The letters identify a long working or ultra-long working distance objective. The vertical distance from the front lens of the objective to the focused specimen (working distance) is much longer than that for a similar magnification standard objective. Such objectives are invaluable for looking up through a culture vessel or Petri dish in inverted biological microscopy.
These letters identify long or ultra-long working distance objectives where the working distance is much longer than standard objectives of similar magnification.

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