I know that the shutter speed should be approximatively as twice as much the fps. However, I know people who tune the shutter speed to any value they need it to achieve the correct exposure. They say that, except for a few cases, using a shutter speed bigger than the recommended value (2X the fps) is not noticeable.
I would like to hear your opinion about this. It is quite important to me, because if this is true, I would avoid using ND filters, as they reduce image quality.
On DSLRs, a fast shutter can actually be better. DSLRs scan the image to sample rather than capturing the whole frame at once. This is why you get a distortion when you do a rapid pan. A faster shutter will result in stopping motion more clearly (less motion blur), which can seem a little harsher than the softness of film, but a sharper image may be preferable. That said, a good ND shouldn't have a significant impact on image quality.
So, my final advice is, if you want crisp frames with little motion blur, use high shutter speed, if you need smooth motion for things moving around a static scene, a high quality ND is probably better. If you simply have a fairly static shot, then shutter is probably the way to go again.
I disagree with the above answers.
Our visual culture and the century of cinema has dictated that we evaluate a 180 degree shutter as "normal", because that is how film cameras have worked almost forever, and that is how most scenes in every movie are shot.
180 degree shutter is the same as 1 over twice the frame rate, or 1/48 for 24fps.
If there is little to no movement in frame, it is true that shutter speed will not create any visible effect, but with fast character movement or almost any but the slowest camera movement a different shutter speed will be noticeable.
It is valid to say that for storytelling reasons you need the shutter to be closed down, but if its just for technical reasons, it will be noticed, and will come through as amateurish. Use ND filters for exteriores, for sure. If you are afraid of quality loss, test them, but every major production uses ND filters, so unless you use cheap plastic ones, you will be fine. Be more wary of color cast than of resolution loss though. Some digital sensors need extra IR filtration on hot day exteriors to maintain color consistency.
Regarding timelapses, the same rule applies. Use 180 degree shutter unless the story calls for something different. This presents an extra challenge. If you need one frame every 10 seconds, it means that each frame must be exposed for 5 seconds, which is a lot for day exteriores, and you will need a lot of ND filtration. I am sure there are many great timelapses that do not follow this rule, but I suggest looking at Kooyanisqatsi, the feature film that made nature timelapses famous. Every shot uses a 180 degree shutter, and it looks gorgeous, cinematic, epic.
In almost all digital video cameras there's no physical shutter, just a circuit that limits the exposure time. As long as that time allows the entire sensor to be illuminated and scanned, you should see no degradation of any individual frame. In this sense, as long as the minimum is met, there should be no visible difference between a fast shutter and an ND. But then there's motion.
With limited motion, fast shutter times and NDs are pretty much equivalent, and trade off against aperture. A significant difference is that closing the aperture increases depth of field, where increasing shutter speed (or using NDs) does not. So aperture and ND have different visual effects, but only you can decide which is better for your shot.
With more rapid motion the difference is more obvious. With fast shutter times motion will seem less fluid, with each frame being crisper, more frozen in time, which is not true for NDs. With normal shuttering, motion tends to blur within the frame, since different positions of a moving object are integrated during the exposure. This is usually more pleasing to my eye, but your aesthetic, or your situation, should be what matters.
