I am certainly an advocate of photographers' rights, but frankly some
of them take the copyright issue to unreasonable extremes. A museum has
every right to be in full control of images of its objects and an
outside photographer, quite frankly, has no business owning rights for
work commissioned by the museum.
  This is a situation in which the institution should insist on a
work-for-hire contract with the photographer: the institution should
retain all rights and should get all images--prints, negatives,
transparencies, what have you--with the exception of perhaps a few
portfolio copies to show future clients. You should expect to pay more
for this arrangement than one in which the photographer gets the
rights--and it's worth it. An institution should not lose control of
images of its collection just because it has no staff photographer.
--David Haberstich