Flash Sync Mode, including Rear Curtain

Cameras typically offer a few flash sync modes. The "curtains" below refer to the focal plane shutter, which has a front curtain which opens to expose the sensor to light, and a rear curtain which then closes, shutting off the light exposure. Shutter exposure duration is the delay that the second curtain follows behind the first curtain. "Sync" means that the flash cannot be triggered unless both curtains are fully open over the full frame. More at Minimum Flash Sync Speed.

Camera Sync modes for flash

• Front Curtain Sync (this menu may be labeled Fill flash on entry models with Auto mode, but it is still just default Front Curtain Sync). This is normal regular default flash sync, where the shutter fully opens, and then the flash triggers immediately. Whereas, any actual Fill flash function is instead done by TTL BL flash metering, or with TTL flash compensation (but not by this menu). Here, Fill sync mode just means the regular sync mode, and specifically means NOT one of these next sync modes.

  This is the normal and default everyday sync mode, which typically enforces a Minimum Shutter Speed with Flash of about 1/60 second when in camera A or P or Auto modes. Because, if using flash, we don't need shutter to be any slower, the flash will illuminate things. Some Nikon cameras (typically those with the Commander) offer an E2 menu, where this Minimum can be set slower, but the only way to make it faster is to go out into brighter light that will actually meter higher. Or use camera M or S mode so you can set the shutter speed directly.

• Rear Curtain Sync causes the flash trigger to wait, and not to trigger the flash until the end of the shutter duration, as the second curtain is about to start closing. Nikon names this mode Rear Curtain Sync, and Canon names it Second Curtain Sync. This is commonly used at very slow shutter speeds to make long blurred trails in the ambient light, and then the flash to freeze the moving subject making the trails. Rear Curtain sync puts subject at the end of the long blur trail, with the trail following the motion naturally, instead of seeing the blur leading out in front of the subject. Front Curtain sync freezes the image of the subject, and then the slow ambient continues to record the ambient blur trails. This Rear Curtain Sync just freezes the subject at the end of the motion blur trail, naturally, instead of the blur trail leading the motion unnaturally. Rear Curtain sync can have no visible effect unless the shutter is slow enough to create some ambient motion blur. In Nikon cameras, Rear Curtain Sync mode also includes Slow Sync (to allow very slow shutter speeds in A or P mode, to help create the blur trail). More below. If a fast shutter speed and there is no ambient blur, then Rear Curtain Sync has no effect.
  We say Rear Curtain sync triggers the flash at the end of the shutter duration, which is adequate understanding for our usage. But technically, the timing must occur early enough to allow the flash duration to pass before the shutter is closed. Speedlights are normally very fast, but are still slow at full power. The camera shutter is what triggers the flash sync. Nikon puts the Rear Curtain control menu in the camera (works with any flash), and Canon puts it in the flash (and that flash can enable it).

  The camera doing Rear Curtain Sync waits to trigger the flash until near the time the shutter closes. So to prevent cutting off the flash short, that means the camera needs to allow for the power duration of the flash to know when to trigger it earlier, to still allow time for the flash to show. But the camera doesn't know the flash duration (varies with power level and which flash). So it would seem the camera timing guess must allow lead time for a longest imagined flash duration situation. For a number, we might say a long flash duration is about 5 milliseconds.

  Also, the focal plane shutter takes the same time to close as to open, and which the Maximum Shutter Sync Speed to open is around 1/200 second (5 milliseconds), for todays shutters. The sum of these two estimated delays is 10 milliseconds. My own notion is that if shutter speed is as fast as about 1/100 second (10 milliseconds), the camera cannot even attempt to try Rear Curtain Sync. I have heard that number said as 1/90 second, but shutters are faster today.

  In reality, the only purpose of Rear Curtain Sync is when a slow shutter speed could otherwise blur motion in ambient light. Rear Curtain sync does not change the motion or the blur, but it triggers the flash at the end of the motion instead of at the start of the motion (to look more natural). Rear Curtain Sync would seem quite pointless for a fast shutter speed, or if HSS flash, or if no motion is present, or if very little ambient light.

• Slow Sync - If without using Slow Sync mode, in camera A or P or Auto modes, the camera normally enforces a Minimum Shutter Speed With Flash of 1/60 second (typical) when it recognizes that flash is used. Advanced Nikon camera models have a menu E2 to allow setting a slower minimum (A or P modes), but the menu default is 1/60 second minimum shutter speed (with flash). If you want a faster shutter speed, use camera mode S or M to set it directly. Or if in camera A or P or Auto mode, you must go out into brighter ambient light that will actually meter brighter.

