The relative power level of each flash at different zoom levels is relatively consistent: the 50mm chart above is so close to the ones for 35mm, 70mm, and 85mm that there's no point in graphing them separately. The only exceptions are at the narrowest zoom position as detailed above, and at the widest setting.
Most of the fixed-reflector flashes are set at a 28mm coverage angle (35mm equivalent), and can't go wider than that without a diffuser, while all of the zoom flashes except the Metz 36 AF-4/AF-5 go as wide as 24mm. At higher focal lengths, the inability to produce a more focused beam severely penalizes the fixed-reflector packages. This is particularly unfair to the tiltable-reflector Promaster flashes, because it is common to use a non-zoomed setting when using bounced light — so this value may be your typical use case.
This chart shows the 28mm fixed-reflector flashes, normalized to the Pentax AF360FGZ set to 28mm.
The Metz 24 AF-1 isn't included, because it's an anomaly, with a fixed 35mm coverage area. I estimate that it's "true" power is roughly equal to the Pentax AF200FG; it's just slightly more focused. The Tumax DSL28 series doesn't have a documented coverage area, but I assume that it is similar.
- Promaster 7200EDF = 1.9×
- Promaster 5550DX = 1.9×
- Promaster 5250DX = 1.2×
- Pentax AF200FG = 0.83×
- K20D/K200D/K-7/K-5 built-in = 0.35×
- K100D/K10D/K2000/K-m/K-x/K-r built-in = 0.25×
Note the oddity with the Promaster figures when compared against the Promaster zoom-head flashes — the 7200EDF does not zoom but Promaster claims a GN of 100' at its fixed 28mm coverage, the same as the 7400EDF zoomed to 50mm. Promaster confirms by e-mail that the 7200EDF does indeed have a more powerful strobe to compensate for its lack of zoom. In fact, at 28mm it is more powerful than all but the most expensive offerings from other companies.
And, not quite related but perhaps important: if wide-angle light is valuable to you, the Metz 58 AF-2 has a built-in wide-angle panel which Metz says diffuses light to cover the field of view of an 8mm lens on APS-C. No other flash in this guide comes close.