What shutter speed are you using? It could be the curtain closing at the same time as the strobe firing. I think the max shutter speed you can use with strobes on a D90 is 1/200.
I just reviewed the parameters of the pictures and think you are right. The shutter speed of the posted image was 1/400. On other images I had 1/320, and the dark part was smaller (curtain not that much closed, makes sense to me). On images with shutter speeds 1/200 or longer, the problem does not seem to exist.
I´m impressed by your profound knowledge and speed of response. I had not been aware of this problem. Thanks a lot!
Where did you get the technical information (1/200 for a D90) from?
Unfortunately, in the menu of my camera you can only define a LONGEST shutter opening when using a strobe. According to this problem, you should also be able to define a SHORTEST opening, and the problem would not occur. But as I shoot on manual settings anyway, now that I am aware of this, problem should be avoidable.
Pictures (without technical problems) of my latest trip to Cocos Island can be seen here: http://www.flickr.co...57628567090599/