  Specifically, this Slow Sync mode ignores any Minimum Shutter Speed With Flash (camera A or P modes), and simply uses whatever slow shutter speed the dim ambient actually meters (in camera A or P mode, Slow Sync mode can have no effect if in S or M camera mode, which simply set shutter speed directly). This slower shutter (the metered ambient) will expose the ambient light properly and naturally, where regular flash sync indoors probably won't. For example, perhaps the ambient without flash is metering 1/4 second, so you think you need flash. Reach up and turn on the flash, and shutter speed jumps to 1/60 second (Minimum), because now you are using flash instead. If Slow Sync, it will not change, it will stay at the 1/4 second. You might use Slow Sync when you want the shutter to stay slow (to increase the ambient exposure, or maybe to capture more ambient motion blurring). Otherwise, this 1/60 second minimum in camera A or P mode is an arbitrary default Minimum Shutter Speed With Flash (it is Not calculated to be about an exposure, and the flash does not care about shutter speed).

• Nikon offers a Red-Eye Reduction sync mode. This is regular front curtain sync, but it delays the shutter about a second while the flash flashes about three times, trying to cause the subjects eye pupils to contract smaller, hoping to prevent or reduce the red eye. You may not like that. 😊  Don't forget it if you turned it on.

  However, the Best way to reduce red eye is to provide more distance between flash and lens, at least about an inch of separation for each foot distance to the subject. Red eye is caused by the direct reflection from the retina inside the eye, and a flash at a wider angle will eliminate that possibility from reflecting back. A tiny compact camera (flash maybe 1/2 inch from the lens) will be terrible about red eye. A hot shoe flash on a DSLR (5 or 6 inches from lens) will be better. Bounce flash or off camera flash will be best of all.

• And there are some combinations of these options offered, for example Nikon Rear Curtain Sync will also be Slow Sync. I think they are separate on Canons.

On Nikon bodies, cameras with two thumb wheels set flash sync mode with the same button used to set Flash Compensation, but just using the other wheel. So it becomes easy to accidentally change sync mode without intending to (wrong wheel). Cameras with one wheel have a different button to set sync mode.

Rear Curtain Sync
for Blur from Ambient Motion Trails

Rear Curtain Sync waits to fire the flash until almost the end of the shutter duration, so that the ambient shutter blur occurs first, which then appears to follow the subject (appears back where it previously used to be, before the flash), which looks more natural to the motion. Only point of Rear curtain sync is to be used with a too-slow shutter speed, to intentionally make the blur trail visible. Rear Curtain Sync simply will not matter at any normal fast shutter speed (if no blur trail).

The three pictures below are a tape dispenser swinging on a string (the motion is to your right in all cases). The ambient room light is morning window light, but not direct sun. D300 settings were ISO 200, 50 mm, 1/20 second f/4.5, with a hot shoe manual flash at four feet. Frankly, 1/20 second seems abysmal to stop motion, but this is indoors, and I am trying to show blur here, blur caused by the ambient light. The SB-800 flash is at 1/32 power, which has a flash duration of 1/17800 second (spec chart in rear of flash manual). The flash certainly stops the motion of the swinging tape dispenser fine (note its shadow), but the 1/20 second shutter still allows the continuous ambient room window light to blur it severely anyway. There are clearly two separate exposures in a flash picture. The flash is very fast, but the much slower shutter continues the continuous ambient exposure much longer after the flash has finished.

Said again: The shutter duration is represented by the longer blur trail of the motion. The instant the flash occurred is represented by the sharper location of the subject at that instant. The point is, Rear Curtain Sync fires the flash last, as the shutter closes, which causes flash to appear to lead the motion, which is more natural.

Some people imagine that the delayed result of rear curtain sync causes a sharp stopped image superimposed on top of the blurred image (so is sharper), which may appear true of the leading edge, but the opposite is true of the trailing edge. So while there definitely are two separate exposures, and the flash does freeze the action when it triggers (and the continuous light continues blurring it), the effect is not "on top" of anything. Each pixel can only contain the one total accumulated pixel exposure value, regardless of when. The only difference is when the flash occurs. Rear Curtain sync only determines if a blur trail follows the subject instead of leading the subject. There is really no point of using Rear Curtain sync except with the slow shutter speeds that can cause the blur trails from brighter ambient.

