Dark bands with Z6 and YS-D1 TTL

Hi! Long time no see and so forth.

I've been doing my first dives with my new rig. Relevant for my question: Nikon Z6, Sea&Sea housing, S&S TTL trigger, 2 x YS-D1 strobes. Mostly it went fine but I do have an intermittent problem, showcased by the two attached images. A shadow region in the bottom of the image with strobes firing. 

What it is not:
It is not occlusion of the strobes by the rig.
It is not, as I first thought, too strong focusing light partly occluded by rig.
It happens with more than one lens.
It is not completely black, rather very underexposed compared to the rest of the image.

It does vary with shutter speed. The broad band occurs at 1/200 s shutter speed, which happens to be the max sync of the camera. The thinner band occurs at 1/160 s. This is not 100 % consistent, I have one 1/200 s picture with a thinner band. But when the dark bands occur they are always at the bottom of the image and almost always with these sizes for the respective shutter speeds.

My hypothesis is that somewhere in the strobe firing chain (camera - TTL trigger - strobes) something misses the timing, so the the physical shutter is blocking part of the image when the strobes are fired. Since both strobes appear to be affected and I've never seen this on land I'm tempted to blame the TTL trigger. I have the camera set to rear curtain sync. It seems the strobes fire a bit early.

Note: the problem is intermittent. 

Have you encountered this before? Do you think my hypothesis is likely?

Looks like a synch problem. Try using longer than 1/200 shutter speeds. Like 1/100 or 1/60.

Have a look at the fine print in your instruction manual.  I have a Sony A7RIV which has an advertised maximum sync speed of 1/250 but I experienced the same problem as you when shooting with it underwater.  After a closer reading of the manual for my camera it turns out that the sync speed of 1/250 is only possible with dedicated Sony flashes or flashes that use Sony TTL (Godox TT685S TTL Flash for Sony in my case).  Other flashes/strobes will only sync up to 1/200.  I have no problems with my Godox flash at 1/250 but can only sync at 1/200 with other brand flashes on land and my Nauticam Flash Trigger underwater.

Just found this DPReview, it seems that there are similar issues with using non-nikon strobes and the sync speed as I have described for my Sony:

https://www.dpreview.com/forums/post/62076439

Thanks.

It may also be the rear curtain sync which is the problem. I checked the reference manual and it mentions that rear curtain sync will not work with studio strobes. I'm guessing my underwater strobes may be considered studio strobes for this purpose. So next time I'll start with changing that setting and see if it helps. 

If it does that will be fine. In the underwater shooting I'm doing I don't really see the need for rear curtain sync anyway.

Trying without rear curtain sync is a good first step.  Ty it out and report back if it continues I guess.

On 7/18/2020 at 8:12 AM, Gudge said:

Have a look at the fine print in your instruction manual.  I have a Sony A7RIV which has an advertised maximum sync speed of 1/250 but I experienced the same problem as you when shooting with it underwater.  After a closer reading of the manual for my camera it turns out that the sync speed of 1/250 is only possible with dedicated Sony flashes or flashes that use Sony TTL (Godox TT685S TTL Flash for Sony in my case).  Other flashes/strobes will only sync up to 1/200.  I have no problems with my Godox flash at 1/250 but can only sync at 1/200 with other brand flashes on land and my Nauticam Flash Trigger underwater.

Strange thing has happened!  I was just testing my camera with my wife's strobes (Inon S2000) and it synced perfectly at 1/250.  I then went and tested it with one of my Retra flashes (old original model) and now it syncs perfectly at 1/250.

I think I'll have to take back my comments about the A7RIV's sync speed back.