I'm using the Di866 with a Nikon D300, I also own two sb600 flash units to compare with.Most concerning to me before I purchased it was its compatibility with the Nikon CLS system, but it turned out that is not a problem.I did most of my testing using my D300 as the Commander.I took one afternoon and well over 200 pics with the Di866.
Size and packaging:see the Youtube video someone has posted (link below).The video shows what's in the box and goes through the menus.
Power:As expected the power is much stronger then my sb600's.I took a picture of some trees at night that were across the street, the sb600 did very little to light up the trees, but the Di did a pretty decent job.
CLS:For test 1, I used two sb600s, commander mode, one in A group and other in B group, shot through umbrellas onto subject; I then replaced one sb600 with the Di866 and took the same shot.Comparably there was very little difference, the Di had just a little brighter result, but not overexposed.
Test 2, I used one sb600, Commander mode in group A (group B was disabled - this becomes an important note).Took shot with sb600 (on the Commander mode, had setting adjusted to -1.7), looked great, replaced with Di866 and it was underexposed.On the d300 Commander mode I had to adjust setting to +1 or +2 to get a good picture.Not sure why it did this so I started adjusting everything, flash position, zoom level, with/without shoot through umbrella, but then I noticed that when I added one sb600 back into the mix it always shot as normal.Then what was really odd was the underexposure problemseemed to be improved just by turning on group B in the Commander mode (with no flashes configured for it).With this turned on (but not used) I was able to reduce my Commander Mode Group A from +1or+2 down to 0.At first this made me wonder if there was something wrong with my camera, but this does not happen with my sb600 flashes.This sounds odd and it is very puzzling.Overall though, I was able to fully control the Di866.
I tested with TTL and M Commander modes both on and off camera and was fully able to control flash.
I used it as a Commander for my other two sb600's.It has groups A, B, and C available (along with M for the master flash).I was able to control the other flashes just as if I was able to do so with the D300 built-in Commander mode.One thing I thought was odd was that group A always had to be enabled, whereas M, B and C you could set it to -- to not use a remote flash set on that channel.Group A's choice was TTL or M but no --.It does not have an AA option like the D300 Commander mode does (but I understand that only works with the sb800 and sb900).
On camera use it worked great and as expected.I even tried it using the high speed sync and work as expected.
Recycle times were fast (using around the house for studio or normal pictures).When I forced the flash at full power (using manual mode), besides being blinded after this test, the recycle times were about 5 seconds (batteries used were Ni-MH and had already taken around 170 pictures.
Features of flash that I like:
It has a second flash!- if you notice in pictures of the flash it has a small white cap on the front, I thought it was a sensor, but it is a flash.You can enable this second flash when you set the main flash to bounce off a ceiling/wall/built in bounce card.The menu called it Sub.Flash and it was available during the TTL or manual settings.
The Di866 can also be use as a remote flash for any camera in manual mode using the flash detection feature.I tested it with a point-n-shoot camera and it fired each time.
In the Settings, there is an off-set correction, where you can set it from -3 to +3 offset which the manual said applied to all modes. I did not test this.
The flash head can be rotated 90 degrees to the left, 180 degrees to the right, and 90 degrees up.
It has a built-in flash diffuser and attached white bounce card.
You can purchase an optional battery pack, the Nikon SD-8A, SD-9, or the Nissin Power Pack Pro-300 (although I have not found the Nissin pack anywhere yet).
The display has a rotate feature to detect the direction the flash is being held and rotates the display.As I was testing out the flash and handholding it a lot, I found this feature to be an annoyance and turned it off.But it would rotate the display right or left, but not upside down.
A big disappointment was that it has no audible tone, I really missed this when using it as a remote flash.Especially when firing it near or at full power, I had to watch the green/red light on the back to ensure it was ready for the next picture (it never needed to fire at this full strength setting in my test environment unless I forced it to or using high speed sync at high speeds, so for most pictures it was always ready for me).
In terms of power, it is comparable to the SB900 although I don't have one to compare it to, in terms of price its comparable (slightly higher) to the SB-600.
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Nissin Di866 Speedlight for Nikon Digital SLR Cameras, Guide number 198Product Description: Nissin is a traditional Japanese flash
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So what is the ring flash? 
