Creating a schema which imports some other schemas

I have 3 basic schemas which both Xerces-C++ and XMLSpy claim to be valid: Nikon.xsd, Olympus.xsd and Pentax.xsd.

Then I try to create a new schema (Camera.xsd) which imports the definitions of the 3 basic schemas in order to create a more complex new element. Both Xerces and XMLSpy show an error which says that types with name nikon:body_type, olympus:lens_type and pentax:manual_adapter_type have not been defined in current nor included/imported schemas.

I cannot find the error which prevents the new schema from being properly validated. Some advice would be appreciated.

Best regards,
David

--------------------- Nikon.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns="http://www.nikon.com"
            targetNamespace="http://www.nikon.com"
            elementFormDefault="qualified"
            attributeFormDefault="qualified">
    <xsd:element name="body_type">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element name="description" type="xsd:string"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>

--------------------- Olympus.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns="http://www.olympus.com"
            targetNamespace="http://www.olympus.com"
            elementFormDefault="qualified"
            attributeFormDefault="qualified">
    <xsd:element name="lens_type">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element name="zoom" type="xsd:string"/>
                <xsd:element name="f-stop" type="xsd:decimal"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>

--------------------- Pentax.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns="http://www.pentax.com"
            targetNamespace="http://www.pentax.com"
            elementFormDefault="qualified"
            attributeFormDefault="qualified">
    <xsd:element name="manual_adapter_type">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element name="speed" type="xsd:string"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>

--------------------- Camera.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns="http://www.camera.org"
            targetNamespace="http://www.camera.org"
            xmlns:nikon="http://www.nikon.com"
            xmlns:olympus="http://www.olympus.com"
            xmlns:pentax="http://www.pentax.com"
            elementFormDefault="qualified"
            attributeFormDefault="qualified">
    <xsd:import namespace="http://www.nikon.com"
                schemaLocation="Nikon.xsd"/>
    <xsd:import namespace="http://www.olympus.com"
                schemaLocation="Olympus.xsd"/>
    <xsd:import namespace="http://www.pentax.com"
                schemaLocation="Pentax.xsd"/>
    <xsd:element name="camera">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element name="body" type="nikon:body_type"/>
                <xsd:element name="lens" type="olympus:lens_type"/>
                <xsd:element name="manual_adapter" type="pentax:manual_adapter_type"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>