<?xml version="1.0"?>
<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron">

   <sch:ns uri="http://www.olympus.com"
           prefix="olympus" />

   <sch:pattern id="Lens-Size-f-Stop-Correlation">

      <sch:rule context="olympus:lens[olympus:zoom='300mm']">

         <sch:assert test="olympus:f-stop &gt;= 1.2">
             A 300mm lens must have an f-stop of at least 1.2
         </sch:assert>

      </sch:rule>

   </sch:pattern>

</sch:schema>