Yes, that's the normal mode and the one strobists prefer.

P-TTL-driven automatic exposure has its virtues but can be too inconsistent for some. Some professional wedding photographers shoot their camera in manual mode because it gives them more consistency and apparently does not slow them down sufficiently to not consider this approach. In the same vein, I'd say manual flash control can work even in dynamic situations if you trained yourself to work that way.

The only way to get automatic exposure support in combination with the V6 would be to use flashes with an "(A)uto" mode and just fire them remotely. But I'm sceptical of how well this would work with multiple flashes.

Would be nice if you could post a pointer to that discussion here, once you have started it. I'm starting to develop patterns, like matching the exposure steps on the camera (say 1/2 stop) and on the V6, and then just matching click counts on both aperture dial and V6 flash power dial whenever I change the aperture and want to keep the same flash exposure.

Thanks a lot for the kind words. I'm glad it is helping someone.

What makes you think that?

Unlike the Godox FT-16s which has some real usability issues, the Cactus V6 supports up to four groups very well.

The V6 does not support automatic exposure support, but for manual power level control the usability is really good (I could think of alternative ways of providing control, but it becomes a matter of preference at some point).

The V6 does not relay any P-TTL information.

It only uses the TTL protocol of the flash in order to support remote power control.

I've just published my Cactus V6 review about a new radio trigger that allows remote manual power control for speedlights in a brand-agnostic manner.

With this trigger you can combine, for instance, a Nikon SB-24, Nikon SB910, Canon 540EZ, Metz 58 AF-2, and Pentax AF 360 FGZ all in one lighting setup while controlling the power levels from your camera.

I think it is really cool that this trigger gives old legacy TTL flashes a new lease of life. If you have one of these TTL flashes, chances are that you can trigger it and remote control its power even if your model is not supported out of the box. The V6 has a "learning" mode with which you can create a flash profile for your flash model. You can then use it in combination with other flashes in an "absolute mode" which makes your power level settings -- now no longer expressed in fractions but in "EV" -- independent of the maximum guide numbers of a flash.

The review goes into a lot more detail.

Happy to answer any questions if I happen to know the answers.

I've just made my Cactus V6 review available. I hope people will find it useful.

The V6 is the perfect on-camera trigger for the RF60 but can also remote control the power levels of many modern digital flashes and a large number of legacy TTL flashes.

I'm not sure what you are objecting to.

Please note that my statement was made in the context of using HSS, i.e., the camera is using a shutter speed higher than 1/180s.

In such an HSS situation, the only way to obtain a trigger signal is optically from a P-TTL flash connected to the camera.

If a pre-A lens disables P-TTL then even that triggering opportunity is gone and in this case you cannot use flash beyond 1/180s at all.

What is the concrete challenge?

There is unfortunately no way around using a P-TTL flash connected to the camera (on the hot-shoe or using a cable), but you could use a Cactus V6 (or Cactus RF60) to optically trigger on the P-TTL flash and thus gain a radio trigger signal from the HSS flash.

You are then free to trigger as many off-camera flashes as you like. The Cactus RF60 offers an HSS sympathy mode that achieves HSS bursts for all manual levels, but you can also use pretty much any flash at full power because with most speedlites at full power the pulse is long enough to illuminate the whole frame.

The Cactus V5 have both functions (including half-press and bulb mode for the shutter release functionality), but if I were you, I'd wait a tiny little bit longer for the Cactus V6. They are going to be awesome.

No, but you have to make sure that the flash and the K20D use the same channel.

You need to put the flash on the K20D's hot-shoe for the channel matching. Have a look into your K20D manual.

That's not true.

The P-TTL flashes issues of the K-5 occur with other flashes as well. Not the fault of the Metz.

The YN560-III is a good flash, but I'd recommend one whose power can be remote controlled via a radio trigger. The YN560-III is rumoured to support this, but the corresponding trigger is yet to be released.

Have a look at the Cactus RF60 as well. It can work as a master flash on the hot-shoe, remote controlling other flashes (via radio), or you can use the Cactus V6 radio trigger to remote control the power of an off-camera RF60 (and many other flashes).