a second, where many are around 1/1000th, and some studio flashes have even longer duration. This can give some pretty strange effects though -- at 1/1600th, with a decent-sized studio flash, the flash can overpower the ambient light to the point that even shooting in broad daylight, you can make it look almost like you were shooting at night, with the sky and most background relatively dark. You do have to be careful, though, to avoid the "deer in the headlights" look. With care, however, you can help isolate the subject, even (for example) with a background that would otherwise be excessively busy and distracting.
  2. As far as I know, the fastest X-sync with a focal plane shutter was at 1/350th (Minolta Maxxum/Dynax/Alpha 9 and 9xi).
  3. Those same cameras did high-speed sync at their maximum shutter speed of 1/12000th.
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A higher flash sync speed is useful if you wish to shoot with flash at large appeture (because you want low DoF) but are shooting at a subject that has bright backlight. A typical instance where this might happen would be a wedding shot with the sun behind the subject. You want flash because you want to bring up the dynamic range, you want large appeture because you want low DoF, and you have bright light so you want a shutter speed faster than 1/250th.
In this case a camera that can do 1/500th sync speed is a life saver and if you were doing wedding photos for a living I guess you would rely on that for brightlight shooting.
Is it a factor in buying desision? If your going to want to use flash in bright light (for dynamic range) and low appeture for DoF then YES. Otherwise NO.

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