Help me explain what happened here?

anonymouscuban

OK... so this is a shot I took a couple weeks ago. Not bad huh? Well, I'm scratching my head trying to figure how the heck I was able to sync my AB800 flash at 1/640s?

The shot was total dumb luck as far as the flash sync goes. We had just come upon this spot. I wanted to get some shots with sun flare so I setup real quick, metering to get the background exposed. Got it and then metered the flash without really considering that my shutter was set faster than 1/250. Took the shot, again, not considering that I'd exceeded the flash sync speed and loved the shot. It wasn't till I got home that I realized the sync speed issue.

:scratch

divamum

I don't know the answer, but just typed "ab800 high speed flash sync" into google and got a bunch of hits of varying kinds - only scanned the articles (since I have neither an AB strobe nor a Nikon system) but seems there's some chatter about it around the interwebz....

Foques

what do you use to trigger your AB?

anonymouscuban

I use Pixel King wireless radio triggers.

anonymouscuban

Found this on the interwebs which may explain what happened:

What PW does, and Pixel claims to do, is trigger the light a little early. Pocket wizard calls this HyperSync.

The sucess of hypersync and similar triggering methods is dependant on the flash duration of the flash and the camera you are usng (and of course the remotes).

Gary752

From looking at the photo, the background was brighter than the subject and you set the exposure for the background, and set your strobe to supply fill, so the shutter speed didn't matter what it was at. At least that is what was explained in the workshop on Creative Live. That is if you are shooting everything in full manual, and not TTL.

GaryB

anonymouscuban

Gary752 wrote: »

From looking at the photo, the background was brighter than the subject and you set the exposure for the background, and set your strobe to supply fill, so the shutter speed didn't matter what it was at. At least that is what was explained in the workshop on Creative Live. That is if you are shooting everything in full manual, and not TTL.

GaryB

Gary,

Everything you say is correct EXCEPT for shutter not mattering. Rather than trying to explain this myself, here is a copy/paste job from the net:

A modern SLR camera has a dual curtain that usually slides from top to bottom. By dual curtain, I mean, one curtain opens sliding from top to bottom and a second second curtain closes behind it also from top to bottom. It is important that the curtains open and close in the same direction so that the frame will be exposed evenly.

Lets say you are taking a picture at 1/30 of a second. The first curtain would open all the way, the flash fires, light hits the sensor, and then 1/30 of a second later the second curtain closes. Keep in mind that the duration of the flash is only about 1/20000th of a second. As long as it fires within the time frame that the first curtain is completely open, and before the second curtain starts to close, life is good.

That's actually a little bit of a simplification. You have to remember that it takes time for the curtain to travel from the top to the bottom of the frame. So the camera has to take that into account when deciding when to start closing the second curtain.

On a Pentax, it takes 1/180th of a second for the curtain to move from one top the frame to the bottom. The only way that the camera can achieve a shutter speed faster than 1/180th is for the second shutter to start closing before the first shutter is even all the way open.

I guess the best way to think of it is, at those higher shutter speeds, the curtain becomes a "slit" that travels across the frame. The higher the shutter speed, the narrower the slit.

Now - How does this relate to sync speed. A cameras "normal maximum sync speed" is the maximum shutter speed in which the camera has the first curtain fully open before the second curtains starts to close. Or in other words, the maximum speed that the camera can operate without going into "slit mode". If you try to take a normal flash photo while the camera is operating in "slit mode". You will actually capture a photograph of the shutter itself. Probably not what you wanted.

The only way that a camera can operate in the "high speed flash" mode. Is to be able to fire the flash multiple times as the slit travels across the frame. That takes quite bit of electronic magic and requires a much more expensive flash. Those flashes have to be in constant communication with the camera during the actual exposure. It's amazing to me that they can even do that!!

Another way to achieve HIGHSPEED sync is to do it totally electronically without involving the mechanical shutter at all.

So there you have it. How a camera works 101. Hope it helped.

jpc

Here's my take... Since the flash only contributed a small part to the exposure, I think you probably are seeing effect from the curtain being partially closed when it fired, but not enough to be as noticeable as if this had been a studio shot where the flash was ALL of your light. In that case, you end up with a dark bar across the bottom of the frame, as I'm sure you're aware.

As far as I know, Auto FP or high-speed sync is the only way to compensate for a shutter speed greater than 1/250, since it sends a steady stream of low-power flashes that hit the subject while the curtains are traveling across the sensor. I don't know of any way to make that happen with a studio flash like the AB, but it sure would be great. PW and The Odin support High-speed sync, but only with a camera / flash setup that also supports it.

Again, if anyone knows a way to make that work with AB's, I'm all ears.

anonymouscuban

jpc wrote: »

Here's my take... Since the flash only contributed a small part to the exposure, I think you probably are seeing effect from the curtain being partially closed when it fired, but not enough to be as noticeable as if this had been a studio shot where the flash was ALL of your light. In that case, you end up with a dark bar across the bottom of the frame, as I'm sure you're aware.

