Inferring that a Camera is a SLR?

Hi Folks,

I am a bit stuck on an inference.  I want to be able to infer that this
document is talking about a SLR:

<Camera>
     <viewFinder rdf:resource="http://www.camera.org#ThroughTheLens"/>
     ...
</Camera>

Here is the piece of knowledge that I am trying to use to make the
inference: 

    "Only SLRs have a ThroughTheLens viewFinder"

When I defined SLR I restricted the value of viewFinder to
'ThroughTheLens', as shown here:

     <owl:Class rdf:ID="SLR">
          <rdfs:subClassOf rdf:resource="#Camera"/>
          <rdfs:subClassOf>
              <owl:Restriction>
                   <owl:onProperty rdf:resource="#viewFinder"/>
                   <owl:hasValue rdf:resource="#ThroughTheLens"/>
              </owl:Restriction>
          </rdfs:subClassOf>
     </owl:Class>

And when I defined viewFinder I stated that it is a FunctionalProperty
and all Cameras have a viewFinder:

     <owl:ObjectProperty rdf:ID="viewFinder">
          <rdf:type rdf:resource="&owl;#FunctionalProperty"/>
          <rdfs:domain rdf:resource="#Camera"/>
          <rdfs:range rdf:resource="#Viewer"/>
     </owl:ObjectProperty>

Initially, I thought that these two OWL statements were sufficient to
allow me to infer for the above instance that the Camera is a SLR.  I
now believe, however, that it only allows me to infer that it "may" be a
SLR.  True?  What do I need to allow me to infer that the Camera "must"
be a SLR?  /Roger