Optical Slaves in Strobes

Hello all,

I was wondering what the synch speed is for most optical slaves integrated
into studio flashes. I was considering using a on camera dedicated flash as
a fill, and to set off strobes. The flashes synch much faster than the
traditional synch (1/4000 or 1/8000 depending on camera flash) versus
1/180th of a second.

Thanks!

Robert Meyers

 

I was wondering if I could use the high speed synch for propretary flashes
to cheat
the max x-synch of the camera. This sounds like the answer is yes?

Thanks!

 

I'm not quite sure I understand your question. If you are shooting
with 35 mm you will have to shoot at your maximum sync speed of your
camera or lower.

Your on camera flash will trigger your slaved flashes instantaneously
(but not quite at the speed of light!).

Some older flash units have quite slow sync speeds. I have used some
older Bowens Monolights that recommend a shutter speed (with medium
format) of no more than 1/400 of a second.

I don't know if this helps.

 

No. You cannot cheat the max x-sync of your camera with studio
flashes. HSS is a different beast altogether.

 

Depends what you're trying to do, but for the most part, the answer
is "no".

Maximum sync speed on most cameras is determined by the shutter
mechanism, and has nothing to do with the flash. The shutter curtains (that
open to expose the film) can only move so fast physically.

There are two shutter curtains in most SLRs, and they both move the
same direction across the film - one opens, say moving left to right, and
the other closes in the same direction, following it. For high shutter
speeds, the first one doesn't even open all the way before the second one
starts to close, and they both move across the film at the same time. The
gap between them is the actual shutter speed, but the film never gets
exposed all at once - it's actually exposed by a slit between the curtains
that passes across the film.

Flash durations are usually extremely brief, much shorter than the
shutter speeds. So when they go off, they only expose the portion of the
film currently uncovered by the shutter curtains, and what you get is a
narrow band on the film that's properly lit - the rest is underexposed.

So maximum sync speeds are actually the highest speed the camera can
manage where both curtains are open all the way at the same time - it
pretty much tells you how fast the curtains can move.

High Speed Sync flashes are made to go off several times in rapid
succession, carefully timed to illuminate each patch of film as that slit
travels. This is pretty hard to get around - it requires precision
communication between camera and flash.

However, this would not stop you from locking the shutter open for a
longer period and using the flash as the sole illumination for the image -
for instance, in a very dark room. How you might accomplish this is up to
you, but the method of flash metering has to be taken into consideration.

Hope this helps. Good luck!


- Al.

 

No. What HSS does is keep the flash on _longer_, so that it continues
to illuminate the subject while the "slit" between the front and rear
curtains travels across the sensor (digital or film).

If you were to use HSS to trigger a typical monolight, you'd end up
with a stripe (probably horizontal) that's fully exposed; the rest
of the shot would show varying degrees of underexposure.

What's the shooting situation where you envision requiring high
shutter speeds?

Different systems offer various options for off-camera HSS, but they
all require dedicated flash units. To be honest, I played with HSS
when I first got my F100 and haven't used it since. If a situation
requiring a high shutter speeds and mix of natural and off-camera
lighting arose tomorrow, I'd either rent a continuous fluorescent
light or use a medium format camera with a leaf shutter lens.

 

1) Studio strobes 'dump' all of the power in the caps when triggered. The
power level chosen sets the amount of cap charging. At low power settings, a
studio strobe dumps in about 1/10,000 to 1/1,000 of a second, at high power
settings, it might be as much as 2 ms. Leaf shutter lenses typically go up to
1/500 so it is a convenient fit. (Some leaf shutters go even faster). Needless
to say, the shutter needs to be fully open when the strobe fires.

The optical signal from the camera strobe to the slave travels about 1 foot per
nanosecond. So negligible. The trigger time from receiving the pulse to
closing the circuit at the slave is less than a microsecond. Negligible.

In short, to use the studio strobes, you must operate your camera at its sync
speed or slower regardless of the sync speed capability of the attachment flashes.

2) 1/4000 (1/8000) synch is really not sync'd at all. Often called High Speed
Sync (HSS), the flash is modulated to output a long series of tiny bursts while
the shutter 'slit' is in movement. The flash actually begins firing before the
first shutter begins moving and is still firing after the rear curtain gets to
the other side. During travel, tiny bursts of light are fired.
http://www.photozone.de/bindex3.html

This is highly inefficient as part (sometimes most) of the 'return' light is
falling on the shutter curtains not film, making a GN 56 m flash into a GN 3 m
flash at 1/8,000.

Again, you cannot use the HSS mode as fill if the shutter speed is above the
sync speed of the shutter if you are triggering studio lights as well.

3) Wireless HSS
On Minolta cameras and flashes, off camera HSS is available (Maxxum 7,
3600HS/5600HS flash).

In the end, HSS is not that useful that often.

Cheers,
Alan

 

I have often seen cameras bost rear curtain sync. What is this and how does
it differ from normal sync?

 

Normal sync: The exposure begins by the 'first' (front) curtain opening up
completely. The entire frame is exposed. At that point in time the flash
contact is fired and the strobe goes off. Some time later (on the order of 1 or
2 msec, or much longer if that is what the photog set), the rear curtain begines
to close.
Advantage: catches action very near the time the flash fired
Disadvantage: if motion is exposed by ambient light, the effect 'leads' the
motion and it looks unnatural.

Rear curtain sync: As above, except that the flash does not fire until the rear
curtain is about to travel. Therefore regardless of the duration of the
exposure, the flash comes at the end. (about 2 ms before the rear curtain begins
to cover the frame.
Advantage: motion streaks appear to trail the action.
Disadvantage: on longer exposures, hard to judge the desired "frozen" by flash
timing.

Cheers,
Alan.

 

so its not a technique for increasing the maximum flash sync speed. That is
actually the multi firing gun and nothing more.

 

Rear sync simply means the flash fires at the end of the full frame exposure,
not at the start of the full frame exposure. It is always at the sync speed or
slower.

By "multi firing gun" I assume you mean HSS, which is the continuous firing of
the flash as the slit crosses the frame (the frame is never full frame exposed
in this case. See: http://www.photozone.de/bindex3.html HSS is usually at
speeds above the sync speed.

Cheers,
Alan

 

my 300d flash sync speed is something like a 60th. there are claims with
550ex it is 250th. I suspect this is hss.

 

Maybe, just maybe, you could fix it up with a Strobex Polikon and a lot of
flash heads - but then you're talking a lot of money, a lot of effort, a lot
of weight and bulk, and probably a lot of experimentation before you got it
right. Certainly not a practical answer.

Otherwise the studio heads would fire once when the first burst from the HSS
head hit their slaves, and wouldn't be even close to being fully re-charged
in time to fire again, never mind catching every one of the bursts, which is
what they'd have to do to make this work.



Peter