Hey,
To debug this problem, a developer will need the smallest possible example of code that demonstrates the problem. That means, I take it, just 2 schemas and a single form. Describe briefly what you expect to happen and what in fact happens. If that example can be done *without* inheriting from Field that'd be good, as it is true that Field is only to be used inside a schema definition and once someone sees that we'll conclude that's the cause of the problem even though it might not be.
It is interesting that in table 4.1 of Philipp W's book it specifically states that Field is the base class for all other fields. So how does one build fields that are noot part of the standard zope.schema?
Yes, you do create new schema fields by subclassing from Field. It's just that we saw you putting a field not in a schema but in what looked like a concrete object. Perhaps we were wrong in reading your code, and this is one reason why you should come up with a minimum example that demonstrates the problem and only that, without a lot of distracting code surrounding it. You're the best suited person to actually create a minimum example. Regards, Martijn