sync speed help

I am now using a Canon EOS-1 Ds Mark II in a Seacam housing. I was using two ds125s (I flooded one and am now contemplating switching strobes). In trying to achieve the sun balls or freeze sun rays, I have noticed that to come close I need to push my shutter speed higher then the max sync speed of 250.

I am curious of the relation ship between published sync speed and what a camera is able to do. Canon tech support is of no help. Canon does have ability to high speed sync with its own flash.

I decided to run some test tonight in my living room with the housing and one ds125. Taking pictures of a plant I noticed that pushing to 320 will give a decent result—some loss of light. As I push the shutter speed up past that I get the bottom of the picture cut off while the top half seemed to have decent exposure, but the same loss of light. The faster I push the speed the more the picture is cut offed. It is as if a card is placed in front of the lens. I then tried to cut the power down on the strobe. The only effect was a poor exposure in the top half of the shot. What is causing the blocked light effect?

I then hooked up my Canon flash and put it in high speed mode. As I pushed the shutter speed up, my picture over all got darker but there was no picture cut off. I could naturally compensate for the darkness by opening the aperture.

So what is my limiting factor here? If it is the dump time of the strobe wouldn’t I have poor exposure throughout the picture not just a cut off bottom? And if the camera can high speed sync with a Canon without cutting off the picture why can’t it do it with an external strobe?

Any suggestions on tricking the camera into letting me high speed sync? Has anyone tried to use a Canon flash u/w and with what results?

Finally I need to open the can of worms, which strobe might be able to dump quicker if in fact that is the issue?

Thanks, mark

Ikelite will repair or replace the strobe if you send it to them. The big circular flash tube of a DS-125 is a limiting factor in flash duration. Full flash requires 1/250 - 1/300th of a second.............

The limiting factor is the shutter on the camera. When the shutter speed is less than the sync speed, (1/250th in your case) the shutter is never completly open.

For example at 1/4000th shutter opens a tiny slit and then moves that slit across the frame. This way each part of the frame is exposed for only 1/4000th of a second. Though each part of the frame is exposed a different 1/4000th of a second.

At slower speeds the slit becomes larger, but moves at the same speed. Thus each part of the frame is exposed longer.

At speeds slower than the sync speed, there is a point in which the entire frame is exposed.

So when the DS-125 fires when the camera is set higher than the sync speed, there is no way the strobe can sync with the camera because there is no time inwich the entire frame is open so when the strobe fires, at least part of the frame will be covered by the shutter.

The faster the shutter, the smaller the slit, more of the frame is covered by the shutter, thus you see a larger dark band.

Canon's flash, with high speed sync, when set to high speed sync more or less emits a constant beem of light long enough for the slit to move all the way across the frame.

So, moral of the story, there is nothing you can do to make your DS-125 or any underwater strobe do high sync faster than 1/250 on your camera, it is a limitation of your camera, not the strobe.

I guess there is a chance that Ike could figure a way to support high speed sync in a camera specific controler like he does for eTTL/iTTL but that wouldnt help you any.

Cameras which do not have a shutter, such as the 1D I and the D70 can sync as fast as you want because they have no shutter so the entire frame is exposed all at once.

I agree that it's a limitation of the camera and not the strobe.

There are many conditions that contribute to capturing sun rays besides strobe sync speed......time of day.....depth.....water conditions, etc.

Here's an example at 1/100 at f6.3

You'll be hard pressed to find a better strobe than the DS-125 and I've got several examples of shots at shutter speeds of 1/500 and higher....the D70 can handle that speed because of an electronic shutter and it shows the capability of the DS-125....

1/500 at f6.3

Karl

William stated that “Canon's flash, with high speed sync, when set to high speed sync more or less emits a constant beem of light long enough for the slit to move all the way across the frame.â€

There is something here that I am just not getting. The duration of the Canon flash being longer for a shorter shutter speed doesn’t seem to make sense to me. If the strobe (ds125) takes 250th of second to dump, and the shutter is only taking 500th of second to move across the plan, then the strobe is firing throughout the duration of the curtain moving and then some. I would think that just the amount of overall light is cut down hitting the sensor. Unless the real problem is the shutter starts moving before the flash fires. Then I could see some of the frame getting clipped off.

Now if this is the case, how does Canon achieve high speed sync? Do they delay the shutter opening? Are all flash’s created equally in terms of how fast they can start to fire.

Thanks, mark

The way canon does very high speed synch, with their own strobes, is to pulse the flash as the shutter slit is moving, so as to try to get full coverage, without overlap. So it does not work by actually getting the shutter fully open, or by leaving the strobe on the whole time.

This takes sub microsecond coordination, to come out right.

Correct, the Canon flash in high speed mode fires a series of pulses as the shutter curtain slit moves across the frame.

Of all the underwater strobes, the ones that can discharge and recycle the fastest have straight short flash tubes. Unfortunately, these flashes don't have the great even round coverage of a circular tube.

I think from the data we've seen here, the strobe that discharges its energy the fastest is the Inon D2000. Unfortunately, that one is an optical slave only - no sync cord.

HTH

James

William stated that “Canon's flash, with high speed sync, when set to high speed sync more or less emits a constant beem of light long enough for the slit to move all the way across the frame.â€

 

There is something here that I am just not getting.  The duration of the Canon flash being longer for a shorter shutter speed doesn’t seem to make sense to me. If the strobe (ds125) takes 250th of second to dump, and the shutter is only taking 500th of second to move across the plan, then the strobe is firing throughout the duration of the curtain moving and then some.  I would think that just the amount of overall light is cut down hitting the sensor.  Unless the real problem is the shutter starts moving before the flash fires.  Then I could see some of the frame getting clipped off. 

 

Now if this is the case, how does Canon achieve high speed sync?  Do they delay the shutter opening?  Are all flash’s created equally in terms of how fast they can start to fire.

 

Thanks, mark

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Yes, I can see what you are saying, but the problem is that even though the DS-125 takes 1/250th for a full dump which would be plenty long to acchieve high speed sync if the output was constant. But stobes do not emit a the same ammount of light for the entire 1/250th of a second.

Look at this

http://wetpixel.com/i.php/full/inon-d-2000...al-explanation/

It shows graphs which show the output of various strobes vs time. It doesnt have the DS-125 but you can see the trend that they all follow. Even if it takes 1/250th to dump all of its light, it dumps most of the light within a fraction of that time, thus by the time the bottom of the frame is exposed the light being put out by the strobe is minimal.

Where canon's flash by making tiny pulses acchieves constant illumination for the entire period.