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Bottom of image underexposed when using TTL


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#1 diverdoug1

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Posted 03 February 2014 - 08:51 AM

I am using a D800 and dual or triple opticaly triggered YS-D1 strobes.  I have been using this set-up successfully using manual strobe seting.  I thought I would play with TTL settings and I noticed that when using TTL, for macro, a strip on the bottom of the image, comprising about 10% of the total image appears to have no flash.  When setting the strobes to TTL I did remember to switch both my camera on-board flash and the strobes to the TTL setting, and batteries are fully charged.   When I switch both back to manual, the issue resolves.  Also, if I leave my on-board camera flash on TTL, and set the strobes to manual with a pre-flash, there is no issue.  I tried slowing my shutter speed from 1/200 to 1/160, but the bottom is still underexposed.    I usually don't use TTL , but am still surious what could be causing this issue.


Edited by diverdoug1, 03 February 2014 - 08:56 AM.


#2 marc1990

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Posted 03 February 2014 - 08:59 AM

You camera may be set to rear flash curtain(second curtain).


Edited by marc1990, 03 February 2014 - 09:01 AM.

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#3 diverdoug1

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Posted 03 February 2014 - 09:07 AM

You camera may be set to rear flash curtain(second curtain).

Nope, but that was a good thought.



#4 BottomTime

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Posted 06 February 2014 - 02:27 PM

I've heard of people having similar problems using wireless triggers.

 

When Heinrich Weinkamp was still making TTL converters, he used to provide a (fairly comprehensive) table of strobe timings that listed the minimum time required between a pre-flash and the main flash. Inon strobes were generally in the 2000 microsecond range while my memory was that the Sea & Sea strobes were considerably longer (I think the YS-110a was ~18000 microseconds).

 

HW provided a second list with the minimum timing requirement of the camera. The timing for the nikon's are faster than most and the D300 was around 8000 microseconds. You're D800 will be atleast as fast as that and will do it's thing without regard for strobes that can't keep up. I suspect that in your case, the rear curtain has started to close before the minimum timing for the strobe has elapsed (10000 microseconds is 1/100s).

 

In the case of using manual powers, then the strobe ignores the preflash and the minimum timing between the pre-flash and the main flash is no longer relevant.

 

This is all speculation freshly squeezed from my butt cheeks though. An Oscilloscope would probably be able to tell you exactly what is going on.


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#5 diverdoug1

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Posted 07 February 2014 - 12:24 AM

So it sounds like it might not be possible to use TTL on the D1 when triggering with fiber optic cables using the D800.  Not a tragedy, but might be good to put out there for potential customers that want TTL capability.



#6 Cerianthus

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Posted 07 February 2014 - 01:41 AM

Dont all slr shutters curtains move sideways rather then horizontal?
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#7 jlyle

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Posted 07 February 2014 - 07:23 AM

With fiber optic cables you may not be able to take advantage of the fastest sync speed on your camera.  If you slow the shutter speed to just under the sync speed, you can use TTL without the problem you have described.  Wired strobes do not have this problem.


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#8 BottomTime

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Posted 07 February 2014 - 09:13 AM

If I'm right, then you should still be able to use TTL, but at lower sync speeds as jlyle said. I would try a TTL shot at 1s. If your still seeing the bar at the bottom of the image then there is something wrong and my theory is bunk. It'll take a little experimentation to figure out what your max sync speed is with FO TTL.

This is something that will vary from camera to camera. My nikon D70 had a preflash/main flash delay that was so long it allowed my wife to blink with 100% accuracy when I used a speed light in TTL mode. I'm guessing that your YS-D1's would have worked with the D70 just fine right up to it's maximum sync speed of 1/500s.

 

 

Dont all slr shutters curtains move sideways rather then horizontal?

I don't know what is most common, but Nikon dSLR's uses focal plane shutters that travel vertically and descend to expose and cover the image sensor. There are FP shutters that travel horizontally but I don't know if any are currently being used in a modern digital camera.


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#9 diverdoug1

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Posted 07 February 2014 - 03:34 PM

As I said originaly, the problem occurs even at 1/160



#10 MikeVeitch

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Posted 08 February 2014 - 03:36 AM

Can you post an example photo?


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#11 Cerianthus

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Posted 08 February 2014 - 04:11 AM

I stand corrected. Looks like horizontal is normally used.
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#12 diverdoug1

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Posted 08 February 2014 - 03:41 PM

I deleted images already, but I will save one from my next dive.



#13 bvanant

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Posted 11 February 2014 - 09:10 AM

I am pretty sure that most modern focal plane shutters are vertical rather than horizontal since the distance to travel is typically 25% or more shorter. To Diverdoug, do you see the same problem on land, or with a Nikon strobe?

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#14 diverdoug1

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Posted 12 February 2014 - 11:05 AM

No, using the Nikon flash above water does not display this problem.



#15 Chogless

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Posted 25 April 2014 - 05:42 AM

Have you tried camera TTL and strobes in either 1 or 2 or TTL modes?  Do you get the same light pattern



#16 Stuart Keasley

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Posted 25 April 2014 - 07:20 AM

This will be down to sync speeds, you're flash is catching the curtain rising at the end of the image (the underexposed bit is the curtain casting a shadow)

What shutter speed were you using?

Edit : just seen, you've tried down to 1/160th... Keep pushing down to 1/120th, 1/100th etc to see where the problem disappears.

Pure guess work, but in TTL mode the strobes need to wait for the flash trigger from the inboard flash to complete in order to know the flash duration time, so sync time will increase over manual?

My personal view, TTL is a waste of time, you just end up chasing your tail trying to get the effect you want.

Edited by Stuart Keasley, 25 April 2014 - 07:36 AM.

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