Does the HX-100V have preflash or red-eye?
buy Yongnuo Yn560, not expensive, more than enough for HX100v, use S2 mode to skip the preflash and it works perfect at least under indoor condition. The only problem is that it is a full manual flash which does not support any kind of TTL auto metering/setting therefore if you wish to conserve it for further use on the hotshoe of a DSLR then you may find a manual flash really slow to operate. But for studio portriat a strobist manual flash is not a problem at all as you will have plenty of time to adjust the settings. Under S2 mode it does not need any radio trigger or additional optical trigger as long as the sensor on the flash can see the internal flash of your HX100v.