You have indeed got things slightly confused, if you don’t mind us saying. So let’s just recap interlacing and progressive:-
Interlacing is a way of dealing with video images that came about decades ago when TV was first designed. Imagine each frame of a moving image (and there are 25 frames per second in the UK, 30 in the US) as comprising many lines of dots. In HD video, that’s 1,080 lines each of 1,920 dots. Interlacing was a way of capturing and displaying these lines by treating each frame as two distinct “halves”, called “fields” (thus 25 frames per second is actually 50 fields per second).
But rather than one field being the top half of a frame and the other field being the bottom, the way it works is that the first field is all the odd-numbered lines, the second field is all the even ones. When the two fields are displayed on a conventional television, each field is displayed one after the other, so quickly that your eye considers it to be a single frame but in reality you’re seeing 50 half-images per second rather than the 25 full images that were scanned by the video camera.
One advantage of this interlacing is that it gives video a nice fluid smooth motion because you get 50 changes of the picture every second (albeit that they’re only half images!)
Progressive is a (newer) way of capturing all the information on all the lines in one single frame (no fields) and is the way that computer screens work. So you get 25 full frames per second. One disadvantage of progressive is that motion can look a little jerky because of there being fewer changes in the image for your eye to see. That said, nobody minds watching films at the cinema and they’re projected at 24 frames per second!
To cut a long story short, to shoot video that will be watched on TV you would traditionally shoot interlaced. This same video can then, if you wish, be “de-interlaced” (combining the two fields into a single frame) for playback on a computer screen.
However, if you are shooting video that you categorically know will only ever be watched on a computer, you can record progressive from the outset, if your videocamera permits, and that will be the best format for computer playback. There’s nothing to stop you also watching this on a telly but you might notice the slightly staccato motion in some instances.
So, you can indeed, quite happily, shoot 50i and then edit it, rendering out with deinterlacing at 25fps for use on a website. Most of the earlier videos on our channel were recorded exactly like this.
However, you can, should you choose and if your editing software supports it (depends on your editor), shoot 25pf which is a genuine 25p frame but encoded into a (50i) interlaced format because of the way the camera works (this is why your edit software needs to understand the format correctly – it has to understand that although the footage looks like 50i, it’s actually 25p!!)
More recent camcorders now offer both 25p recording (that’s genuinely saved as a 25p file) and even 50p (which is best of all because you then have the choice of rendering it out as either 50p, 25p or 50i without any deinterlacing necessary). Unfortunately, the HF-G25 does not offer these newer formats.
In short, you’re fine to use 50i (but deinterlace to 25p when you render out). Or, try 25pf but make sure your editing software understands what the format means.