triggering Inon z240 with Nauticam NA-D500 optical trigger

Hi 

i have as technical question concerning NA-D500 and D500 triggering Inon z240 flashes. I just bought a second hand NA-D500 housing for my Nikon D500
I realize I I want to use the fast sync mode (250fp*) I have a shadow on the lower half of the image from the curtain. I used the z240 with D7000 and D800 without problems using 1/320 SHUTTER SPEED. Now I use the optical led trigger system without the optional TTL converter. The camera triggers the flash so far no problem. But somehow the camera does not realize that a flash is connected and I cannot use the fastest sync mode. so the shortest shutter I can use is 1/250. If I use 1/320 (250fp*) the curtain is making a shadow in the image. Also when using the internal flash in D7000 for example. the shutter speed is automatically limited to 1/320. Now I can accidentally put on a faster shutter speed as the camera does not notice I use a flash. Is there any way to make the camera realize a flash is used and thereby use 250FP*? Also how can I assure that  the internal output of the flash trigger of the cameras at a minimum in order to save battery? Thanxx ++ in advance for suggestions

Hello.  I too use a Nauticam and D500.  The D500 as far as I can tell does not have a control for the flash trigger light intensity.  This is because the D500 does not have a built-in flash. 

The shutter speed with an external flash should be no faster than 1/250, set on e1 - Custom Settings Menu, and e5 Modelling Flash off to save current. 

As it is easy to change the shutter speed accidentally by turning the rear control knob, it is a good idea to keep an eye on the shutter speed as shown in the viewfinder, or on the camera's settings by pressing Info from time to time. 

Both cameras have 1/250 sync speed
The d7200 had an internal flash with auto fps mode this increases the speed to 1/320
The d500 has no internal flash so it stops at 1/250
If you see a black line it means the camera is capturing the rear shutter window

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Z240 performs just a mono flash, it cannot produce FP flicker 40 khz.  Camera shutter window must be fully open at the moment of mono flash, to get uniform exposure along the shot. Nikon mechanical shutter fully opens window maximum at 1/250, not faster. That is why 1/250 is official "sync speed". If user sets 1/320 ... etc., the window never fully opens, it looks like an open strip going along the shot (rear curtain becomes visible on the picture), and it needs FP flicker long flash then.

So, shooting by Nikon with underwater strobes, user must set only up to 1/250 speeds, to get uniformly lighted picture. Yes, 1/320 is threshold value, dark strip is slightly visible (or almost invisible) on some Nikons, but anyway, it is not recommended for common case.