I have got the SG-3ir
but .....
that diffusor plate built in on SB-910 -- is that eqviv to the soft dome?
Not really: The wide plate is just for wide angle lenses. It thows the light at a wider angle but doesn't do much diffusion.
The dome is semi opal so it does diffuse. It's small so it doesn't have much effect past 1 to 1.5 meters unless it's used in a small room with white walls then it acts as a bare bulb. At 1:2 to 1:1 the dome makes a large softbox. This is about the size of the softbox to the size of the subject.
I find the dome for my SB-800 gives a hazy bright sun effect at about 1 meter but it's still directional. It's not like when a hazy bright condition where the light come from everywhere.
Many people say the dome is useless and it just waste power. This is true at distance. This is because of the small size. When you get far enough away the dome is small enough to act as a near point source. It's amusing to see PJs use the dome with a 70-200/2.8 zoom. At these distances the dome is wasting around 2 stops of light.
I use the soft pouch that came with my SB-800 and keep the dome on for storage. For use I generally pop the dome off and put it in the soft case if the subject is more than 1.5 meters away. I use the stand frequently to bounce the speedlight off white ceilings. I place the SB-800 where I can and aim it where it will cause the light to fall from the direction I want. I generally don't use the dome when bouncing as that doesn't give directional light.
I hope this makes sense, I just got up.
Dave Hartman
There is one "Got ya!" when setting up my cameras as commander. You have to press the OK button to exit the menu or the changes are ignored. I have a D800 and D300s. I set any group I'm not using, group B and Master to "--" or off so the commander doesn't send pre-flashes to groups that aren't there.