Today, I have conducted a followup test to see the effect on the true field of view for 1.25" and 2" focuser.
Strange is that, my 2" Pentax SMC XL does provide more true field of view than my 20mm Widescan Type III and also my 40mm Plossl in my Tele Vue Ranger which has only a 1.25" focuser.
That said, it's nothing strange that my C5 and my Borg can deliver more field of view with a 2" eyepiece.
I have also done some tests with my Meade prism, chromatic abberation really jumps in together with other distortions and a softer image. It's not to be used for any serious observation.