I used it at a wedding reception in a pretty dark restaurant and even at 3/4 brightness it was more than enough light on subjects that were within 10' or so. I was using a pretty fast lens with a Canon T2i (which isn't a full frame sensor) and it was fine. Hope that helps.
The unit did not come with instructions so I had to figure out how the F550 battery fit to begin with, trial and error. This batter requires that the outer back cover on the light be removed. The battery does not require the adapter that is included with the light. The charged batter has slots that need to be engaged with the "teeth" on the light. The battery should be inserted on the right hand side of the battery compartment and then slid to the left. Two pins on the light need to slide into the holes on the left side of the battery. Once the battery is firmly in place with the teeth engaged and the pins inserted, the back cover should be snapped into place on the light. It works great!
I really don't know but I suspect it is for a certain battery that you would purchase separately. I had the same problem trying to figure that out, but came to conclusion I didn't need it anyway. I had ordered the NP-F550 battery or I use AA batteries. I agree very cheap.
IT will fit the hot shoe are you looking for a adjustable light brightness control???
I love mine i use it as a fill fight and studio strobes i made a tube with a foil center and blocked off the rest for the ring light effect
I have 4 will buy more when i need them