I received my Fisheye 500DX from Reef Photo today. It's bigger than I remember the HG20DX being. A good bit bigger. It's almost like having a third strobe, in terms of bulk.
It seemed pretty nice, and I didn't have the problem HarryM did with his copy - fairly clearly he got a defective light. It didn't seem quite as bright as I was expecting, so being nerdy, I decided to test. I did my best to replicate the test conditions described in the Backscatter Focus Light Shootout
. I shot a white surface at maximum output at a distance of 12" and noted the metering at 100 ISO. I assume the article images were shot at 100 ISO - the photos are missing their EXIF data, so I can't say for certain, and the article doesn't say.
As you can see, beam width is very similar. The article specifies a white balance of 3200K, which makes the 500DX's beam appear very blue, even though it's white to the eye. I make the beam at about 20" in diameter at 12", where the article says the HG20DX is 18". However, the 500DX has a very definite 5" diameter hot spot, where the HG20DX's beam is very uniform.
I'm less certain of the metering. I did try and duplicate the article, but even a slight difference in distance from the camera will make a definite difference in exposure. I measured 1/60th at f/10 and 100 ISO. 2 stops wider implies the 500DX only puts out 1/4 the light of the HG20DX, despite drawing about half the power. I can see why the 1000DX exists, though using 8 AA's instead of an expensive proprietary battery seems like a real advantage. I can easily stock a spare set of NiMH AA's for a lot less money than the $150 price of an extra lithium battery.
I also tried the light at minimum power, and metered at f/4 and 400 ISO. That's 4 stops, or about 6% of maximum brightness. Fisheye claims 150 minutes with 2000mAh batteries at full power, so 40 hours at minimum power? I suspect less, but I'm not really interested in testing that.
What matters in practice? I've only done a few dives with a real focus light, instead of relying on my dive buddy illuminating the target with her primary dive light. Beam width definitely matters, since you can't count on the light being precisely aimed at your target at macro distances, and your subject may not be center-frame at longer distances. The light doesn't have to be strobe-bright, it's not supposed
to be a source of light in your photo, just enough for autofocus to work reliably.
At macro distances, beam brightness is almost a disadvantage, since it might add a hotspot to your photo. For longer distances, brightness matters, but a bluer LED beam almost certainly penetrates water better than a ~3600K incandescent light like the HG20DX. I think a really fair comparison would be underwater, and distances of 3' or so.