Originally posted by Robert N Des, many thanks for your comments on Fresnel extenders. From your experience of the MagBeam, what typical f stop exposure gain do you get from this device. I am considering it for wildlife photography.
I haven't done any comparison, Robert, but based on my subjective impression I would say that Digitalis's estimate of 1.5 stops may be a little conservative. There is a noticeable gain in the throw of the light - that is, at say 10m or more the light output is still quite strong. So much so, in fact, that it's often necessary to wind back the flash output (I have a Metz 58-AF1, which is GN 58).
The MagBeam is well-designed and robust, and there are three extension options. Its main disadvantage is its weight. With a hefty flash like the Metz I probably end up with about 1kg or so perched in the hotshoe of the K-3. I previously used a JJC one, which was light weight, cheap and effective but rather flimsy - the plastic attachment mount broke quite easily. The third one I have used (and still use) is the Rogue Safari, which concentrates the beam from the popup flash. It's cheap, lightweight, simple and surprisingly effective with the K-3:
Rogue Safari Flash Extender - PentaxForums.com. The main problem is that it produces a lot of eye shine, because the beam is too close to the lens.