code: public byte getFieldAsByte(int tag, int index) {
          Integer i = (Integer)fieldIndex.get(Integer.valueOf(tag));
          byte [] b = fields[i.intValue()].getAsBytes();
          return b[index];
      }
docs: Returns the value of a particular index of a given tag as a
  byte.  The caller is responsible for ensuring that the tag is
  present and has type TIFFField.TIFF_SBYTE, TIFF_BYTE, or
  TIFF_UNDEFINED.