The definition of lewdness may not be obvious, but
that doesn't mean that we
aren't capable of coming up with a good one to use within the context of the
law.
The definition in the context of UK law is simply what a "reasonable
person" considers indecent, there is no set definition. The sentencing
guidelines talk about "erotic posing" (as the least serious form of
indecency, there are obviously more serious forms that are far easier
to define).
I'd like to hear your definition, especially with
regard to what type
of exhibitionist acts a parent can legally convince his young daughter to
perform. I don't much care for assertions of what "plenty of people"
believe if they aren't attached to explanations as to *why* those people
believe that.
My definition of "erotic" would require there to be a sexual element.
Nudity is not automatically sexual, there would need to be something
about the pose which could be reasonably associated with sex.