Clearly the wired mic is a non-starter. As soon as you or the angler moves, the wire’s going to yank away from his clothes or pull out of the camera, or you’re both going to trip over the wire and so on.
Your Rode – or any – shotgun mic will give you freedom of movement but you’ll still need to be in quite close to the subject to hear what he’s saying. Don’t forget, a shotgun merely rejects sound from the sides and behind. It doesn’t actually enhance or zoom in on sound from the front. So for the sound to be good, you still want the mic no more than a few feet away from the subject and pointing directly at him, which could be very tricky when he moves. As soon as he goes “off axis” (ie not pointing directly at the mic) you’ll lose all decent sound.
Having said that, whatever else you do, I suggest it would certainly still be worth having a shotgun on the camera to pick up a clean recording of the ambient sound which will enhance your video but the Rode may or may not be appropriate. You can fit conventional hotshot gear on the G25 but it’s a nonstandard size so you need to add a small adapter (£10 ish) and I wouldn’t want to put anything too heavy on it as it could strain the adapter and mount. The Rode mic also has XLR connections which you’d need to convert to non-XLR for the Canon, either the crude way (with a cable which simply cuts one of the XLR connectors) or the proper way which is with an adapter box from the likes of Beachtek. Oh, and the mic will have to be self-powered too as the Canon does not provide phantom power or even any kind of plug-in power.
If budget is not an issue then the best option is a skilled sound operator with the mic on a boom pole held over the subject’s head; that way he can move as the subject moves and always capture good sound.
Without that, I think you’re going to have to invest in a radio mic pack and put a radio mic with lavalier on the fisherman. The only slight issue with that is that if turns his head, you will get a slight loss of pick-up but a decent omni-directional tie-clip should be OK.
There are various radio mic systems out there but the cheap ones tend to be terrible. The most widely used “entry level” decent quality set is the Sennheiser G3. Beware, there are assorted versions of this with different frequency bands according to different country’s licensing requirements. The UK version is Channel 38 but this requires a licence from JFMG. There is a legally available version that includes a licence-free band (Channel 70) but you have to specify this at time of order. And note that because it’s licence free, lots of people can use it so you’re not guaranteed a clear signal though I would have thought on a riverbank the chances of interference are negligible…
Finally, are you sure the HF-G25 is the right camera for you? You may find the XA10 – which includes XLR audio sockets and a mic mount – is a better bet, albeit at a greater price. And if you can wait a month or so, there is new XA20 which has various improvements (better screen, zoom, stabilisation etc)
Meanwhile I recommend a quick look at these videos:
http://www.youtube.com/watch?v=Y63sin6vvfs (discusses the power issue with external mics)
http://www.youtube.com/watch?v=XVHNe-6runQ (all about the audio on the HF G25)
http://www.youtube.com/watch?v=Le8F2H-Hebw (mentions and shows the shoe adapter)
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