Most models allow one to turn off P-TTL mode and simply control the flash manually. This is done by setting the flash to output at a certain fraction of its full power, normally ¹⁄₁, ½, ¼, ⅛, etc.
Metz is the clear winner here, with the top models having 25 settings all the way down to ¹⁄₂₅₆ power in third-stop increments, and even the 50 AF-1 has eight steps down to ¹⁄₁₂₈. The 44 AF-1 gets an honorable mention with four choices.
The top-of-the-line Pentax AF540FGZ, Sigma EF-530 DG Super and EF-610 DG Super, and Promaster 7500EDF flashes each have seven steps to ¹⁄₆₄, followed by the Pentax AF360FGZ and Tumax DPT5 at six steps to ¹⁄₃₂, and the Tumax DPT3 at five steps to ¹⁄₁₆.
Of course, since the AF360FGZ has about half the power of the AF540FGZ, ¹⁄₃₂ of the former is about the same power as ¹⁄₆₄ of the latter.
The Sigma EF-530 DG ST and EF-610 DG ST allow only full or ¹⁄₁₆ power, and the Pentax AF200FG can only be manually fired at full power. The Metz 5000-series flashes can be equipped with a separate module (the FTM5000) which enables manual flash at full or ¹⁄₁₆ power. The Metz 36 AF-4/AF-5 and Promaster 7200EDF and 7400EDF models to have no manual mode at all.