Pentax K-1, Pentax AF540FGZ II, Cactus V6 II - Guide number table for HSS

edited February 2019 in V6 II Suggestions
Hi, everyone,
I have following equipment:

Pentax K-1
Pentax AF540FGZ II
2 x Cactus V6 II

I've noticed that the output in HSS mode is different from that decribed in the manufacturer's tables, the Cactus V6 II allows the flash reach a higher guide number @1/250 s and the function seems decreasing by √2 while doubling shutter speed.

You're supposed to use the guide number because in HSS mode the illumination is continuous and constant beween the two courtains and a light meter woldn't work.

Could you provide a guide number table for HSS in the next edition of the user's manual?


  • edited February 2019
    HSS is a continuous light in the short period of time. It also depends on the camera F value and shutter speed to determine the power level in HSS. It also depends on the camera system and flash system. Therefore, there is no guide number table for HSS.
    Ray Chan

    Senior Product Specialist
    Harvest One Limited
  • I guess that's not exact.

    Both Pentax and Cactus provide a Guide Number table for HSS in their flashes user's manual, respectively AF360/540FGZ II page 59, and RF60X page 35.

    The difference between a HSS flash and a normal flash is that the exposure is affected by shutter speed, we can anyway use a guide number, which is just a relationship.
  • edited February 2019
    Chernobog said:

    The difference between a HSS flash and a normal flash is that the exposure is affected by shutter speed, we can anyway use a guide number,...

    Yes, for HSS, the GN numbers would depend on the shutter speed (as well as the zoom setting and the power level).

    As you observed, though, you can obtain the GN for a particular shutter speed by dividing the GN that holds for half the shutter speed by √2. So it is easy to create an approximation of the HSS GN table yourself, as long as you can (roughly) establish the GN for the longest shutter speed possible.

    Note, though, that the RF60X does not behave as you probably expect in HSS mode regarding power levels. The difference in power output between full stop changes is not a full stop when the RF60X operates in HSS mode. It seems that Cactus opted to spread the HSS power output range from 1/1-1/16 to something like 1/1-1/128, giving up the correct output relationship between nominal power levels.

    Other products limit the use of HSS to a range like 1/1-1/16; I'm assuming due to limitations of flash tubes that cannot maintain an HSS burst at very low output values. Instead of going that route, Cactus seem to have spread the available output range over the full nominal power range, so don't expect your GN calculations to hold as they would in non-HSS mode.

  • edited February 2019
    Thank you. Through empiricism I've seen that the obtained GN is precise enough to avoid repeated attempts during a shooting, this was the purpose. I've already noticed that RF60X works in a different way, as specified on the manual, but I haven't bought a RF60X yet.
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