Slow Sync / Rear Curtain

Hey everyone. I've recently been learning about and experimenting with
slow sync flash and have been attempting various special effects photos.
I thought I fully understood the diference between slow sync and rear
curtain, but yesterday my D70s decided to throw a spanner in the works!
I had the flash set to 'rear' in A mode, and did around a 1s exposure. I
noticed it was flashing at the start AND end of the exposure. Well that
threw me to start with, but I recovered from that and read in the manual
that in order to just get the flash at the end (which is what I
wanted), it needs to be in P or S mode. Well I'm not sure why things are
like that, but whatever, I set it to S mode. Well I STILL got 2 flashes
and not just the one. Can anyone give me a clue why that was please? I
have other questions regarding the results I got yesterday but it's
probably best to re-research that myself.

Cheers, Geoff.

Ed Ruf (REPLY to E-MAIL IN SIG!) wrote:
Preflash to determine exposure?
--
Ed Ruf Lifetime AMA# 344007 )
http://EdwardGRuf.com


I don't think it was that because that normally happens immediately
before the main flash and is barely detectable. Also, this first flash
is happening once the shutter is open, so ruins the effect I'm trying to
acheive by having rear curtain set.

Geoff wrote:

Hey everyone. I've recently been learning about and experimenting with
slow sync flash and have been attempting various special effects photos.
I thought I fully understood the diference between slow sync and rear
curtain, but yesterday my D70s decided to throw a spanner in the works!
I had the flash set to 'rear' in A mode, and did around a 1s exposure. I
noticed it was flashing at the start AND end of the exposure. Well that
threw me to start with, but I recovered from that and read in the manual
that in order to just get the flash at the end (which is what I
wanted), it needs to be in P or S mode. Well I'm not sure why things are
like that, but whatever, I set it to S mode. Well I STILL got 2 flashes
and not just the one. Can anyone give me a clue why that was please? I
have other questions regarding the results I got yesterday but it's
probably best to re-research that myself.

Probably pre-flash flash metering.

1) Did the images look okay?

2) If you determine the flash power seperately (incident flash meter)
and you can set your (attachement) flash power manually, then you can
get rid of the pre-flash.

Cheers,
Alan.


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Geoff wrote:

Ed Ruf (REPLY to E-MAIL IN SIG!) wrote:

Preflash to determine exposure?
--
Ed Ruf Lifetime AMA# 344007 )
http://EdwardGRuf.com



I don't think it was that because that normally happens immediately
before the main flash and is barely detectable. Also, this first flash
is happening once the shutter is open, so ruins the effect I'm trying to
acheive by having rear curtain set.

Are you sure of that? If that were so, then you would have enough
exposure from the first flash to freeze the moving subject slightly.

Note that in the presence of some light sources, such as mercury vapour
lamps, a 1/10S exposure will result in about 6 weak images of the moving
subject and then one last fully exposed frozen subject image.

Post some images of whatever you're getting.

Cheers,
Alan

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Alan Browne wrote:
Probably pre-flash flash metering.

1) Did the images look okay?

2) If you determine the flash power seperately (incident flash meter)
and you can set your (attachement) flash power manually, then you can
get rid of the pre-flash.


Thanks for both your replies Alan. The images did look ok but I wasn't
aiming for a particular effect really, more just experimenting and was
confused when I saw the first flash. With the camera set in normal front
curtain mode, I thought the flash did do a quick pre-flash that was
barely detectable. In fact I guess that as the shutter is open for
longer and it's set to rear curtain, that just means that the pre-flash
was noticable in my case last night. Hmmm....Ok I think I get it now! So
is the first flash actually taking place before the shutter is open? I
didn't thnik it was but maybe I just assumed that.

I can't honestly say if it's causing a problem with the images or not,
as I'm just trying to understand things at this point rather than
guessing, so I can then plan exactly what images I want, then set the
camera up as I think would be right for the situation. I never like
guessing because I want to know why I'm getting the effect I'm getting.

Geoff.

Kennedy McEwen wrote:
I don't think it was that because that normally happens immediately
before the main flash and is barely detectable.

It normally happens immediately before the main flash in front curtain
sync, but it has to happen before the mirror goes up, so it will be a
long time before the main flash with rear curtain sync, and will appear
right at the start of the exposure (actually fractionally before the
exposure commences).

Also, this first flash is happening once the shutter is open, so ruins
the effect I'm trying to acheive by having rear curtain set.

Are you sure it is happening *after* the shutter is open, and not just
before it opens? In other words, does the first flash appear on the
image, resulting in two sharp images of moving objects, one at the
beginning of the movement trail and one at the end?

If it does then you either aren't properly setting rear curtain sync or
there is a fault with your camera. :-(


I've only just posted something that your post has just backed up, so it
all makes sense to me now. I kinda worked out what was going on as I was
writing my last post, and you've assured me I was right. As you and Alan
suggest, the first flash isn't going off before the shutter opens, it
can't be. I assumed it was for some reason but I guess that isn't the
case. In fact now I'm aware of that it also helps me understand why some
of my test images weren't exposed correctly (I was messing about zooming
out while the shutter was open to get some nice streaking effects around
the main image).

Thanks for your help,
Geoff.

Geoff wrote:

Alan Browne wrote:

Probably pre-flash flash metering.

1) Did the images look okay?

2) If you determine the flash power seperately (incident flash meter)
and you can set your (attachement) flash power manually, then you can
get rid of the pre-flash.



Thanks for both your replies Alan. The images did look ok but I wasn't
aiming for a particular effect really, more just experimenting and was
confused when I saw the first flash. With the camera set in normal front
curtain mode, I thought the flash did do a quick pre-flash that was
barely detectable. In fact I guess that as the shutter is open for
longer and it's set to rear curtain, that just means that the pre-flash
was noticable in my case last night. Hmmm....Ok I think I get it now! So
is the first flash actually taking place before the shutter is open? I
didn't thnik it was but maybe I just assumed that.

Yes. In film cameras with TTL flash, flash is controlled by
continuously measuring the flash light being received by the film.
There are sensors in the film chamber that catch the light reflected off
of the film, integrate that and then shutoff the flash when the right
amount of flash light is received.

This is not possible with digital cameras as there is no useful
reflection from the sensor. Off-The-Film (OTF) doesn't work.

So, pre-flash is used. It fires before the mirror lifts and the
metering system in the prism assembly (used for available light
meterering) are used to measure the pre-flash light return from the
subject area. This is often integrated with the focus distance of the
lens. The flash power is calculated and then that "dose" is used during
the shot. The usual problems with depending on subject reflectivity,
actual subject position v. sensor position, etc. apply.

I can't honestly say if it's causing a problem with the images or not,
as I'm just trying to understand things at this point rather than
guessing, so I can then plan exactly what images I want, then set the
camera up as I think would be right for the situation. I never like
guessing because I want to know why I'm getting the effect I'm getting.

Post examples of what you're getting.

Cheers,
Alan.


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.