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Old Jan 3, 2006, 1:11 PM   #1
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Can anyone please tell me what the difference is between rear-curtain flash sync and slow flash sync on the D70s?

In P and A mode, you can select either slow sync mode or rear-curtain slow sync, while in M and S mode you get only the rear curtain slow sync.

I assumed that rear curtain syncmeans the flash triggers just before the 2nd curtain moves across the sensor.What I do not understand is how this differs from slow sync, which sounds pretty much the same to me (allowing a balance between ambient light and flash under low lighting conditions). If the flash triggers just after the 1st curtain clears the sensor, how can the camera determine the right exposure to allow ambient light to fill the picture? Surely the slow syncmust thereforebe a rear-curtain flash:?

Does anyone know?

Thanks, Vincent
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Old Jan 3, 2006, 4:41 PM   #2
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Rear curtain flash fires at the end of the exposure, rather than at the beginning. When shooting a moving subject, this will cause the streaks to appear behind the subject rather than in front. This is a more realistic effect as it conveys the sense of motion better. This is described in the manual, as well as when to use both types of flash.
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Old Jan 4, 2006, 1:49 AM   #3
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Thanks, but the manual is as clear as mud on this one. The description only talks about using natural light combined with flash (for slow sync) and the trailing lights of a moving vehicle (for rear-curtain sync). But both are meant for low light conditions where an object or people needs to be exposed in conjunction with naturally lit background.

I think that rear-curtain sync can be used for both types of shots, and I therefore do not understand why Nikon offers both settings. I therefore assume I am missing something!

Has anyone used both modes and understands the difference between them?

Vincent
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Old Jan 4, 2006, 5:30 AM   #4
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Sundance kid wrote:
Quote:
In P and A mode, you can select either slow sync mode or rear-curtain slow sync, while in M and S mode you get only the rear curtain slow sync.

This is the wrong way round. In P and A mode you get Slow Rear Curtain only. Slow sync has no meaning in S and M modes as in these modes you set the shutter speed so it is as fast or slow as you set it. In P and A mode the camera sets the shutter speed and without slow sync set on it will be set to a speed between 1/30 1/500 creating a flash only exposure. In slow sync mode it will set shutter speeds as slow as 30 seconds to also expose the background that is not lit by the flash.

In P and A modes you can select either slow sync or slow rear curtain sync so that is slow first curtain or slow rear curtain sync.

In S and M modes you can select the default first curtain sync or rear curtain sync and set the shutter speed as you want it.

At slow shutter speeds any moving subjects will have a trail in front of the flash lit exposurewith the default front curtain, with rear curtain these are behind. At faster exposure speeds using front or rear curtain sync is largely irrelevent as you don't get movement blur caused by ambient exposure.

So to summarise slow sync determines how long the shutter is open, rear sync determines at which point in the exposure the flash fires.




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Old Jan 4, 2006, 6:53 AM   #5
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A-ha:idea:. Now it is clear! Thanks for the detailed explanation; now I can go and play some more!

Vincent
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Old Oct 16, 2013, 12:01 PM   #6
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Quote:
Originally Posted by Nagasaki View Post
Sundance kid wrote:


This is the wrong way round. In P and A mode you get Slow Rear Curtain only. Slow sync has no meaning in S and M modes as in these modes you set the shutter speed so it is as fast or slow as you set it. In P and A mode the camera sets the shutter speed and without slow sync set on it will be set to a speed between 1/30 1/500 creating a flash only exposure. In slow sync mode it will set shutter speeds as slow as 30 seconds to also expose the background that is not lit by the flash.

In P and A modes you can select either slow sync or slow rear curtain sync so that is slow first curtain or slow rear curtain sync.

In S and M modes you can select the default first curtain sync or rear curtain sync and set the shutter speed as you want it.

At slow shutter speeds any moving subjects will have a trail in front of the flash lit exposurewith the default front curtain, with rear curtain these are behind. At faster exposure speeds using front or rear curtain sync is largely irrelevent as you don't get movement blur caused by ambient exposure.

