I am thinking to buy the M1-L wRD16-II .. in the site has a option to change the rail to 210mm ... which rail that be changed ... the horizontal or vertical rail ? ... And what is the function of this 210mm rail ( I mean .. why I need to change to 210mm ) ?
BTW .. what is the weight of the M1-L wRD16-II ?
what is your equipment in use?
The default package comes with 210mm lower rail and 170mm upper rail. It should fits most people. If you use very big/ long wide angle lens such as the 24-70mm F2.8, you need 210mm upper rail. If you don't use battery grip, you just swap the upper rail with the lower rail. if you use fisheye lens you need 170mm upper rail unless you use the QRC-40B to slide the camera on the rail.
So the safest bet is use both 210mm rails and QRC-40B together with QRC-40A.