Originally posted by Christian thanks for all the info. I'm not particuarly sure what all that meant, so still not sure which of the two is strongest... although they're both prob on a par I guess?
What's auto flash?
At maximum power they're probably nearly equivalent.
When in P-TTL mode, the AF-540 tends to be "lazy" when mounted to the K10D, likely resulting in the perception that it is far less powerful than the Sigma, as it consistently underexposes even though it has the power delivery capability to properly expose the picture.
Note that a few other threads have indicated that some K10D metering problems were fixed in 1.30 despite no mention of it in the changelog for that firmware rev. The 540 may be fine now when used with the K10D - As I type this, I'm topping off my K10D's battery in preparation for a 1.30 upgrade to check.
As to "auto flash" - Most older flashes that didn't do TTL had a bult-in light sensor that allowed the flash itself to meter the exposure. You would set aperture on your camera and the flash would do all the rest. Most TTL (and P-TTL) flashes have removed this sensor, one of the 540's saving graces is that Pentax left the auto sensor on the flash so it can be used in this mode, which usually performs more consistently than P-TTL when mounted on a K10D. Of course, once you remove P-TTL the AF540 becomes not much better than a much cheaper Vivitar 285HV.