Actually the traditional 180° shutter speed for 25 FPS is 1/50 not 1/60. Having a 180° degree shutter simply means having the shutter open for half the time of the frame advance rate. In old cine cameras they literally had a circular disk with a slot cut in in that rotated at the same revolutions per second as the frame rate. The standard slot would be open for 180°, giving a shutter time of half. This allows just the right amount of blurring of moving subjects for it to look natural when played back at the same fps.
If you shoot at faster than 180° then what happens is the moving parts of the image look sharper than they really ought to. The classic example of this effect was first famously used when filming the opening half hour or so of Saving Private Ryan, but is also used in the series Band of Brothers, and Pacific, to give that very sharp look that we tend to get with the release of lots of adrenaline to the system. In those circumstances our vision becomes heightened, and we can see and react events that would otherwise be beyond our ability to see. So actually the video isn't "jaggedy" it is simply sharper. The effect can be quite good, as long as it's not over used. They often use it in shows like Top Gear when cars are throwing up lots of loose gravel etc, in either slow mo, or normal speed.
When it comes to shooting in bright light using such slow shutter speeds, ISO 100 is actually too fast. This is when a digital camera with native ISO 25 and/or 50 would be very helpful. Personally for what I shoot most lowering the ISO range by two stops would be of great benefit, but preferably not just by building a two stop ND into the body. Ideally with most lenses you want to be shooting one or two stops down for best performance, although for fast primes I guess you really want to be shooting around ƒ/5.6 to ƒ/8. I would not routinely want to be shooting at apertures smaller than ƒ/11. So I would also suggest that investing in some ND filters, or a variable ND which will allow you to control the light entering the camera when you cannot reduced the ISO any more.
I shoot aviation and a lot of the time I need to be able to shoot at 1/160s and slower so that I can get a decent amount of propblur. Some helicopters need shutter speeds of 1/60 to show any significant amount of rotor blur, and again at ISO 100 this can mean shooting at ƒ/22 even on days that are a bit overcast. The one thing that shooting at such small apertures will show you is the amount of sensor dust and other stuff you have on your sensor. It can be pretty horrific, and almost none of it will show if you shoot at ƒ/8 or wider.
Alan
