code: public float getFieldAsFloat(int tag, int index) {
          Integer i = (Integer)fieldIndex.get(Integer.valueOf(tag));
          return fields[i.intValue()].getAsFloat(index);
      }
docs: Returns the value of a particular index of a given tag as a
  float.  The caller is responsible for ensuring that the tag is
  present and has numeric type (all but TIFF_UNDEFINED and
  TIFF_ASCII).