Originally posted by Photodana I'm looking to buy a Metz 64 AF-1 because it has more power than the Cactus RF60x as well as my Pentax AF540FGZ (The two have close to the same power output, I suspect).
Note that flash manufacturers specify the guide number of a flash (GN 64 in the case of the Metz 64 AF-1) when using the longest zoom setting. In case of the Metz 64 AF-1 that zoom setting is 200mm, giving the Metz some advantage over flashes with a standard maximum setting of 105mm.
Fortunately, Metz specify the
GN numbers of the Metz 64 AF-1 for each of its zoom settings in the manual. You'll notice that at 105mm, the Metz 64 AF-1 "only" has a GN of 60. That is insignificantly more powerful (~1/3 stop) than a GN 56 flash.
Note that at 35mm, the Metz 64 AF-1 has a GN of 36, whereas the
Metz 58 AF-2 has a GN of 35. So when used at lower zoom settings, these flash models are functionally equivalent, despite the apparent difference in maximum GN spec.
The only time you are actually going to reap an advantage from a Metz 64 AF-1 in terms of power is when using it without any modifier, directly (not bounced) at its full zoom setting. The maximum zoom setting can be a good way to bridge more distance (when using a respectively long focal length lens) but whether you'll like the harsh look of flash from an unmodified small flash head is up to you. The only other advantage of a long zoom setting I can think of is to create a smaller spot, when you are trying to control the light to a small area for some reason.
Mcgregni is quite right in recommending to double check compatibility between the Metz 64 AF-1 (for Pentax) and the Cactus V6II, in particular for the X-TTL variant you are aiming to use. My expectation would be that it is supported, but it is better to double check.
@Mcgregni, if you are running the X-TTL variant on your V6II, perhaps you could check whether a Metz 64 AF-1 profile is present? If you have the opportunity to check this, perhaps you could also look for the Metz 58 AF-1 and 50 AF-1 which are two other flashes Photodana has expressed interest for?