Hi,
I have been trying to work out what is the safe focal length without vignetting for the Professional Kit with 2 filters altogether (three slots in total - 2 filters + tandem adaptor).
I have a 77mm WA adaptor and one holder at the moment and I am considering buying the upgrade kit and sq polariser.
I made some simple tests and calculations (FOV etc.) and if the two holders are connected with as I think (two slots at the back, one on the front) with max thickness of 30mm, the FOV is 112deg and should not produce vignetting @18mm. In fact with the trial I did I can see vignetting all the way up to 22mm when the holder is at and angle to the frame.
Also for 18mm I worked out that it should be safe to have the holder sticking out by as much as 37mm.
But it seems that this distance works for the Nodal Point, not the front of the glass.
Can anybody share some info with me, as I would like to decide if there is any point in spending money on the round CPL which is much more expensive or I could simply get the upgrade kit and the sq CPL
