So what's the advantage of 50i? And why does every man and his dog seems to say it's better to use 50i for fast action stuff?
Dave.
The more information the better in motion recording. But this depends on too many factors now when you are producing video for more than one type of display. I and my dog say "Use 25P and don't worry about it". If you had 50P or 60P then it would be better and no hassle. Using non-interlaced recording makes less work for you, less work for the viewer, and ensures that everyone sees the video without nasty artifacts.
I shoot 30P for NTSC clients and 25P for PAL clients. I never shot interlaced as it just causes all kinds of issues and more work to make sure the end viewer sees the intended quality.
The eye can see the difference between higher frame rates. It is just that the minimum frame rate to help the brain fool itself into seeing fluid motion is around 18fps and 24fps to ensure the appearance of fluid motion. Of course people can see motion better with higher frame rates, just ask any gamer why they spend $400 or more on a video card.
The frame rates for PAL and NTSC were chosen to get the best motion appearance out of the signal bandwidth provided. The interlace idea was a work around to using phosphors in the CRTs. At 25/30FPS the phosphor at the top of the screen that was "written" to by the beam would start to fade before the beam finished drawing the whole image and the rolling flicker was annoying. If they used a phosphor mix that had longer persistence then the image retained so ghosting from the previous and the image got blurred. So the idea was to write half the image very quickly on every other line, then go back and re-draw the second part of the image, the in between lines, before the image could fade too much and this alleviated the rolling flicker effect. So this interlacing was a compromise for video and the technology of 60 years ago. Therefore, interlacing is old and useless now and should not be used!
Shutter speeds for lower frame rates are slow to compensate for the loss of temporal information between frames. If you use 1/1000 shutter speed then you have a nice series of frozen very sharp frames and the film/video can look non-fluid. If you use a slow shutter rate then the blurring effect on the moving object helps the brain fool itself into seeing more fluid motion.
Interlaced video corrected for non-interlaced output will just look the same as non-interlaced output in the first place if you use simple de-interlacing techniques. The more sophisticated techniques will throw away half the spacial resolution locally to remove the comb effect and fill in the missing information with interpolated lines so that things look nice and smooth. You gain temporal resolution for the trade off of loss of spatial resolution. The really advanced techniques can clean up most of the problems but there is always a loss of information somewhere but balanced in a way so the eye is fooled.
For the purposes of a video blog that will be viewed on mostly non-interlaced display devices, interlaced is a complication without any benefit. Again, the only reason to use interlaced is if you intend the final output to be viewed solely on CRT TVs!