When you say that the f-number is not that important, I think it is.
It will have to a specific aperture size (f-number) that combined with the 8sec shutter speed, gives you the correct overall exposure. Won't it. A random f stop probably won't work.
Did you mean it wasn't important when it comes to depth of field?
I said this so as not to put to much brain thinking into such a shot.
I.E. if you just set the camera to auto on an exposure to allow an 8 sec shot it may go from small f-number to large f-number to get the exposure up to the required time and you will still get the ghost effect to work. It may get blurry towards the back of the shot but for the overall effect this will add or enhance the image in some cases.
Even up at f-32 the shot will still work.
For the more experienced like yourself then yes the f-number would be best at f-8.
Depth of field will be better if thought out before the shot but not essential to still produce a ghost effect.
Most people use a zoom and no-one I know uses fixed prime lenses bar me.
I took this shot with about 5 different settings on the day as the students were only expected to give me a ghost image effect without any help from programs.
I set out in the assignment they only had to show a believable ghost effect and back when I did it cameras were still only up at 6 megapixels. The 8 &10 megapixels came later.
Image shows 2004, Manual, ISO 100, f-8, 8 sec exposure. Canon D300.
Actual lens used I cannot tell you and have no way of remembering.
YES, I USED A CANON, the shame and horror
The actual outcome and look of the image in their assignment was the final marks for them, the actual camera, lens or any camera setting were not relevant to the assignment for the students. Even today it is not relevant to be a master photographer to get a required shot, only a basic understanding of how to point and shoot. Students would use images in so small a final setting you would never see any problems.
Multimedia, technology and design does not require a degree in camera settings and only needs a basic understanding of camera settings.
As a professional photographer, then yes these camera settings matter and in fact form the basis of how well you will survive in the real world.