On Nikon DSLR, Rear Curtain sync will also ignore the 1/60 second minimum shutter speed menu limit with flash (in camera A or P modes), and will select the SLOW shutter speed the ambient actually meters (called Slow Sync, which allows metered slow shutter speed. On Nikons, Rear Curtain sync includes Slow Sync). Rear Curtain Sync mode is intended for SLOW shutter speeds, it only has use for SLOW shutter speeds which can produce the blur that Rear Curtain addresses (I think Canon does not automatically combine Rear and Slow).

Stopping Motion without Blur

Note that if we had wanted to eliminate the blurring, we would eliminate the continuous ambient light by using maximum shutter sync speed (around 1/200 second) instead of a very slow 1/20 second. That fast shutter keeps out the continuous light, which is generally easy indoors, but it is brighter and more difficult outdoors in sunlight. Sure the shutter now is set to be 1/10 of the longer duration, which is faster, but not really enough for the action this close. But at 1/200 second, also the continuous ambient light is now weakened to be over three stops more dim. Its blur is simply too dark to be seen. The shutter speed does not affect the flash (because the speedlight flash is faster than the shutter speed), so the flash exposure was unchanged, but the ambient exposure was decimated, taking the blur out with it.

Saying it again, as it is very important, and slightly tricky. The 10x faster shutter should make a blur trail that is 1/10 as long. The tape dispenser is 3.2 inches long, and at 1/20 second, here the blur trail above seems about that same length too. So for a 10x faster shutter speed, we would expect the blur trail to be reduced to about 0.3 inches long then (still wider than a pencil), which would still be a blurred picture in this close view. Which would be obviously true if we also opened the aperture the same three stops to create an "equivalent exposure" of the ambient. But we didn't open the aperture! The flash saw the same aperture and still provided the same exposure (not affected by shutter speed). But the ambient became over three stops more dim, not at all equivalent, due to faster shutter speed. Light that is too dim to be visible adds no visible blur. The picture is sharper and the background shadow is darker, NOT because the 1/200 second shutter was able to freeze anything, but only because the 1/200 second shutter kept the continuous ambient light off of it (no addition of two sources) — letting the speedlight do its fast work. This is a significant principle.

Repeating song and dance for third picture: It is NOT that the 1/200 second helped enough to freeze the motion of the swinging string (it didn't). The speedlight flash (1/17800 second) was more than fast enough for that. The benefit is that the 1/200 second shutter speed kept the continuous ambient light out, too dim to blur the action. This was easy indoors, in more dim ambient light. It is harder to ignore bright full sun, but sports action flash pictures do often intentionally darken the ambient background somewhat, for this same reason — to minimize blurring due to ambient (and to make the subject stand out against the background). High Speed Photography with speedlight flash does the same, reducing ambient enough to not blur the motion the speedlight already stopped.

The third picture is all the same, the same flash, same room, same motion, just without any contribution from the continuous ambient light, due to the faster 1/200 second shutter speed shutting it out. It seems very important to realize this distinction. It is not that the 1/200 second is fast and stopped the motion (1/200 would still be insufficient speed to do that when up this close). The flash stopped the motion — 1/32 power is 1/17800 second duration. What happened was that the ambient was made to be very dark and invisible at 1/200 second exposure, when it simply didn't matter any more. No visible blurring because it was dark. This is a biggie to realize and know, very usable.

And it is easy to do indoors with dim ambient, but much harder in bright sun, because the maximum shutter sync speed is limited, and the flash power has limited range. Cameras vary some, but 1/200 second is the ballpark of as fast as the focal plane shutter can open to sync flash.

Auto FP flash (HSS): Note that so-called High Speed Sync (FP mode) cannot help do this, since the HSS flash itself becomes continuous light then too, same as the sun. The faster shutter can help the sunlight exposure, but the FP flash cannot stop motion, and the FP flash distance range is reduced. Rear Curtain Sync has no meaning for FP mode, and no possible action or benefit (FP flash begins before shutter opens, and remains on until after shutter closes). Continuous light has no motion stopping ability anyway. FP shutter speed is faster than 1/200, but still nothing like the speedlight already does. A fast shutter speed will attenuate both HSS flash and sun the same (both are continuous). etc, etc. Auto FP flash mode is about using wide aperture with flash in sunlight, and is not about the flash freezing motion (a fast shutter speed can help in bright ambient, if the FP flash is bright enough then).