[web2py] Any difference passing a string vs. a field for arg #2 in IS_IN_DB?

[MichaelF]

The doc shows two calling patterns for IS_IN_DB, the difference being the 
type of the second positional argument:

The example in section 3.6 shows:
db.comment.image_id.requires = IS_IN_DB(db, db.image.id, '%(title)s')

Section 6.5 has a table for default validators:
IS_IN_DB(db,table.field,format)

Section 7.6.2 *Database Validators* (and other places) shows:
db.dog.owner.requires = IS_IN_DB(db, 'person.id', 
'%(name)s', zero=T('choose one'))

Is there a functional difference in passing the string vs. passing the 
field?