Lots of videos shot with DSLRs look pretty awful to my eyes when the frame includes rapid motion. Stroboscopic, or stutter, effects destroy any pretence at a natural look.
But those helpful people over at the Canon Professional Network have just published the article 25F Progressive video: the cinema show
which contains some handy hints, particularly on the second page. While the article is not particularly DSLR focussed I think much of it also applies:
Shooting 25F: Camera handling
How do you avoid flickering or strobe-like images? If you have played around with progressive video before, you have probably wondered why those Hollywood movies, shot at only 24 frames per second look so steady and ‘peaceful’ in comparison with your own. Well, besides a considerably lower budget, that’s because progressive filming is all about camera handling.
The same handheld shot that worked fine in 50i may end up somewhat disappointing when it was made with your camera set to 25F. Due to the lower refresh rate of the image, a shaky camera style is much more unpleasant to watch in 25F than it is in 50i. So you’ll have to make an effort to make slower better-controlled camera moves and take it a little easy on the panning.
Objects that move horizontally through the frame can result in a very noticeable 'stutter' and ruin your shot. Instead, try to follow your subject while leaving the background slightly out of focus. This will create a motion blur that makes everything run smoothly.
You can also change the composition of the shot. For example, instead of having your subject move across the frame, you could have it move at an angle to the camera, say, along a roughly diagonal line.
Shooting 25F: Shutter speeds
In most situations a good shutter speed for 25F footage is 1/50sec. With that speed you get smoothly moving images, while retaining enough detail in slow motion. Using a higher shutter speed will give you lots of detail but it can also cause serious stutter and will definitely be uneasy on the eye. When capturing high-speed motion, as in sports, a smaller shutter speed will give you very crisp images, although they can look somewhat artificial. This, of course, could be exactly the effect you are going for.
A big advantage of shooting 25F is that you can go down to a shutter speed of 1/25sec, which gives you an extra f-stop of exposure. This makes 25F ideal for shooting in low light conditions. With a 1/25sec shutter speed you need to take into account that you will have a lot more motion blur with moving images. The picture looks very smooth but moving objects can lack detail.
Definitely worth giving the whole article
a read, though, and kudos to Canon for producing it.