I recently posted this review on Amazon. I cannot attest to the actual brightness of the lights, but they are both clearly brighter than my "500 lumen" lights, Over the course of a one-hour dive, I did not notice any appreciable dimming; it may well have happened, but not so much that it was noticeable.
My bottom line is they are both good lights, differing mainly in the switch location.
My bottom line is they are both good lights, differing mainly in the switch location.
Reviewed in the United States on March 28, 2023
I have a D530 and a D550. The only substantive difference is the on-off switch is a button on the side of the D530, but on the tail of the D550. I prefer the side-button of the D530 if the light is hanging rom a chest d-ring and just turned on occasionally in the daytime to look into a hole or under a ledge; and in prefer the tail-button if the light is "always on" as during a night dive, or if worn in a hand sock. Both lights will work in day or night, of course, and both last easily for a full dive if kept on. If I were only going to have one of these lights, it would be the D550, because the side button does not work well in a sock.
I am truly impressed with the build quality of both lights, the beam patterns (which are almost identical), and the brightness. I just returned from two weeks in Indonesia, and had both lights with me on all dives. No issues.
