Thyme writes: Hi i recently bought a Canon Sx1 IS and heard that it is possible to mount filters on it.
Which size is it possible to put on?
If you are talking about using the Lensmate SX filter holder:
This will accept 58mm filters.
...The lensonly goes down to f 1/8
Don't know quite what that means -- that is not a standard aperture #.
Go out on a sunny day and set the camera to aperture priority mode (whatever Canon calls it) and set the smallest lens aperture (largest f/number) you have. Now check out what shutter speed the camera is choosing. That will give you a good idea about how many more stops the light would have to be cut down to give you the long exposure times you mentioned. Then, shop for an ND filter(s) that would get you there.
Can they be combined or would this result in too much vignetting?
I have no idea. You could probably stack a couple of filters, but the more you put on, the longer the focal length you will have to use to avoid vignetting.
If you post that question in the Canon Forum, somebody there might have had some experience with this.