Correct Owl representation

I am confused about the representation in OWL for the following

<owl:DatatypeProperty rdf:ID="focal-length">
        <owl:equivalentProperty rdf:resource="#size"/>
        <rdfs:domain rdf:resource="#Lens"/>
        <rdfs:range rdf:resource="&xsd;#string"/>
</owl:DatatypeProperty>

and

<owl:DatatypeProperty rdf:ID="f-stop">
        <owl:equivalentProperty rdf:resource="#aperture"/>
        <rdfs:domain rdf:resource="#Lens"/>
        <rdfs:range rdf:resource="&xsd;#string"/>
</owl:DatatypeProperty>

and then use it as ,

<Lens>
    <focal-length>75-300mm zoom</focal-length>
   <f-stop>4.5-5.6</f-stop>
</Lens>

Would the above be incorrect representation in OWL ?

Would it be correct to do it like this  

<Lens>
    <focal-length rdf:datatype="&xsd;string">75-300mm zoom</focal-length>
   <f-stop rdf:datatype="&xsd;string">4.5-5.6</f-stop>
</Lens>

If this is correct, then what is the rationale behind specifying range 
as "String" in defining the property ?

-Monika
-- 
**>><<**>><<**>><<**>><<**>><<**>><<**>><<**
Monika Solanki
Software Technology Research Laboratory(STRL)
De Montfort University
Hawthorn building, H00.18
The Gateway
Leicester LE1 9BH, UK

phone: +44 (0)116 250 6170 intern: 6170
email: monika@dmu.ac.uk
web: http://www.cse.dmu.ac.uk/~monika
**>><<**>><<**>><<**>><<**>><<**>><<**>><<**

Received on Friday, 26 September 2003 14:17:23 UTC