As far as I know, Auto FP or high-speed sync is the only way to compensate for a shutter speed greater than 1/250, since it sends a steady stream of low-power flashes that hit the subject while the curtains are traveling across the sensor. I don't know of any way to make that happen with a studio flash like the AB, but it sure would be great. PW and The Odin support High-speed sync, but only with a camera / flash setup that also supports it.

Again, if anyone knows a way to make that work with AB's, I'm all ears.

Yeah JPC... that I am aware, there isn't any studio strobes that can do high speed sync and is why I asked the question.

I've been thinking about it a bit and I think your explanation is valid. Also take into consideration that the Octobox was also serving as a 4 foot reflector. I think the combo of ambient and reflected light, in addition to any strobe light that was captured, helped to mask the curtain drag.

ziggy53

anonymouscuban wrote: »

Found this on the interwebs which may explain what happened:

What PW does, and Pixel claims to do, is trigger the light a little early. Pocket wizard calls this HyperSync.

The sucess of hypersync and similar triggering methods is dependant on the flash duration of the flash and the camera you are usng (and of course the remotes).

This is almost certainly the answer. Your Alien Bee AB800 has a rather long flash duration when it's not at full power. By triggering at the right time the flash duration can overlap the shutter when it's open.

HSS/FP flash stretches it's duration by rapidly "strobing" at reduced power through the shutter travel, unlike your AB800 case where your flash just has a long "tail" behind the primary pulse.

BTW, shutter travel is a constant rate on focal-plane shutters. The shutter travels just as fast at a "fast" shutter speed setting as it does at a slower shutter speed setting. The "effective" shutter speed is derived from the width between front and rear shutter components.

More information here:

http://en.wikipedia.org/wiki/Focal-plane_shutter

http://webs.lanset.com/rcochran/flash/hss.html

jpc

That would mean that the flash duration of the B800 would have to be longer than 1/640, right?

Even at 1/32 power (which I'm sure is far less than he used), the AB800 is 1/1650 t.5 and 1/550 at t.1.

The t.1 time is longer than 1/640, but that's all the way to the end of the pulse. Now factor in the additional time to fire the flash early and I think you're right back to uneven flash across the frame that just isn't that noticeable.

Dreadnote

For what it's worth, I can sync my Einsteins all the way to 1/8000 if they are at full power, and that is without playing with the PW Hypersync settings but just leaving it on auto. If your triggers behave similarly then that would easily be reproducible. Do you know the power setting that your AB was set to?

anonymouscuban

Dreadnote wrote: »

For what it's worth, I can sync my Einsteins all the way to 1/8000 if they are at full power, and that is without playing with the PW Hypersync settings but just leaving it on auto. If your triggers behave similarly then that would easily be reproducible. Do you know the power setting that your AB was set to?

Honestly, I don't remember what the flash power was set at. I can guess but that won't help.

Dreadnote

anonymouscuban wrote: »

Honestly, I don't remember what the flash power was set at. I can guess but that won't help.

Well all I can say is fire off a few shots at above sync shutter speeds and see what you get. I am willing to bet that you get better than 250th using wireless triggers. Also, is the timing on that brand of trigger adjustable like it is on a PW. If so you can dial it in to work with different sync speeds. The longer the flash duration the more options you have. PW has a bit on info (mostly a commercial) with Tyler Stableford on the idea here:

http://www.pocketwizard.com/inspirations/profiles/tyler_stableford/

However determining if any of it is applicable to to your system will no doubt require some playing with them.

Dreadnote

Also seems that the Pixel King website says that the product will support studio strobe sync up to 1/8000th using the PC port. (though it appears to be translated from Mandarin or Korean )

ziggy53

To understand how hypersync works, these links should help:

http://wiki.pocketwizard.com/index.php?title=Understanding_HyperSync_and_High_Speed_Sync

http://wiki.pocketwizard.com/index.php?title=HyperSync

http://www.paulcbuff-techforum.com/viewtopic.php?f=8&t=1580

Bryce Wilson

Dreadnote wrote: »

Also seems that the Pixel King website says that the product will support studio strobe sync up to 1/8000th using the PC port. (though it appears to be translated from Mandarin or Korean )

WARNING...

'fore ya go out and get these because of the high speed sync abilities, be advised that TTL exposure compensation doesn't work with the camera set to manual. Kinda ruins the whole thing for me.

divamum

depends on the flash settings and flash model Bryce, not the. camera mode, at least on my Canon gear (I also use. PKs although with speeedlghts). I use with manual exposure + ettl and flash ex comp works great. I can also dial it in from the camera in M flash mode on my 430e, ii. but only the ii series; doesn't work with earlier models.

biggest drawback to them is no ratio groups for more than one flash, but other than that they're great.