Chris T wrote:
Would that honor go to a Nikon unit, or a Canon unit? .... Or, perhaps, to a Sony, or to a Pentax unit? ... Or would these all be bested by a Third Party unit, in fact?
What do you mean by "on-camera flash unit"?
If you mean the "built in" flash that many cameras have.... well those generally suck. They're quite weak, in the worst possible place for redeye and ugly shadow problems, plus they draw heavily on the camera's power source.
If you mean "accessory flash" that typically connect via a hot shoe on the camera (can be mounted in that shoe... or might be used off-camera via a cord or wirelessly).... those vary in power but all are rated by Guide Number (GN), usually at ISO 100. Some use a "telephoto" setting to determine guide number, done by a Fresnel lens in front of the flash tube that adjusts to better accommodate different lens focal lengths (such as 24mm, 35mm, 50mm, 85mm, 105mm). So long as ISO and lens focal length are the same, you CAN compare across various brands even though they're operations are dedicated to different, specific camera systems.
Guide Numbers are stated in feet or meters. It's basically the maximum distance the flash can reach (at ISO 100).
I don't know how other brands name their flashes, but most Canon Speedlite model names give an immediate clue of the flash's power (in meters):
For example, top of the line Canon 600EX has a GN of 60 meters (197 feet).
The mid-size Canon 430EX has approx. GN 47 (141 feet).
Among the smaller current Canon flash are the 270EX series (GN 27 meters/88 feet).
Even the tiniest Canon Speedlite, the 90EX, follows this naming scheme. That has a GN of 9 meters/30 feet.
There have been other Canon Speedlite in the past including 580EX, 550EX, 420EX, 380EX and 320EX. They're all similarly named.
Often third party flash for Canon use the same or very similar model names, giving similar clue to the flash's power. But even if they don't there's almost always a specification somewhere stating the GN.
For sake of comparison, the flashes built into cameras typically only have a GN of about 4.5 meters/15 feet... maybe 6 meters/20 feet or a little further at best. And, even in the hotshoe accessory Speedlites are positioned farther from the lens axis than built-ins, making for less redeye and better shadow effects. But it's also possible to put them higher up and/or off to the side on a flash bracket, attached to the camera via an off-camera shoe cord - or even off-camera entirely and control them wirelessly - to further manage and improve lighting effects. Many also have heads that tilt or rotate to bounce the light and there are many accessory light modifying devices available for them. They also have their own power source, instead of putting a heavy drain on the camera's power source. This usually makes for much faster recycling than built-ins, too.