Yeah, the limiting factor for
true field is the diameter of the field stop (ie: the diameter of the maximum "cone" of light that can make it through the end of the eyepiece without being cut off by the field stop). For a 32mm diameter eyepiece, this is around 27mm.
This is why a 32mm FL plossl gives the biggest possible true field for a 32mm diameter eyepiece, and why a 40mm plossl (or longer) at 32mm diameter has a reduced
apparent field. If it had a full 50 degree field there would be vignetting, or light cut-off from the field stop.
Al Nagler has written an interesting article about all this. See:
http://www.skyandtelescope.com/astro...epiece-primer/
- Dean