Originally posted by eversun The only available radio-optical trigger system.
What do you mean by "radio-optical"?
Probably not the approach taken by the Aokatec AK-TTL triggers, correct?
If you are simply talking about a mixed radio / optical operation then other trigger systems support that as well. Not supporting full Nikon CLS features for any optically communicating flashes, though. If that is desired then the Aokatec system would be applicable.
Originally posted by eversun that means you need only a pair to work many flashes. no need to buy receivers for all flashes.
Again, that is not unique to Acon.
Originally posted by eversun the only available trigger to make Pentax to Nikon CLS adaptor
Note that the Cactus system also supports the mixing of brands. In other words, one can use a Nikon, Canon, etc. flash in combination with a Pentax camera. If by "adapter" you mean a conversion from camera hot-shoe to a flash mounted on the camera-mounted trigger then to the best of my knowledge even that is supported by the Cactus V6II.
Originally posted by eversun that means you can nikon flashes only buying a transmitter.
You need at least one receiver.
A single transmitter cannot directly control Nikon flashes.
Again, I don't see why you think the Acon system is different to other systems with respect to how many units are needed for standard firing. If, for some reason, optical communication is used between most flashes then the Aokatec approach would provide native support (i.e., not just standard firing).
It doesn't make much sense, though, to achieve the benefits of radio-triggering only for one flash while the rest have to suffer the limitations of optical triggering.
If one assumes radio-control for each flash then there are a number of systems which can achieve brand interoperability.