So to summarise slow sync determines how long the shutter is open, rear sync determines at which point in the exposure the flash fires.
I know this is an old thread but could anyone give me an example where one would want or need to use front curtain sync? Thanks! (Brand new to this forum but I have been lurking )
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Old Oct 17, 2013, 7:02 AM   #7
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Very rarely do I use rear curtain sync (perhaps when I am capturing a moving vehicle at night...not often). Mostly I use the front curtain synch when taking indoor flash pictures: it fires the flash at the beginning of the shutter cycle. This freezes any motion of the subject and depending on the exposure timing, will allow for the background to be exposed for a nice balanced image. If I were to use Rr curtain sync in this instance, I would likely see ghosting of the subjects since any motion blur or subject movement will be captured prior to the flash firing.

Jehan
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Old Oct 17, 2013, 12:37 PM   #8
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Thanks very much for your thoughtful reply Jehan. I am wondering though, what keeps the sensor from capturing movement AFTER the flash on front curtain sync? It just seems like it wouldn't matter either way since motion would be captured in either instance. In fact, I was under the impression that the sensor is going to remember the most recent thing it is exposed to most clearly, i.e.; the flash firing at the end would seem to record things most clearly…. What am I missing?

Danny
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Old Oct 17, 2013, 7:36 PM   #9
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Danny, Not sure I can explain the science behind it. But I think when using front curtain sync, the flash freezes the subject in the foreground and for the short time that the shutter remains open, any residual motion goes unnoticed because of the brightness of the subject. See Example 1 below. I/m sure if you kept the shutter open for a long time under front curtain sync, motion blur would be captured and visible.

Example 2 uses Rear Curtain Sync where I exposed the subject twirling a sparkler and then at the end of the exposure the flash fired providing some lighting to the subject. But notice how motion blur was captured (visible on the lettering on the pant leg). Perhaps some one else reading this thread can explain the science better!

Jehan

Example 1 (Fr. Curtain Sync)


Example 2 (Rr. Curtain Sync)
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Old Oct 18, 2013, 9:09 AM   #10
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Quote:
Originally Posted by dvs.pictures View Post
I am wondering though, what keeps the sensor from capturing movement AFTER the flash on front curtain sync? It just seems like it wouldn't matter either way since motion would be captured in either instance. In fact, I was under the impression that the sensor is going to remember the most recent thing it is exposed to most clearly, i.e.; the flash firing at the end would seem to record things most clearly…. What am I missing?

Danny

Using flash is like taking two pictures at the same time. The 1st picture is the picture that is exposed by the flash unit. The 2nd picture is the picture that is exposed by the ambient / available light. Its just that they get recorded in the same final picture.

The reason this is is because of something called the flash-sync speed. It is the shutter speed that your camera max's out at when using flash. For Nikon it is usually about 1/200 sec or 1/250 sec. [Look up flash-sync speed and the other related topic high-speed flash sync or FP sync flash.]

So . . . when you use flash, the flash is only on for say about 1/10,000 of a second. That's as long as it needs to contribute enough light to the picture. But because of flash-sync speed requirements . . . the shutter remains open longer . . . ie. 1/200 sec. So even though the flash (1st picture) is done . . . the camera (ie the sensor) is still capturing ambient light (2nd picture) and it is contributing to the final shot.

Usually . . . in dark situations . . . the flash on near subjects contributes a lot more to the picture than the ambient light. That is when you get the pictures of people posing in front of something at night, but that something never shows up because its too dark. This is the perfect use for slow-sync flash . . . as keeping the shutter opening longer allows that 2nd picture to absorb more of that ambient light onto the sensor. (But the risk is that your main subject is in some light and there might be motion blur there.) Its a balancing act as most things are with photography.

That is "what keeps the sensor capturing movement after the flash on front curtain sync."

Ways to reduce (darken) the 2nd picture may be to use a lower iso, so the sensor is less able to pick up the ambient light . . . [Or reduce the ambient light (ie turn the lights off) . . . like people do when photographing water drops / splashes . . .]

Ways to increase (lighten) the 2nd picture may be to use higher iso or . . . slow sync speed flash . . . [Or just increase the ambient lighting.]

But as far as front and rear curtain sync goes . . . its a matter of which picture you want to happen first (and how they line up in the final combined picture.) Do you want picture 2 to happen first and then picture 1 - rear curtain sync. Or do you want picture 1 and then picture 2 - front curtain sync.


Sorry for the long winded post . . . but as soon as I started thinking about flash as taking two pictures at the same time . . . it made a lot more sense to me.


Take care & Happy Shooting!
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Last edited by tacticdesigns; Oct 18, 2013 at 9:20 AM.
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